Appendix A - Reference Manual (p. 191)

A1 - Introduction

This manual describes the C language specified by the draft submitted to ANSI on 31 October, 1988, for approval as "American National Standard for Information Systems — Programming Language C, X3.159-1989." The manual is an interpretation of the proposed standard, not the Standard itself, although care has been taken to make it a reliable guide to the language.

For the most part, this document follows the broad outline of the Standard, which in turn follows that of the first edition of this book, although the organization differs in detail. Except for renaming a few productions, and not formalizing the definitions of the lexical tokens or the preprocessor, the grammar given here for the language proper is equivalent to that of the Standard.

Throughout this manual, commentary material is indented and written in smaller type, as this is. Most often these comments highlight ways in which ANSI Standard C differs from the language defined by the first edition of this book, or from refinements subsequently introduced in various compilers.

A2 - Lexical Conventions

A program consists of one or more translation units stored in files. It is translated in several phases, which are described in §A12. The first phases do low-level lexical transformations, carry out directives introduced by lines beginning with the # character, and perform macro definition and expansion. When the preprocessing of §A12 is complete, the program has been reduced to a sequence of tokens.

A2.1 - Tokens

There are six classes of tokens: identifiers, keywords, constants, string literals, operators, and other separators. Blanks, horizontal and vertical tabs, newlines, formfeeds, and comments as described below (collectively, "white space") are ignored except as they separate tokens. Some white space is required to separate otherwise adjacent identifiers, keywords, and constants.

If the input stream has been separated into tokens up to a given character, the next token is the longest string of characters that could constitute a token.

A2.2 - Comments

The characters /* introduce a comment, which terminates with the characters */. Comments do not nest, and they do not occur within string or character literals.

A2.3 - Identifiers

An identifier is a sequence of letters and digits. The first character must be a letter; the underscore _ counts as a letter. Upper and lower case letters are different. Identifiers may have any length, and for internal identifiers, at least the first 31 characters are significant; some implementations may make more characters significant. Internal identifiers include preprocessor macro names and all other names that do not have external linkage (§A11.2). Identifiers with external linkage are more restricted: implementations ' may make as few as the first six characters as significant, and may ignore case distinctions.

A2.4 - Keywords

The following identifiers are reserved for use as keywords, and may not be used otherwise:

auto	double	int	struct
break	else	long	switch
case	enum	register	typedef
char	extern	return	union
const	float	short	unsigned
continue	for	signed	void
default	goto	sizeof	volatile
do	if	static	while

Some implementations also reserve the words fortran and asm.

The keywords const, signed, and volatile are new with the ANSI standard; enum and void are new since the first edition, but in common use; entry, formerly reserved but never used, is no longer reserved.

A2.5 - Constants

There are several kinds of constants. Each has a data type; §A4.2 discusses the basic types.

constant:
    integer-constant
    character-constant
    floating-constant
    enumeration-constant

A2.5.1 - Integer Constants

An integer constant consisting of a sequence of digits is taken to be octal if it begins with 0 (digit zero), decimal otherwise. Octal constants do not contain the digits 8 or 9. A sequence of digits preceded by 0x or 0X (digit zero) is taken to be a hexadecimal integer. The hexadecimal digits include a or A through f or F with values 10 through 15.

An integer constant may be suffixed by the letter u or U, to specify that it is unsigned. It may also be suffixed by the letter l or L to specify that it is long.

The type of an integer constant depends on its form, value and suffix. (See §A4 for a discussion of types.) If it is unsuffixed and decimal, it has the first of these types in which its value can be represented: int, long int, unsigned long int. If it is unsuffixed octal or hexadecimal, it has the first possible of these types: int, unsigned int, long int, unsigned long int. If it is suffixed by u or U, then unsigned int, unsigned long int. If it is suffixed by l or L, then long int, unsigned long int.

The elaboration of the types of integer constants goes considerably beyond the first edition, which merely caused large integer constants to be long. The U suffixes are new.

A2.5.2 - Character Constants

A character constant is a sequence of one or more characters enclosed in single quotes, as in 'x'. The value of a character constant with only one character is the numeric value of the character in the machine's character set at execution time. The value of a multi-character constant is implementation-defined.

Character constants do not contain the ' character or newlines; in order to represent them, and certain other characters, the following escape sequences may be used.

newline           NL (LF)   \n            backslash         \     \\
horizontal tab    HT        \t            question mark     ?     \?
vertical tab      VT        \v            single quote      '     \'
backspace         BS        \b            double quote      "     \"
carriage return   CR        \r            octal number      ooo   \ooo
formfeed          FF        \f            hex number        hh    \xhh
audible alert     BEL       \a

The escape \ooo consists of the backslash followed by 1, 2, or 3 octal digits, which are taken to specify the value of the desired character. A common example of this construction is \0 (not followed by a digit), which specifies the character NUL. The escape \xhh consists of the backslash, followed by x, followed by hexadecimal digits, which are taken to specify the value of the desired character. There is no limit on the number of digits, but the behavior is undefined if the resulting character value exceeds that of the largest character. For either octal or hexadecimal escape characters, if the implementation treats the char type as signed, the value is sign-extended as if cast to char type. If the character following the \ is not one of those specified, the behavior is undefined.

In some implementations, there is an extended set of characters that cannot be represented in the char type. A constant in this extended set is written with a preceding L, for example L'x', and is called a wide character constant. Such a constant has type wchar_t, an integral type defined in the standard header <stddef.h>. As with ordinary character constants, octal or hexadecimal escapes may be used; the effect is undefined if the specified value exceeds that representable with wchar_t.

Some of these escape sequences are new, in particular the hexadecimal character representation. Extended characters are also new. The character sets commonly used in the Americas and western Europe can be encoded to fit in the char type; the main intent in adding wchar_t was to accommodate Asian languages.

2.5.3 - Floating Constants

A floating constant consists of an integer part, a decimal point, a fraction part, an e or E, an optionally signed integer exponent and an optional type suffix, one of f, F, l, or L. The integer and fraction parts both consist of a sequence of digits. Either the integer part or the fraction part (not both) may be missing; either the decimal point or the e and the exponent (not both) may be missing. The type is determined by the suffix; F or f makes it float, L or l makes it long double; otherwise it is double.

Suffixes on floating constants are new.

2.5.4 - Enumeration Constants

Identifiers declared as enumerators (see §A8.4) are constants of type int.

2.6 - String Literals

A string literal, also called a string constant, is a sequence of characters surrounded by double quotes, as in "...". A string has type "array of characters" and storage class static (see §A4 below) and is initialized with the given characters. Whether identical string literals are distinct is implementation-defined, and the behavior of a program that attempts to alter a string literal is undefined.

Adjacent string literals are concatenated into a single string. After any concatenation, a null byte \0 is appended to the string so that programs that scan the string can find its end. String literals do not contain newline or double-quote characters; in order to represent them, the same escape sequences as for character constants are available.

As with character constants, string literals in an extended character set are written with a preceding L, as in L"...". Wide-character string literals have type "array of wchar_t." Concatenation of ordinary and wide string literals is undefined.

The specification that string literals need not be distinct, and the prohibition against modifying them, are new in the ANSI standard, as is the concatenation of adjacent string literals. Wide-character string literals are new.

A3 - Syntax Notation

In the syntax notation used in this manual, syntactic categories are indicated by <angled> type, and literal words and characters in typewriter style. Alternative categories are usually listed on separate lines; in a few cases, a long set of narrow alternatives is presented on one line, marked by the phrase "one of." An optional terminal or nonterminal symbol carries the subscript "opt," so that, for example,

{ <expression>_opt }

means an optional expression, enclosed in braces. The syntax is summarized in §A13.

Unlike the grammar given in the first edition of this book, the one given here makes precedence and associativity of expression operators explicit.

A4 - Meaning of Identifiers

Identifiers, or names, refer to a variety of things: functions; tags of structures, unions, and enumerations; members of structures or unions; enumeration constants; typedef names; and objects. An object, sometimes called a variable, is a location in storage, and its interpretation depends on two main attributes: its storage class and its type. The storage class determines the lifetime of the storage associated with the identified object; the type determines the meaning of the values found in the identified object. A name also has a scope, which is the region of the program in which it is known, and a linkage, which determines whether the same name in another scope refers to the same object or function. Scope and linkage are discussed in §A11.

A4.1 - Storage Class

There are two storage classes: automatic and static. Several keywords, together with the context of an object's declaration, specify its storage class. Automatic objects are local to a block (§A9.3), and are discarded on exit from the block. Declarations within a block create automatic objects if no storage class specification is mentioned, or if the auto specifier is used. Objects declared register are automatic, anq are (if possible) stored in fast registers of the machine.

Static objects may be local to a block or external to all blocks, but in either case retain their values across exit from and reentry to functions and blocks. Within a block, including a block that provides the code for a function, static objects are declared with the keyword static. The objects declared outside all blocks, at the same level as function definitions, are always static. They may be made local to a particular translation unit by use of the static keyword; this gives them internal linkage. They become global to an entire program by omitting an explicit storage class, or by using the keyword extern; this gives them external linkage.

A4.2 - Basic Types

There are several fundamental types. The standard header <limits.h> described in Appendix B defines the largest and smallest values of each type in the local implementation. The numbers given in Appendix B show the smallest acceptable magnitudes.

Objects declared as characters (char) are large enough to store any member of the execution character set. If a genuine character from that set is stored in a char object, its value is equivalent to the integer code for the character, and is non-negative. Other quantities may be stored into char variables, but the available range of values, and especially whether the value is signed, is implementation-dependent.

Unsigned characters declared unsigned char consume the same amount of space as plain characters, but always appear non-negative; explicitly signed characters declared signed char likewise take the same space as plain characters.

unsigned char type does not appear in the first edition of this book, but is in common use. signed char is new.

Besides the char types, up to three sizes of integer, declared short int, int, and long int, are available. Plain int objects have the natural size suggested by the host machine architecture; the other sizes are provided to meet special needs. Longer integers provide at least as much storage as shorter ones, but the implementation may make plain integers equivalent to either short integers, or long integers. The int types all represent signed values unless specified otherwise.

Unsigned integers, declared using the keyword unsigned, obey the laws of arithmetic modulo $2^n$ where $n$ is the number of bits in the representation, and thus arithmetic on unsigned quantities can never overflow. The set of non-negative values that can be stored in a signed object is a subset of the values that can be stored in the corresponding unsigned object, and the representation for the overlapping values is the same.

Any of single precision floating point (float), double precision floating point (double), and extra precision floating point (long double) may be synonymous, but the ones later in the list are at least as precise as those before.

long double is new. The first edition made long float equivalent to double; the locution has been withdrawn.

Enumerations are unique types that have integral values; associated with each enumeration is a set of named constants (§A8.4). Enumerations behave like integers, but it is common for a compiler to issue a warning when an object of a particular enumeration type is assigned something other than one of its constants, or an expression of its type.

Because objects of these types can be interpreted as numbers, they will be referred to as arithmetic types. Types char, and int of all sizes, each with or without sign, and also enumeration types, will collectively be called integral types. The types float, double, and long double will be called floating types.

The void type specifies an empty set of values. It is used as the type returned by functions that generate no value.

A4.3 - Derived Types

Besides the basic types, there is a conceptually infinite class of derived types constructed from the fundamental types in the following ways:

• arrays of objects of a given type;
• functions returning objects of a given type;
• pointers to objects of a given type;
• structures containing a sequence of objects of various types;
• unions capable of containing any one of several objects of various types.

In general these methods of constructing objects can be applied recursively.

A4.4 - Type Qualifiers

An object's type may have additional qualifiers. Declaring an object const announces that its value will not be changed; declaring it volatile announces that it has special properties relevant to optimization. Neither qualifier affects the range of values or arithmetic properties of the object. Qualifiers are discussed in §A8.2.

A5 - Objects and Lvalues

An object is a named region of storage; an lvalue is an expression referring to an object. An obvious example of an lvalue expression is an identifier with suitable type and storage class. There are operators that yield lvalues: for example, if E is an expression of pointer type, then *E is an lvalue expression referring to the object to which E points. The name "lvalue" comes from the assignment expression E1 = E2 in which the left operand E1 must be an lvalue expression. The discussion of each operator specifies whether it expects lvalue operands and whether it yields an lvalue.

A6 - Conversions

Some operators may, depending on their operands, cause conversion of the value of an operand from one type to another. This section explains the result to be expected from such conversions. §A6.5 summarizes the conversions demanded by most ordinary operators; it will be supplemented as required by the discussion of each operator.

A6.1 - Integral Promotion

A character, a short integer, or an integer bit-field, all either signed or not, or an object of enumeration type, may be used in an expression wherever an integer may be used. If an int can represent all the values of the original type, then the value is converted to int; otherwise the value is converted to unsigned int. This process is called integral promotion.

A6.2 - Integral Conversions

Any integer is converted to a given unsigned type by finding the smallest nonnegative value that is congruent to that integer, modulo one more than the largest value that can be represented in the unsigned type. In a two's complement representation, this is equivalent to left-truncation if the bit pattern of the unsigned type is narrower, and to zero-filling unsigned values and sign-extending signed values if the unsigned type is wider.

When any integer is converted to a signed type, the value is unchanged if it can be represented in the new type and is implementation-defined otherwise.

A6.3 - Integer and Floating

When a value of floating type is converted to integral type, the fractional part is discarded; if the resulting value cannot be represented in the integral type, the behavior is undefined. In particular, the result of converting negative floating values to unsigned integral types is not specified.

When a value of integral type is converted to floating, and the value is in the representable range but is not exactly representable, then the result may be either the next higher or next lower representable value. If the result is out of range, the behavior is undefined.

A6.4 - Floating Types

When a less precise floating value is converted to an equally or more precise floating type, the value is unchanged. When a more precise floating value is converted to a less precise floating type, and the value is within representable range, the result may be either the next higher or the next lower representable value. If the result is out of range, the behavior is undefined.

A6.5 - Arithmetic Conversions

Many operators cause conversions and yield result types in a similar way. The effect is to bring operands into a common type, which is also the type of the result. This pattern is called the usual arithmetic conversions.

• First, if either operand is long double, the other is converted to long double.
• Otherwise, if either operand is double, the other is converted to double.
• Otherwise, if either operand is float, the other is converted to float.
• Otherwise, the integral promotions are performed on both operands; then, if either operand is unsigned long int, the other is converted to unsigned long int.
• Otherwise, if one operand is long int and the other is unsigned int, the effect depends on whether a long int can represent all values of an unsigned int; if so, the unsigned int operand is converted to long int; if not, both are converted to unsigned long int.
• Otherwise, if one operand is long int, the other is converted to long int.
• Otherwise, if either operand is unsigned int, the other is converted to unsigned int.
• Otherwise, both operands have type int.

There are two changes here. First, arithmetic on float operands may be done in single precision, rather than double; the first edition specified that all floating arithmetic was double precision. Second, shorter unsigned types, when combined with a larger signed type, do not propagate the unsigned property to the result type; in the first edition, the unsigned always dominated. The new rules are slightly more complicated, but reduce somewhat the surprises that may occur when an unsigned quantity meets signed. Unexpected results may still occur when an unsigned expression is compared to a signed expression of the same size.

A6.6 - Pointers and Integers

An expression of integral type may be added to or subtracted from a pointer; in such a case the integral expression is converted as spe~ified in the discussion of the addition operator (§A7.7).

Two pointers to objects of the same type, in the same array, may be subtracted; the result is converted to an integer as specified in the discussion of the subtraction operator (§A7.7).

An integral constant expression with value 0; or such an expression cast to type void *, may be converted, by a cast, by assignment, or by comparison, to a .pointer of any type. This produces a null pointer that is equal to another null pointer of the same type, but unequal to any pointer to a function or object.

Certain other conversions involving pointers are permitted, but have implementationdependent aspects. They must be specified by an explicit type-conversion operator, or cast (§§A7.5 and A8.8).

A pointer may be converted to an integral type large enough to hold it; the required size is implementation-dependent. The mapping function is also implementation-dependent.

An object of integral type may be explicitly converted to a pointer. The mapping always carries a sufficiently wide integer converted from a pointer back to the same pointer, but is otherwise implementation-dependent.

A pointer to one type may be converted to a pointer to another type. The resulting pointer may cause addressing exceptions if the subject pointer does not refer to an object suitably aligned in storage. It is guaranteed that a pointer to an object may be converted to a pointer to an object whose type requires less or equally strict storage alignment and back again without change; the notion of "alignment" is implementationdependent, but objects of the char types have least strict alignment requirements. As described in §A6.8, a pointer may also be converted to type void * and back again without change.

A pointer may be converted to another pointer whose type is the same except for the addition or removal of qualifiers (§§A4.4, A8.2) of the object type to which the pointer refers. If qualifiers are added, the new pointer is equivalent to the old except for restrictions implied by the new qualifiers. If qualifiers are removed, operations on the underlying object remain subject to the qualifiers in its actual declaration.

Finally, a pointer to a function may be converted to a pointer to another function type. Calling the function specified by the converted pointer is implementationdependent; however, if the converted pointer is reconverted to its original type, the result is identical to the original pointer.

A6.7 - Void

The (nonexistent) value of a void object may not be used in any way, and neither explicit nor implicit conversion to any non-void type may be applied. Because a void expression denotes a nonexistent value, such an expression may be used only where the value is not required, for example as an expression statement (§A9.2) or as the left operand of a comma operator (§A7.18).

An expression may be converted to type void by a cast. For example, a void cast documents the discarding of the value of a function call used as an expression statement.

void did not appear in the first edition of this book, but has become common since.

A6.8 - Pointers to Void

Any pointer to an object may be converted to type void * without loss of information. If the result is converted back to the original pointer type, the original pointer is recovered. Unlike the pointer-to-pointer conversions discussed in §A6.6, which generally require an explicit cast, pointers may be assigned to and from pointers of type void * and may be compared with them.

This interpretation of void * pointers is new; previously, char * pointers played the role of generic pointer. The ANSI standard specifically blesses the meeting of void * pointers with object pointers in assignments and relationals, while requiring explicit casts for other pointer mixtures.

A7 - Expressions

The precedence of expression operators is the same a.s the order of the major subsections of this section, highest precedence first. Thus, for example, the expressions referred to as the operands of + (§A7.7) are those expressions defined in §§A7.1-A7.6. Within each subsection, the operators have the same precedence. Left- or rightassociativity is specified in each subsection for the operators discussed therein. The grammar in §A13 incorporates the precedence and associativity of the operators.

The precedence and associativity of operators is fully specified, but the order of evaluation of expressions is, with certain exceptions, undefined, even if the subexpressions involve side effects. That is, unless the definition of an operator guarantees that its operands are evaluated in a particular order, the implementation is free to evaluate operands in any order, or even to interleave their evaluation. However, each operator combines the values produced by its operands in a way compatible with the parsing of the expression in which it appears.

This rule revokes the previous freedom to reorder expressions with operators that are mathematically commutative and associative, but can fail to be computationally associative. The change affects only floating-point computations near the limits of their accuracy, and situations where overflow is possible.

The handling of overflow, divide check, and other exceptions in expression evaluation is not defined by the language. Most existing implementations of C ignore overflow in evaluation of signed integral expressions and assignments, but this behavior is not guaranteed. Treatment of division by 0, and all floating-point exceptions, varies among implementations; sometimes it is adjustable by a non-standard library function.

A7.1 - Pointer Generation

If the type of an expression or subexpression is "array of $T$," for some type $T$, then the value of the expression is a pointer to the first object in the array, and the type of the expression is altered to ''pointer to $T$." This conversion does not take place if the expression is the operand of the unary & operator, or of ++, --, sizeof, or as the left operand of an assignment operator or the . operator. Similarly, an expression of type "function returning $T$," except when used as the operand of the & operator, is converted to "pointer to function returning $T$."

A7.2 - Primary Expressions

Primary expressions are identifiers, constants, strings, or expressions in parentheses.

primary-expression:
    identifier
    constant
    string
    ( expression )

An identifier is a primary expression, provided it has been suitably declared as discussed below. Its type is specified by its declaration. An identifier is an lvalue if it refers to an object (§A5) and if its type is arithmetic, structure, union, or pointer.

A constant is a primary expression. Its type depends on its form as discussed in §A2.5.

A string literal is a primary expression. Its type is originally "array of char" (for wide-character strings, "array of wchar_t"), but following the rule given in §A7.1, this is usually modified to "pointer to char" (wchar_t) and the result is a pointer to the first character in the string. The conversion also does not occur in certain initializers; see §A8.7.

A parenthesized expression is a primary expression whose type and value are identical to those of the unadorned expression. The presence of parentheses does not affect whether the expression is an lvalue.

A7.3 - Postfix Expressions

The operators in postfix expressions group left to right.

postfix-expression:
    primary-expression
    postfix-expression [ expression ]
    postfix-expression ( argument-expression-list_opt )
    postfix-expression . identifier
    postfix-expression -> identifier
    postfix-expression ++
    postfix-expression --

argument-expression-list:
    assignment-expression
    argument-expression-list , assignment-expression

A7.3.1 - Array References

A postfix expression followed by an expression in square brackets is a postfix expression denoting a subscripted array reference. One of the two expressions must have type "pointer to $T$', where $T$ is some type, and the other must have integral type; the type of the subscript expression is $T$. The expression E1[E2] is identical (by definition) to *((E1)+(E2)). See §A8.6.2 for further discussion.

A7.3.2 - Function Calls

A function call is a postfix expression, called the function designator, followed by parentheses containing a possibly empty, comma-separated list of assignment expressions (§A7.17), which constitute the arguments to the function. If the postfix expression consists of an identifier for which no declaration exists in the current scope, the identifier is implicitly declared as if the declaration

extern int <identifier>();

had been given in the innermost block containing the function call. The postfix expression (after possible implicit declaration and pointer generation, §A7.1) must be of type "pointer to function returning $T$," for some type $T$, and the value of the function call has type $T$.

In the first edition, the type was restricted to "function," and an explicit * operator was required to call through pointers to functions. The ANSI standard blesses the practice of some existing compilers by permitting the same syntax for calls to functions and to functions specified by pointers. The older syntax is still usable.

The term argument is used for an expression passed by a function call; the term parameter is used for an input object (or its identifier) received by a function definition, or described in a function declaration. The terms "actual argument (parameter)" and "formal argument (parameter)" respectively are sometimes used for the same distinction.

In preparing for the call to a function, a copy is made of each argument; all argument-passing is strictly by value. A function may change the values of its parameter objects, which are copies of the argument expressions, but these changes cannot affect the values of the arguments. However, it is possible to pass a pointer on the understanding that the function may change the value of the object to which the pointer points.

There are two styles in which functions may be declared. In the new style, the types of parameters are explicit and are part of the type of the function; such a declaration is also called a function prototype. In the old style, parameter types are not specified. Function declaration is discussed in §§A8.6.3 and A10.1.

If the function declaration in scope for a call is old-style, then default argument promotion is applied to each argument as follows: integral promotion (§A6.1) is performed on each argument of integral type, and each float argument is converted to double. The effect of the call is undefined if the number of arguments disagrees with the number of parameters in the definition of the function, or if the type of an argument after promotion disagrees with that of the corresponding parameter. Type agreement depends on whether the function's definition is new-style or old-style. If it is old-style, then the comparison is between the promoted type of the argument of the call, and the promoted type of the parameter; if the definition is new-style, the promoted type of the argument must be that of the parameter itself, without promotion.

If the function declaration in scope for a call is new-style, then the arguments are converted, as if by assignment, to the types of the corresponding parameters of the function's prototype. The number of arguments must be the same as the number of explicitly described parameters, unless the declaration's parameter list ends with the ellipsis notation (, ...) . In that case, the number of arguments must equal or exceed the number of parameters; trailing arguments beyond the explicitly typed parameters suffer default argument promotion as described in the preceding paragraph. If the definition of the function is old-style, then the type of each parameter in the prototype visible at the call must agree with the corresponding parameter in the definition, after the definition parameter's type has undergone argument promotion.

These rules are especially complicated because they must cater to a mixture of old- and new-style functions. Mixtures are to be avoided if possible.

The order of evaluation of arguments is unspecified; take note that various compilers differ. However, the arguments and the function designator are completely evaluated, including all side effects, before the function is entered. Recursive calls to any function are permitted.

A7.3.3 - Structure References

A postfix expression followed by a dot followed by an identifier is a postfix expression. The first operand expression must be a structure or a union, and the identifier must name a member of the structure or union. The value is the named member of the structure or union, and its type is the type of the member. The expression is an lvalue if the first expression is an lvalue, and if the type of the second expression is not an array type.

A postfix expression followed by an arrow (built from - and >) followed by an identifier is a postfix expression. The first operand expression must be a pointer to a structure or a union, and the identifier must name a member of the structure or union. The result refers to the named member of the structure or union to which the pointer expression points, and the type is the type of the member; the result is an lvalue if the type is not an array type.

Thus the expression E1->MOS is the same as (*E1).MOS. Structures and unions are discussed in §A8.3.

In the first edition of this book, it was already the rule that a member name in such an expression had to belong to the structure or union mentioned irt the postfix expression; however, a note admitted that this rule was not firmly enforced. Recent compilers, and ANSI, do enforce it.

A7.3.4 - Postfix Incrementation

A postfix expression followed by a ++ or -- operator is a postfix expression. The value of the expression is the value of the operand. After the value is noted, the operand is incremented (++) or decremented (--) by 1. The operand must be an lvalue; see the discussion of additive operators (§A7.7) and assignment (§A7.17) for further constraints on the operand and details of the operation. The result is not an lvalue.

A7.4 - Unary Operators

Expressions with unary operators group right-to-left.

unary-expression:
    postfix-expression
    ++ unary-expression
    -- unary-expression
    unary-operator cast-expression
    sizeof unary-expression
    sizeof ( type-name )

unary-operator: one of
    &   *   +   -   ~   !

A7.4.1 - Prefix Incrementation Operators

A unary expression preceded by a ++ or -- operator is a unary expression. The operand is incremented (++) or decremented (--) by 1. The value of the expression is the value after the incrementation (decrementation). The operand must be an I value; see the discussion of additive operators (§A7.7) and assignment (§A7.17) for further constraints on the operand and details of the operation. The result is not an lvalue.

A7.4.2 - Address Operator

The unary & operator takes the address of its operand. The operand must be an lvalue referring neither to a bit-field nor to an object declared as register, or must be of function type. The result is a pointer to the object or function referred to by the lvalue. If the type of the operand is T, the type of the result is "pointer to $T$."

A7.4.3 - Indirection Operator

The unary * operator denotes indirection, and returns the object or function to which its operand points. It is an lvalue if the operand is a pointer to an object of arithmetic, structure, union, or pointer type. If the type of the expression is "pointer to $T$," the type of the result is $T$.

A7.4.4 - Unary Plus Operator

The operand of the unary + operator must have arithmetic type, and the result is the value of the operand. An integral operand undergoes integral promotion. The type of the result is the type of the promoted operand.

The unary + is new with the ANSI standard. It was added for symmetry with unary -.

A7.4.5 - Unary Minus Operator

The operand of the unary - operator must have arithmetic type, and the result is the negative of its operand. An integral operand undergoes integral promotion. The negative of an unsigned quantity is computed by subtracting the promoted value from the largest value of the promoted type and adding one; but negative zero is zero. The type of the result is the type of the promoted operand.

A7.4.6 - One's Complement Operator

The operand of the ~ operator must have integral type, and the result is the one's complement of its operand. The integral promotions are performed. If the operand is unsigned, the result is computed by subtracting the value from the largest value of the promoted type. If the operand is signed, the result is computed by converting the promoted operand to the corresponding unsigned type, applying ~, and converting back to the signed type. The type of the result is the type of the promoted operand.

A7.4.7 - Logical Negation Operator

The operand of the ! operator must have arithmetic type or be a pointer, and the result is 1 if the value of its operand compares equal to 0, and 0 otherwise. The type of the result is int.

A7.4.8 - Sizeof Operator

The sizeof operator yields the number of bytes required to store an object of the type of its operand. The operand is either an expression, which is not evaluated, or a parenthesized type name. When sizeof is applied to a char, the result is 1; when applied to an array, the result is the total number of bytes in the array. When applied to a structure or union, the result is the number of bytes in the object, including any padding required to make the object tile an array: the size of an array of $n$ elements is n times the size of one element. The operator may not be applied to an operand of function type, or of incomplete type, or to a bit-field. The result is an unsigned integral constant; the particular type is implementation-defined. The standard header <stddef.h> (see Appendix B) defines this type as size_t.

A7.5 - Casts

A unary expression preceded by the parenthesized name of a type causes conversion of the value of the expression to the named type.

cast-expression:
    unary-expression
    ( type-name ) cast-expression

This construction is called a cast. Type names are described in §A8.8. The effects of conversions are described in §A6. An expression with a cast is not an lvalue.

A7.6 - Multiplicative Operators

The multiplicative operators *, /, and % group left-to-right.

multiplicative-expression:
    cast-expression
    multiplicative-expression * cast-expression
    multiplicative-expression / cast-expression
    multiplicative-expression % cast-expression

The operands of * and / must have arithmetic type; the operands of % must have integral type. The usual arithmetic conversions are performed on the operands, and predict the type of the result.

The binary * operator denotes multiplication.

The binary / operator yields the quotient, and the % operator the remainder, of the division of the first operand by the second; if the second operand is 0, the result is undefined. Otherwise, it is always true that (a / b) * b + a % b is equal to a. If both operands are non-negative, then the remainder is non-negative and smaller than the divisor; if not, it is guaranteed only that the absolute value of the remainder is smaller than the absolute value of the divisor.

7.7 - Additive Operators

The additive operators + and - group left-to-right. If the operands have arithmetic type, the usual arithmetic conversions are performed. There are some additional type possibilities for each operator.

additive-expression:
    multiplicative-expression
    additive-expression + multiplicative-expression
    additive-expression - multiplicative-expression

The result of the + operator is the sum of the operands. A pointer to an object in an array and a value of any integral type may be added. The latter is converted to an address offset by multiplying it by the size of the object to which the pointer points. The sum is a pointer of the same type as the original pointer, and points to another object in the same array, appropriately offset from the original object. Thus if P is a pointer to an object in an array, the expression P + 1 is a pointer to the next object in the array. If the sum pointer points outside the bounds of the array, except at the first location beyond the high end, the result is undefined.

The provision for pointers just beyond the end of an array is new. It legitimizes a common idiom for looping over the elements of an array.

The result of the - operator is the difference of the operands. A value of any integral type may be subtracted from a pointer, and then the same conversions and conditions as for addition apply.

If two pointers to objects of the same type are subtracted, the result is a signed integral value representing the displacement between the pointed-to objects; pointers to successive objects differ by 1. The type of the result depends on the implementation, but is defined as ptrdiff_t in the standard header <stddef.h>. The value is undefined unless the pointers point to objects within the same array; however if P points to the last member of an array, then (P + 1) - P has value 1.

7.8 - Shift Operators

The shift operators << and >> group left-to-right. For both operators, each operand must be integral, and is subject to the integral promotions. The type of the result is that of the promoted left operand. The result is undefined if 'the right operand is negative, or greater than or equal to the number of bits in the left expression's type.

shift-expression:
    additive-expression
    shift-expression << additive-expression
    shift-expression >> additive-expression

The value of E1 << E2 is E1 (interpreted as a bit pattern) left-shifted E2 bits; in the absence of overflow, this is eqqivalent to multiplication by $2^{E2}$. The value of E1 >> E2 is E1 right-shifted E2 bit positions. The right shift is equivalent to division by $2^{E2}$ if E1 is unsigned or if it has a non-negative value; otherwise the result is implementation-<lefined.

7.9 - Relational Operators

The relational operators group left-to-right, but this fact is not useful; a < b < c is parsed as (a < b) < c, and a < b evaluates to either 0 or 1.

relational-expression:
    shift-expression
    relational-expression < shift-expression
    relational-expression > shift-expression
    relational-expression <= shift-expression
    relational-expression >= shift-exprfssion

The operators < (less), > (greater), <= (less or equal) and >= (greater or equal) all yield 0 if the specified relation is false and 1 if it is true. The type of the result is int. The usual arithmetic coqversions are performed on arithmetic operands. Pointers to objects of the same type (ignoring any qualifiers) may be compared; the result depends on the relative locations in the address space of the pointed-to objects. Pointer comparison is defined only for parts of the same object: if two pointers point to the same simple object, they compare equal; if the pointers are to members of the same structure, pointers to objects declared later in the structure compare higher; if the pointers are to members of the same union, they compare equal; if the pointers refer to members of an array, the comparison is equivalent to comparison of the corresponding subscripts. If P points to the last member of an array, then P + 1 compares higher than P, even though P + 1 points outside the array. Otherwise, pointer comparison is undefined.

These rules slightly liberalize the restrictions stated in the first edition, by permitting comparison of pointers to different members of a structure or union. They also legalize comparison with a pointer just off the end of an array.

7.10 - Equality Operators

equality-expression:
    relational-expression
    equality-expression == relational-expression
    equality-expression != relational-expression

The == (equal to) and the != (not equal to) operators are analogous to the relational operators except for their lower precedence. (Thus a < b == c < d is 1 whenever a < b and c < d have the same truth-value.)

The equality operators follow the same rules as the relational operators, but permit additional possibilities: a pointer may be compared to a constant integral expression with value 0, or to a pointer to void. See §A6.6.

7.11 - Bitwise AND Operator

AND-expression:
    equality-expression
    AND-expression & equality-expression

The usual arithmetic conversions are performed; the result is the bitwise AND function of the operands. The operator applies only to integral operands.

7.12 - Bitwise Exclusive OR Operator

exclusive-OR-expression:
    AND-expression
    exclusive-OR-expression ^ AND-expression

The usual arithmetic conversions are performed; the result is the bitwise exclusive OR function of the operands. The operator applies only to integral operands.

7.13 - Bitwise Inclusive OR Operator

inclusive-OR-expression:
    exclusive-OR-expression
    inclusive-OR-expression | exclusive-OR-expression

The usual arithmetic conversions are performed; the result is the bitwise inclusive OR function of its operands. The operator applies only to integral operands.

7.14 - Logical AND Operator

logical-AND-expression:
    inclusive-OR-expression
    logical-AND-expression && inclusive-OR-expression

The && operator groups left-to-right. It returns 1 if both its operands compare unequal to zero, 0 otherwise. Unlike &, && guarantees left-to-right evaluation: the first operand is evaluated, including all side effects; if it is equal to 0, the value of the expression is 0. Otherwise, the right operand is evaluated, and if it is equal to 0, the expression's value is 0, otherwise 1.

The operands need not have the same type, but each must have arithmetic type or be a pointer. The result is int.

7.15 - Logical OR Operator

logical-OR-expression:
    logical-AND-expression
    logical-OR-expression || logical-AND-expression

The || operator groups left-to-right. It returns 1 if either of its operands compares unequal to zero, and 0 otherwise. Unlike |, || guarantees left-to-right evaluation: the first operand is evaluated, including all side effects; if it is unequal to 0, the value of the expression is 1. Otherwise, the right operand is evaluated, and if it is unequal to 0, the expression's value is 1, otherwise 0.

The operands need not have the same type, but each must have arithmetic type or be a pointer. The result is int.

7.16 - Conditional Operator

conditional-expression:
    logical-OR-expression
    logical-OR-expression ? expression : conditional-expression

The first expression is evaluated, including all side effects; if it compares unequal to 0, the result is the value of the second expression, otherwise that of third expression. Only one of the second and third operands is evaluated. If the second and third operands are arithmetic, the usual arithmetic conversions are performed to bring them to a common type, and that is the type of the result. If both are void, or structures or unions of the same type, or pointers to objects of the same type, the result has the common type. If one is a pointer and the other the constant 0, the 0 is converted to the pointer type, and the result has that type. If one is a pointer to void and the other is another pointer, the other pointer is converted to a pointer to void, and that is the type of the result.

In the type comparison for pointers, any type qualifiers (§A8.2) in the type to which the pointer points are insignificant, but the result type inherits qualifiers from both arms of the conditional.

7.17 - Assignment Expressions

There are several assignment operators; all group right-to-left.

assignment-expression:
    conditional-expression
    unary-expression assignment-operator assignment-expression

assignment-operator: one of
    =   *=   /=   %=   +=   -=   <<=   >>=   &=   ^=   |=

All require an lvalue as left operand, and the lvalue must be modifiable: it must not be an array, and must not have an incomplete type, or be a function. Also, its type must not be qualified with const; if it is a structure or union, it must not have any member or, recursively, submember qualified with const. The type of an assignment expression is that of its left operand, and the value is the value stored in the left operand after the assignment has taken place.

In the simple assignment with =, the value of the expression replaces that of the object referred to by the lvalue. One of the following must be true: both operands have arithmetic type, in which case the right operand is converted to the type of the left by the assignment; or both operands are structures or unions of the same type; or one operand is a pointer and the other is a pointer to void; or the left operand is a pointer and the right operand is a constant expression with value 0; or both operands are pointers to functions or objects whose types are the same except for the possible absence of const or volatile in the right operand.

An expression of the form E1 op = E2 is equivalent to E1 = E1 op (E2) except that E1 is evaluated only once.

7.18 - Comma Operator

expression:
    assignment-expression
    expression , assignment-expression

A pair of expressions separated by a comma is evaluated left-to-right, and the value of the left expression is discarded. The type and value of the result are the type and value of the right operand. All side effects from the evaluation of the left operand are completed before beginning evaluation of the right operand. In contexts where comma is given a special meaning, for example in lists of function arguments (§A 7.3.2) and lists of initializers (§A8.7), the required syntactic unit is an assignment expression, so the comma operator appears only in a parenthetical grouping; for example,

f(a, (t = 3, t + 2), c)

has three arguments, the second of which has the value 5.

7.19 - Constant Expressions

Syntactically, a constant expression is an expression restricted to a subset of operators:

constant-expression:
    conditional-expression

Expressions that evaluate to a constant are required in several contexts: after case, as array bounds and bit-field lengths, as the value of an enumeration constant, in initializers, and in certain preprocessor expressions.

Constant expressions may not contain assignments, increment or decrement operators, function calls, or comma operators, except in an operand of sizeof. If the constant expression is required to be integral, its operands must consist of integer, enumeration, character, and floating constants; casts must specify an integral type, and any floating constants must be cast to an integer. This necessarily rules out arrays, indirection, address-of, and structure member operations. (However, any operand is permitted for sizeof.)

More latitude is permitted for the constant expressions of initializers; the operands may be any type of constant, and the unary & operator may be applied to external or static objects, and to external or static arrays subscripted with a constant expression. The unary & operator can also be applied implicitly by appearance of unsubscripted arrays and functions. Initializers must evaluate either to a constant or to the address of a previously declared external or static object plus or minus a constant.

Less latitude is allowed for the integral constant expressions after #if; sizeof expressions, enumeration constants, and casts are not permitted. See §A12.5.

A8 - Declarations

Declarations specify the interpretation given to each identifier; they do not neces· sarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions. Declarations have the form

declaration:
    declaration-specifiers init-declarator-list_opt;

The declarators in the init-declarator-list contain the identifiers being declared; the declaration-specifiers consist of a sequence of type and storage class specifiers.

declaration-specifiers:
    storage-class-specifier declaration-specifiers_opt
    type-specifier declaration-specifiers_opt
    type-qualifier declaration-specifiers_opt

init-declarator-list:
    init-declarator
    init-declarator-list , init-declarator

init-declarator:
    declarator
    declarator = initializer

Declarators will be discussed later (§A8.5); they contain the names being declared. A declaration must have at least one declarator, or its type specifier must declare a structure tag, a union tag, or the members of an enumeration; empty declarations are not permitted.

A8.1 - Storage Class Specifiers

The storage class specifiers are:

storage-class-specifier:
    auto
    register
    static
    extern
    typedef

The meanings of the storage classes were discussed in §A4.

The auto and register specifiers give the declared objects automatic storage class, and may be used only within functions. Such declarations also serve as definitions and cause storage to be reserved. A register declaration is equivalent to an auto declaration, but hints that the declared objects will be accessed frequently. Only a few objects are actually placed into registers, and only certain types are eligible; the restrictions are implementation-dependent. However, if an object is declared register, the unary & operator may not be applied to it, explicitly or implicitly.

The rule that it is illegal to calculate the address of an object declared register, but actually taken to be auto, is new.

The static specifier gives the declared objects static storage class, and may be used either inside or outside functions. Inside a function, this specifier causes storage to be allocated, and serves as a definition; for its effect outside a function, see §A11.2.

A declaration with extern, used inside a function, specifies that the storage for the declared objects is defined elsewhere; for its effects outside a function, see §A11.2.

The typedef specifier does not reserve storage and is called a storage class specifier only for syntactic convenience; it is discussed in §A8.9.

At most one storage class specifier may be given in a declaration. If none is given, these rules are used: objects declared inside a function are taken to be auto; functions declared within a function are taken to be extern; objects and functions declared outside a function are taken to be static, with external linkage. See §§A10-A11.

A8.2 - Type Specifiers

The type-specifiers are

type-specifier:
    void
    char
    short
    int
    long
    float
    double
    signed
    unsigned
    struct-or-union-specifier
    enum-specifier
    typedef-name

At most one of the words long or short may be specified together with int; the meaning is the same if int is not mentioned. The word long may be specified together with double. At most one of signed or unsigned may be specified together with int or any of its short or long varieties, or with char. Either may appear alone, in which case int is understood. The signed specifier is useful for forcing char objects to carry a sign; it is permissible but redundant with other integral types.

Otherwise, at most one type-specifier may be given in a declaration. If the typespecifier is missing from a declaration, it is taken to be int.

Types may also be qualified, to indicate special properties of the objects being declared.

type-qualifier:
    const
    volatile

Type qualifiers may appear with any type specifier. A const object may be initialized, but not thereafter assigned to. There are no implementation-independent semantics for volatile objects.

The const and volatile properties are new with the ANSI standard. The purpose of const is to announce objects that may be placed in read-only memory, and perhaps to increase opportunities for optimization. The purpose of volatile is to force an implementation to suppress optimization that could otherwise occur. For example, for a machine with memory-mapped input/output, a pointer to a device register might be declared as a pointer to volatile, in order to prevent the compiler from removing apparently redundant references through the pointer. Except that it should diagnose explicit attempts to change const objects, a compiler may ignore these qualifiers.

A8.3 - Structure and Union Declarations

A structure is an object consisting of a sequence of named members of various types. A union is an object that contains, at different times, any one of several members of various types. Structure and union specifiers have the same form.

struct-or-union-specifier:
    struct-or-union identifier_opt { struct-declaration-list }
    struct-or-union identifier

struct-or-union:
    struct
    union

A struct-declaration-list is a sequence of declarations for the members of the structure or union:

struct-declaration-list:
    struct-declaration
    struct-declaration-list struct-declaration

struct-declaration:
    specifier-qualifier-list struct-declarator-list ;

specifier-qualifier-list:
    type-specifier specifier-qualifier-list_opt
    type-qualifier specifier-qualifier-list_opt

struct-declarator-list:
    struct-declarator
    struct-declarator-list , struct-declarator

Usually, a struct-declarator is just a declarator for a member of a structure or union. A structure member may also consist of a specified number of bits. Such a member is also called a bit-field, or merely field; its length is set off from the declarator for the field name by a colon.

struct-declarator:
    declarator
    declarator_opt : constant-expression

A type specifier of the form

struct-or-union identifier { struct-declaration-list }

declares the identifier to be the tag of the structure or union specified by the list. A subsequent declaration in the same or an inner scope may refer to the same type by using the tag in a specifier without the list:

struct-or-union identifier

If a specifier with a tag but without a list appears when the tag is not declared, an incomplete type is specified. Objects with an incomplete structure or union type may be mentioned in contexts where their size is not needed, for example in declarations (not definitions), for specifying a pointer, or for creating a typedef, but not otherwise. The type becomes complete on occurrence of a subsequent specifier with that tag, and containing a declaration list. Even in specifiers with a list, the structure or union type being declared is incomplete within the list, and becomes complete only at the } terminating the specifier.

A structure may not contain a member of incomplete type. Therefore, it is impossible to declare a structure or union containing an instance of itself. However, besides giving a name to the structure or union type, tags allow definition of self-referential structures; a structure or union may contain a pointer to an instance of itself, because pointers to incomplete types may be declared.

A very special rule applies to declarations of the form

struct-or-union identifier ;

that declare a structure or union, but have no declaration list and no declarators. Even if the identifier is a structure or union tag already declared in an outer scope (§A11.1), this declaration makes the identifier the tag of a new, incompletely-typed structure or union in the current scope.

This recondite rule is new with ANSI. It is intended to deal with mutuallyrecursive structures declared in an inner scope, but whose tags might already be declared in the outer scope.

A structure or union specifier with a list but no tag creates a unique type; it can be referred to directly only in the declaration of which it is a part.

The names of members and tags do not conflict with each other or with ordinary variables. A member name may not appear twice in the same structure or union, but the same member name may be used in different structures or unions.

In the first edition of this book, the names of structure and union members were not associated with their parent. However, this association became common in compilers well before the ANSI standard.

A non-field member of a structure or union may have any object type. A field member (which need not have a declarator and thus may be unnamed) has type int, unsigned int, or signed int, and is interpreted as an object of integral type of the specified length in bits; whether an int field is treated as signed is implementationdependent. Adjacent field members of structures are packed into implementationdependent storage units in an implementation-dependent direction. When a field following another field will not fit into a partially-filled storage unit, it may be split between units, or the unit may be padded. An unnamed field with width 0 forces this padding, so that the next field will begin at the edge of the next allocation unit.

The ANSI standard makes fields even more implementation-dependent than did the first edition. It is advisable to read the language rules for storing bit-fields as "implementation-dependent" without qualification. Structures with bit-fields may be used as a portable way of attempting to reduce the storage required for a structure (with the probable cost of increasing the instruction space, and time, needed to access the fields), or as a non-portable way to describe a storage layout known at the bit level. In the second case, it is necessary to understand the rules of the local implementation.

The members of a structure have addresses increasing in the order of their declarations. A non-field member of a structure is aligned at an addressing boundary depending on its type; therefore, th(;re may be unnamed holes in a structure. If a pointer to a structure is cast to the type of a pointer to its first member, the result refers to the first member.

A union may be thought of as a structure all of whose members begin at offset 0 and whose size is sufficient to contain any of its members. At most one of the members can be stored in a union at any time. If a pointer to a union is cast to the type of a pointer to a member, the result refers to that member.

A simple example of a structure declaration is

struct tnode {
  char tword[20];
  int count;
  struct tnode *left;
  struct tnode *right;
};

which contains an array of 20 characters, an integer, and two pointers to similar structures. Once this declaration has been gi~en, the declaration

struct tnode s, *sp;

declares s to be a structure of the given sort and sp to be a pointer to a structure of the given sort. With these declarations, the expression

sp->count

refers to the count field of the structure to which sp points;

s.left

refers to the left subtree pointer of the structure s; and

s.right->tword[0]

refers to the first character of the. tword member of the right subtree of s.

In general, a member of a union may not be inspected unless the value of the union has been assigned using that same member. However, one special guarantee simplifies the use of unions: if a union contains several structures that share a common initial s<:quence, and if the union currently contains one of these structures, it is permitted to refer to the common initial part of any of the contained structures. For example, the following is a legal fragment:

union {
  struct {
    int type;
  } n;
  struct {
    int type;
    int intnode;
  } ni;
  struct {
    int type;
    float floatnode;
  } nf;
} u;
...
u.nf.type = FLOAT;
u.nf.floatnode = 3.14;
...
if (u.n.type == FLOAT)
  ... sin(u.nf.floatnode) ...

A8.4 - Enumerations

Enumerations are unique types with values ranging over a set of named constants called enumerators. The form of an enumeration specifier borrows from that of structures and unions.

enum-specifier:
    enum identifier_opt { enumerator-list }
    enum identifier

enumerator-list:
    enumerator
    enumerator-list , enumerator

enumerator:
    identifier
    identifier = constant-expression

The identifiers in an enumerator list are declared as constants of type int, and may appear wherever constants are required. If no enumerators with = appear, then the values of the corresponding constants begin at 0 and increase by 1 as the declaration is read from left to right. An enumerator with = gives the associated identifier the value specified; subsequent identifiers continue the progression from the assigned value.

Enumerator names in the same scope must all be distinct from each other and from ordinary variable names, but the values need not be distinct.

The role of the identifier in the enum-specifier is analogous to that of the structure tag in a struct-specifier; it names a particular enumeration. The rules for enumspecifiers with and without tags and lists are the same as those for structure or union specifiers, except that incomplete enumeration types do not exist; the tag of an enumspecifier without an enumerator list must refer to an in-scope specifier with a list.

Enumerations are new since the first edition of this book, but have been part of the language for some years.

A8.5 - Declarators

Declarators have the syntax:

declarator:
    pointer_opt direct-declarator

direct-declarator:
    identifier
    ( declarator )
    direct-declarator [ constant-expression_opt ]
    direct-declarator ( parameter-type-list )
    direct-declarator ( identifier-list_opt )

pointer:
    * type-qualifier-list_opt
    * type-qualifier-list_opt pointer

type-qualifier-list:
    type-qualifier
    type-qualifier-list type-qualifier

The structure of declarators resembles that of indirection, function, and array expressions; the grouping is the same.

A8.6 - Meaning of Declarators

A list of declarators appears after a sequence of type and storage class specifiers. Each declarator declares a unique main identifier, the one that appears as the first alternative of the production for direct-declarator. The storage class specifiers apply directly to this identifier, but its type depends on the form of its declarator. A declarator is read as an assertion that when its identifier appears in an expression of the same form as the declarator, it yields an object of the specified type.

Considering only the type parts of the declaration specifiers (§A8.2) and a particular declarator, a declaration has the form "T D," where T is a type and D is a declarator. The type attributed to the identifier in the various forms of declarator is described inductively using this notation.

In a declaration T D where D is an unadorned identifier, the type of the identifier is T.

In a declaration T D where D has the form

( D1 )

then the type of the identifier in D1 is the same as that of D. The parentheses do not alter the type, but may change the binding of complex declarators.

A8.6.1 - Pointer Declarators

In a declaration T D where D has the form

* type-qualifier-list_opt D1

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the identifier of D is "type-modifier type-qualifier-list pointer to T." Qualifiers following * apply to pointer itself, rather than to the object to which the pointer points.

For example, consider the declaration

int *ap[];

Here ap[] plays the role of D1; a declaration "int ap[]" (below) would give ap the type "array of int," the type-qualifier list is empty, and the type-modifier is "array of." Hence the actual declaration gives ap the type "array of pointers to int."

As other examples, the declarations

int i, *pi, *const cpi = &i;
const int ci = 3, *pci;

declare an integer i and a pointer to an integer pi. The value of the constant pointer cpi may not be changed; it will always point to the same location, although the value to which it refers may be altered. The integer ci is constant, and may not be changed (though it may be initialized, as here.) The type of pci is "pointer to const int," and pci itself may be changed to point to another place, but the value to which it points may not be altered by assigning through pci.

A8.6.2 - Array Declarators

In a declaration T D where D has the form

D1 [constant-expression_opt]

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the identifier of D is "type-modifier array of T." If the constant-expression is present, it must have integral type, and value greater than 0. If the constant expression specifying the bound is missing, the array has an incomplete type.

An array may be constructed from an arithmetic type, from a pointer, from a structure or union, or from another array (to generate a multi-dimensional array). Any type from which an array is constructed must be complete; it must not be an array or structure of incomplete type. This implies that for a multi-dimensional array, only the first dimension may be missing. The type of an object of incomplete array type is completed by another, complete, declaration for the object (§A10.2), or by initializing it (§A8.7). For example,

float fa[17], *afp[17];

declares an array of float numbers and an array of pointers to float numbers. Also,

static int x3d[3][5][7];

declares a static three-dimensional array of integers, with rank 3 x 5 x 7. In complete detail, x3d is an array of three items; each item is an array of five arrays; each of the latter arrays is an array of seven integers. Any of the expressions x3d, x3d[i], x3d[i][j], x3d[i][j][k] may reasonably appear in an expression. The first three have type "array," the last has type int. More specifically, x3d[i][j] is an array of 7 integers, and x3d[i] is an array of 5 arrays of 7 integers.

The array subscripting operation is defined so that E1[E2] is identical to *(E1+E2). Therefore, despite its asymmetric appearance, subscripting is a commutative operation. Because of the conversion rules that apply to + and to arrays (§§A6.6, A7.1, A7.7), if E1 is an array and E2 an integer, then E1[E2] refers to the E2-th member of E1.

In the example, x3d[i][j][k] is equivalent to *(x3d[i][j] + k). The first subexpression x3d[i][j] is converted by §A7.1 to type "pointer to array of integers;" by §A7.7, the addition involves multiplication by the size of an integer. It follows from the rules that arrays are stored by rows Oast subscript varies fastest) and that the first subscript in the declaration helps determine the amount of storage consumed by an array, but plays no other part in subscript calculations.

A8.6.3 - Function Declarators

In a new-style function declaration T D where D has the form

D1(parameter-type-list)

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the identifier of D is "type-modifier function with arguments parameter-type-list returning T."

The syntax of the parameters is

parameter-type-list:
    parameter-list
    parameter-list , ...

parameter-list:
    parameter-declaration
    parameter-list , parameter-declaration

parameter-declaration:
    declaration-specifiers declarator
    declaration-specifiers abstract-declarator_opt

In the new-style declaration, the parameter list specifies the types of the parameters. As a special case, the declarator for a new-style function with no parameters has a parameter type list consisting solely of the keyword void. If the parameter type list ends with an ellipsis ", ...", then the function may accept more arguments than the number of parameters explicitly described; see §A7.3.2.

The types of parameters that are arrays or functions are altered to pointers, in accordance with the rules for parameter conversions; see §A10.1. The only storage class specifier permitted in a parameter's declaration specifier is register, and this specifier is ignored unless the function declarator heads a function definition. Similarly, if the declarators in the parameter declarations contain identifiers and the function declarator does not head a function definition, the identifiers go out of scope immediately. Abstract declarators, which do not mention the identifiers, are discussed in §A8.8.

In an old-style function declaration T D where D has the form

D1(identifier-list_opt)

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the identifier of D is "type-modifier function of unspecified arguments returning T." The parameters (if present) have the form

identifier-list:
    identifier
    identifier-list , identifier

In the old-style declarator, the identifier list must be absent unless the declarator is used in the head of a function definition (§A10.1). No information about the types of the parameters is supplied by the declaration.

For example, the declaration

int f(), *fpi(), (*pfi)();

declares a function f returning an integer, a function fpi returning a pointer to an integer, and a pointer pfi to a function returning an integer. In none of these are the parameter types specified; they are old-style.

In the new-style declaration

int strcpy(char *dest, const char *source), rand(void);

strcpy is a function returning int, with two arguments, the first a character pointer, and the second a pointer to constant characters. The parameter names are effectively comments. The second function rand takes no arguments and returns int.

Function declarators with parameter prototypes are, by far, the most important language change introduced by the ANSI standard. They offer an advantage over the "old-style" declarators of the first edition by providing error-detection and coercion of arguments across function calls, but at a cost: turmoil and confusion during their introduction, and the necessity of accommodating both forms. Some syntactic ugliness was required for the sake of compatibility, namely void as an explicit marker of new-style functions without parameters.

The ellipsis notation ", ..." for variadic functions is also new, and, together with the macros in the standard header <stdarg.h>, formalizes a mechanism that was officially forbidden but unofficially condoned in the first edition.

These notations were adapted from the C++ language.

A8.7 - Initialization

When an object is declared, its init-declarator may specify an initial value for the identifier being declared. The initializer is preceded by =, and is either an expression, or a list of initializers nested in braces. A list may end with a comma, a nicety for neat formatting.

initializer:
    assignment-expression
    { initializer-list }
    { initializer-list , }

initializer-list:
    initializer
    initializer-list , initializer

All the expressions in the initializer for a static object or array must be constant expressions as described in §A7.19. The expressions in the initializer for an auto or register object or array must likewise be constant expressions if the initializer is a brace-enclosed list. However, if the initializer for an automatic object is a single expression, it need not be a constant expression, but must merely have appropriate type for assignment to the object.

The first edition did not countenance initialization of automatic structures, unions, or arrays. The ANSI standard allows it, but only by constant constructions unless the initializer can be expressed by a simple expression.

A static object not explicitly initialized is initialized as if it (or its members) were assigned the constant 0. The initial value of an automatic object not explicitly initialized is undefined.

The initializer for a pointer or an object of arithmetic type is a single expression, perhaps in braces. The expression is assigned to the object.

The initializer for a structure is either an expression of the same type, or a braceenclosed list of initializers for its members in order. Unnamed bit-field members are ignored, and are not initialized. If there are fewer initializers in the list than members of the structure, the trailing members are initialized with 0. There may not be more initializers than members.

The initializer for an array is a brace-enclosed list of initializers for its members. If the array has unknown size, the number of initializers determines the size of the array, and its type becomes complete. If the array has fixed size, the number of initializers may not exceed the number of members of the array; if there are fewer, the trailing members are initialized with 0.

As a special case, a character array may be initialized by a string literal; successive characters of the string initialize successive members of the array. Similarly, a wide character literal (§A2.6) may initialize an array of type wchar_t. If the array has unknown size, the number of characters in the string, including the terminating null character, determines its size; if its size is fixed, the number of characters in the string, not counting the terminating null character, must not exceed the size of the array.

The initializer for a union is either a single expression of the same type, or a braceenclosed initializer for the first member of the union.

The first edition did not allow initialization of unions. The "first-member" rule is clumsy, but is hard to generalize without new syntax. Besides allowing unions to be explicitly initialized in at least a primitive way, this ANSI rule makes definite the semantics of static unions not explicitly initialized.

An aggregate is a structure or array. If an aggregate contains members of aggregate type, the initialization rules apply recursively. Braces may be elided in the initialization as follows: if the initializer for an aggregate's member that is itself an aggregate begins with a left brace, then the succeeding comma-separated list of initializers initializes the members of the subaggregate; it is erroneous for there to be more initializers than members. If, however, the initializer for a subaggregate does not begin with a left brace, then only enough elements from the list are taken to account for the members of the subaggregate; any remaining members are left to initialize the next member of the aggregate of which the subaggregate is a part.

For example,

int x[] = { 1, 3, 5 };

declares and initializes x as a 1-dimensional array with three members, since no size was specified and there are three initializers.

float y[4][3] = {
  { 1, 3, 5},
  { 2, 4, 6},
  { 3, 5, 7},
};

is a completely-bracketed initialization: 1, 3, and 5 initialize the first row of the array y[0], namely y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early, and therefore the elements of y[3] are initialized with 0. Precisely the same effect could have been achieved by

float y[4][3] = {
  1, 3, 5, 2, 4, 6, 3, 5, 7
};

The initializer for y begins with a left brace, but that for y[0] does not; therefore three elements from the list are used. Likewise the next three are taken successively for y[1] and then for y[2]. Also,

float y[4][3] = {
  { 1 }, { 2 }, { 3 }, { 4 }
};

initializes the first column of y (regarded as a two-dimensional array) and leaves the rest 0.

Finally,

char msg[] = "Syntax error on line %s\n";

shows a character array whose members are initialized with a string; its size includes the terminating null character.

A8.8 - Type Names

In several contexts (to specify type conversions explicitly with a cast, to declare parameter types in function declarators, and as an argument of sizeof) it is necessary to supply the name of a data type. This is accomplished using a type name, which is syntactically a declaration for an object of that type omitting the name of the object.

type-name:
    specifier-qualifier-list abstract-declarator_opt

abstract-declarator:
    pointer
    pointer_opt direct-abstract-declarator

direct-abstract-declarator:
    ( abstract-declarator )
    direct-abstract-declarator_opt [ constant-expression_opt ]
    direct-abstract-declarator_opt ( parameter-type-list_opt )

It is possible to identify uniquely the location in the abstract-declarator where the identifier would appear if the construction were a declarator in a declaration. The named type is then the same as the type of the hypothetical identifier. For example,

int
int *
int *[3]
int (*)[]
int *()
int (*[])(void)

name respectively the types "integer," "pointer to integer," "array of 3 pointers to integers," "pointer to an array of an unspecified number of integers," "function of unspecified parameters returning pointer to integer," and "array, of unspecified size, of pointers to functions with no parameters each returning an integer."

A8.9 - Typedef

Declarations whose storage class specifier is typedef do not declare objects; instead they define identifiers that name types. These identifiers are called typedef names.

typedef-name:
    identifier

A typedef declaration attributes a type to each name among its declarators in the usual way (see §8.6). Thereafter, each such typedef name is syntactically equivalent to a type specifier keyword for the associated type.

For example, after

typedef long Blockno, *Blockptr;
typedef struct { double r, theta; } Complex;

the constructions

Blockno b;
extern Blockptr bp;
Complex z, *zp;

are legal declarations. The type of b is long, that of bp is "pointer to long," and that of z is the specified structure; zp is a pointer to such a structure.

typedef does not introduce new types, only synonyms for types that could be specified in another way. In the example, b has the same type as any other long object.

Typedef names may be redeclared in an inner scope, but a non-empty set of type specifiers must be given. For example,

extern Blockno;

does not redeclare Blockno, but

extern int Blockno;

does.

A8.10 - Type Equivalence

Two type specifier lists are equivalent if they contain the same set of type specifiers, taking into account that some specifiers can be implied by others (for example, long alone implies long int). Structures, unions, and enumerations with different tags are distinct, and a tagless union, structure, or enumeration specifies a unique type.

Two types are the same if their abstract declarators (§A8.8), after expanding any typedef types, and deleting any function parameter identifiers, are the same up to equivalence of type specifier lists. Array sizes and function parameter types are significant.

A9 - Statements

Except as described, statements are executed in sequence. Statements are executed for their effect, and do not have values. They fall into several groups.

statement:
    labeled-statement
    expression-statement
    compound-statement
    selection-statement
    iteration-statement
    jump-statement

A9.1 - Labeled Statements

Statements may carry label prefixes.

labeled-statement:
    identifier : statement
    case constant-expression : statement
    default : statement

A label consisting of an identifier declares the identifier. The only use of an identifier label is as a target of goto. The scope of the identifier is the current function. Because labels have their own name space, they do not interfere with other identifiers and cannot be redeclared. See §A11.1.

Case labels and default labels are used with the switch statement (§A9.4). The constant expression of case must have integral type.

Labels in themselves do not alter the flow of control.

A9.2 - Expression Statement

Most statements are expression statements, which have the form

expression-statement:
    expression_opt ;

Most expression statements are assignments or function calls. All side effects from the expression are completed before the next statement is executed. If the expression is missing, the construction is called a null statement; it is often used to supply an empty body to an iteration statement or to place a label.

A9.3 - Compound Statement

So that several statements can be used where one is expected, the compound statement (also called "block") is provided. The body of a function definition is a compound statement.

compound-statement:
    { declaration-list_opt statement-list_opt }

declaration-list:
    declaration
    declaration-list declaration

statement-list:
    statement
    statement-list statement

If an identifier in the declaration-list was in scope outside the block, the outer declara· tion is suspended within the block (see §A11.1), after which it resumes its force. An identifier may be declared only once in the same block. These rules apply to identifiers in the same name space (§A11); identifiers in different name spaces are treated as distinct.

Initialization of automatic objects is performed each time the block is entered at the top, and proceeds in the order of the declarators. If a jump into the block is executed, these initialization& are not performed. Initializations of static objects are performed only once, before the program begins execution.

A9.4 - Selection Statements

Selection statements choose one of several flows of control.

selection-statement:
    if ( <expression> ) <statement>
    if ( <expression> ) <statement> else <statement>
    switch ( <expression> ) <statement>

In both forms of the if statement, the expression, which must have arithmetic or pointer type, is evaluated, including all side-effects, and if it compares unequal to 0, the first substatement is executed. In the second form, the second substatement is executed if the expression is 0. The else ambiguity is resolved by connecting an else with the last encountered else-less if at the same block nesting level.

The switch statement causes control to be transferred to one of several statements depending on the value of an expression, which must have integral type. The substate· ment controlled by a switch is typically compound. Any statement within the substatement may be labeled with one or more case labels (§A9.1). The controlling expression undergoes integral promotion (§A6.1), and the case constants are converted to the promoted type. No two of the case constants associated with the same switch may have the same value after conversion. There may also be at most one default label associated with a switch. Switches may be nested; a case or default label is associated with the smallest switch that contains it.

When the switch statement is executed, its expression is evaluated, including all side effects, and compared with each case constant. If one of the case constants is equal to the value of the expression, control passes to the statement of the matched case label. If no case constant matches the expression, and if there is a default label, control passes to the labeled statement. If no case matches, and if there is no default, then none of the substatements of the switch is executed.

In the first edition of this book, the controlling expression of switch, and the case constants, were required to have int type.

A9.5 - Iteration Statements

Iteration statements specify looping.

iteration-statement:
    while ( <expression> ) <statement>
    do <statement> while ( <expression> ) ;
    for ( <expression>_opt ; <expression>_opt ; <expression>_opt ) <statement>

In the while and do statements, the substatement is executed repeatedly so long as the value of the expression remains unequal to 0; the expression must have arithmetic or pointer type. With while, the test, including all side effects from the expression, occurs before each execution of the statement; with do, the test follows each iteration.

In the for statement, the first expression is evaluated once, and thus specifies initialization for the loop. There is no restriction on its type. The second expression must have arithmetic or pointer type; it is evaluated before each iteration, and if it becomes equal to 0, the for is terminated. The third expression is evaluated after each iteration, and thus specifies a re-initialization for the loop. There is no restriction on its type. Side-effects from each expression are completed immediately after its evaluation. If the substatement does not contain continue, a statement

for ( expression1 ; expression2 ; expression3 ) statement

is equivalent to

expression1 ;
while ( expression2 ) {
    statement
    expression3 ;
}

Any of the three expressions may be dropped. A missing second expression makes the implied test equivalent to testing a non-zero constant.

A9.6 - Jump Statements

Jump statements transfer control unconditionally.

jump-statement:
    goto identifier ;
    continue ;
    break ;
    return expression_opt

In the goto statement, the identifier must be a label (§A9.1) located in the current function. Control transfers to the labeled statement.

A continue statement may appear only within an iteration statement. It causes control to pass to the loop-continuation portion of the smallest enclosing such statement. More precisely, within each of the statements

a continue not contained in a smaller iteration statement is the same as goto contin.

A break statement may appear only in an iteration statement or a switch statement, and terminates execution of the smallest enclosing such statement; control passes to the statement following the terminated statement.

A function returns to its caller by the return statement. When return is followed by an expression, the value is returned to the caller of the function. The expression is converted, as if by assignment, to the type returned by the function in which it appears.

Flowing off the end of a function is equivalent to a return with no expression. In either case, the returned value is undefined.

A10 - External Declarations

The unit of input provided to the C compiler is called a translation unit; it consists of a sequence of external declarations, which are either declarations or function definitions.

translation-unit:
    external-declaration
    translation-unit external-declaration

external-declaration:
    function-definition
    declaration

The scope of external declarations persists to the end of the translation unit in which they are declared, just as the effect of declarations within blocks persists to the end of the block. The syntax of external declarations is the same as that of all declarations, except that only at this level may the code for functions be given.

A10.1 - Function Definitions

Function definitions have the form

function-definition:
    declaration-specifiers_opt declarator declaration-list_opt compound-statement

The only storage-class specifiers allowed among the declaration specifiers are extern or static; see §A11.2 for the distinction between them.

A function may return an arithmetic type, a structure, a union, a pointer, or void, but not a function or an array. The declarator in a function declaration must specify explicitly that the declared identifier has function type; that is, it must contain one of the forms (see §A8.6.3)

direct-declarator ( parameter-type-list )
direct-declarator ( identifier-list_opt )

where the direct-declarator is an identifier or a parenthesized identifier. In particular, it must not achieve function type by means of a typedef.

In the first form, the definition is a new-style function, and its parameters, together with their types, are declared in its parameter type list; the declaration-list following the function's declarator must be absent. Unless the parameter type list consists solely of void, showing that the function takes no parameters, each declarator in the parameter type list must contain an identifier. If the parameter type list ends with ", ..." then the function may be called with more arguments than parameters; the va_arg macro mechanism defined in the standard header <stdarg.h> and described in Appendix B must be used to refer to the extra arguments. Variadic functions must have at least one named parameter.

In the second form, the definition is old-style: the identifier list names the parameters, while the declaration list attributes types to them. If no declaration is given for a parameter, its type is taken to be int. The declaration list must declare only parameters named in the list, initialization is not permitted, and the only storage-class specifier possible is register.

In both styles of function definition, the parameters are understood to be declared just after the beginning of the compound statement constituting the function's body, and thus the same identifiers must not be redeclared there (although they may, like other identifiers, be redeclared in inner blocks). If a parameter is declared to have type "array of type," the declaration is adjusted to read "pointer to type;" similarly, if a parameter is declared to have type "function returning type," the declaration is adjusted to read "pointer to function returning type." During the call to a function, the arguments are converted as necessary and assigned to the parameters; see §A7.3.2.

New-style function definitions are new with the ANSI standard. There is also a small change in the details of promotion; the first edition specified that the declarations of float parameters were adjusted to read double. The difference becomes noticeable when a pointer to a parameter is generated within a function.

A complete example of a new-style function definition is

int max(int a, int b, int c)
{
  int m;

  m = (a > b) ? a : b;
  return (m > c) ? m : c;
}

Here int is the declaration specifier; max(int a, int b, int c) is the function's declarator, and { ... } is the block giving the code for the function. The corresponding old-style definition would be

int max(a, b, c)
int a, b, c;
{
  /* ... */
}

where now int max (a, b, c) is the declarator, and int a, b, c; is the declaration list for the parameters.

A10.2 - External Declarations

External declarations specify the characteristics of objects, functions and other identifiers. The term "external" refers to their location outside functions, and is not directly connected with the extern keyword; the storage class for an externally-declared object may be left empty, or it may be specified as extern or static.

Several external declarations for the same identifier may exist within the same translation unit if they agree in type and linkage, and if there is at most one definition for the identifier.

Two declarations for an object or function are deemed to agree in type under the rules discussed in §A8.10. In addition, if the declarations differ because one type is an incomplete structure, union, or enumeration type (§A8.3) and the other is the corresponding completed type with the same tag, the types are taken to agree. Moreover, if one type is an incomplete array type (§A8.6.2) and the other is a completed array type, the types, if otherwise identical, are also taken to agree. Finally, if one type specifies an old-style function, and the other an otherwise identical new-style function, with parameter declarations, the types are taken to agree.

If the first external declaration for a function or object includes the static specifier, the identifier has internal linkage; otherwise it has external linkage. Linkage is discussed in §A11.2.

An external declaration for an object is a definition if it has an initializer. An external object declaration that does not have an initializer, and does not contain the extern specifier, is a tentative definition. If a definition for an object appears in a ttanslation unit, any tentative definitions are treated merely as redundant declarations. If no definition for the opject appears in the translation unit, all its tentative definitions become a single definition with initializer 0.

Each object must have exactly one definition. For objects with internal linkage, this rule applies separately to each translation unit, because internally-linked objects are unique to a translation unit. For objects with external linkage, it applies to the entire program.

Although the one-definition rule is formulated somewhat differently in the first edition of this book, it is in effect identical to the one 'Stated here. Some implementations relax it by generalizing the notion of tentative definition. In the alternate formulation, which is usual in UNIX systems and recognized as a common extension by the Standard, all the tentative definitions for an externallylinked object, throughout all the translation units of a program, are considered together instead of in each translation unit separately. If a definition occurs somewhere in the program, then the tentative definitions become merely declarations, but if no definition appears, then all its tentative definitions become a definition with initializer 0.

A11 - Scope and Linkage

A program need not all be compiled at one time: the source text may be kept in several files containing translation units, and precompiled routines may be loaded from libraries. Communication among the functions of a program may be carried out both through calls and through manipulation of external data.

Therefore, there are two kinds of scope to consider: first, the lexical scope of an identifier, which is the region of the program text within which the identifier's characteristics are understood; and seci:>nd, the scope associated with objects and functions with external linkage, which determines the connections between identifiers in separately compiled translation units.

A11.1 - Lexical Scope

Identifiers fall into several name spaces that do not interfere with one another; the same identifier may be used for different purposes, even in the same scope, if the uses are in different name spaces. These classes are: objects, functions, typedef names, and enum constants; labels; tags of structures, unions, and enumerations; and members of each structure or union individually.

These rules differ in several ways from those described in the first edition of this manual. Labels did not previoulsly have their own name space; tags of structures and unions each had a separate space, and in some implementations enumeration tags did as well; putting different kinds of tags into the same space is a new restriction. The most important departure from the first edition is that each structure or union creates a separate name space for its members, so that the same name may appear in several different structures. This rule has been common practice for several years.

The lexical scope of an object or function identifier in an external declaration begins at the end of its declarator and persists to the end of the translation unit in which it appears. The scope of a parameter of a function definition begins at the start of the block defining the function, and persists through the function; the scope of a parameter in a function declaration ends at the end of the declarator. The scope of an identifier declared at the head of a block begins at the end of its declarator, and persists to the end of the block. The scope of a label is the whole of the function in which· it appears. The scope of a structure, union, or enumeration tag, or an enumeration constant, begins at its appearance in a type specifier, and persists to the end of the translation unit (for declarations at the external level) or to the end of the block (for declarations within a function).

If an identifier is explicitly declared at the head of a block, including the block constituting a function, any declaration of the identifier outside the block is suspended until the end of the block.

A11.2 - Linkage

Within a translation unit, all declarations of the same object or function identifier with internal linkage refer to the same thing, and the object or function is unique to that translation unit. All declarations for the same object or function identifier with external linkage refer to the same thing, and the object or function is shared by the entire program.

As discussed in §A10.2, the first external declaration for an identifier gives the identifier internal linkage if the static specifier is used, external linkage otherwise. If a declaration for an identifier within a block does not include the extern specifier, then the identifier has no linkage and is unique to the function. If it does include extern, and an external declaration for the identifier is active in the scope surrounding the block, then the identifier has the same linkage as the external declaration, and refers to the same object or function; but if no external declaration is visible, its linkage is external.

A12 - Preprocessing

A preprocessor performs macro substitution, conditional compilation, and inclusion of named files. Lines beginning with #, perhaps preceded by white space, communicate with this preprocessor. The syntax of these lines is independent of the rest of the language; they may appear anywhere and have effect that lasts (independent of scope) until the end of the translation unit. Line boundaries are significant; each line is analyzed individually (but see §A12.2 for how to adjoin lines). To the preprocessor, a token is any language token, or a character sequence giving a file name as in the #include directive (§A12.4); in addition, any character not otherwise defined is taken as a token. However, the effect of white space characters other than space and horizontal tab is undefined within preprocessor lines.

Preprocessing itself takes place in several logically successive phases that may, in a particular implementation, be condensed.

1. First, trigraph sequences as described in §A12.1 are replaced by their equivalents. Should the operating system environment require it, newline characters are introduced between the lines of the source file.
2. Each occurrence of a backslash character \ followed by a newline is deleted, thus splicing lines (§A12.2).
3. The program is split into tokens separated by white-space characters; comments are replaced by a single space. Then preprocessing directives are obeyed, and macros (§§A12.3-A12.10) are expanded.
4. Escape sequences in character constants and string literals (§§A2.5.2, A2.6) are replaced by their equivalents; then adjacent string literals are concatenated.
5. The result is translated, then linked together with other programs and libraries, by collecting the necessary programs and data, and connecting external function and object references to their definitions.

A12.1 - Trigraph Sequence

The character set of C source programs is contained within seven-bit ASCII, but is a superset of the ISO 646-1983 Invariant Code Set. In order to enable programs to be represented in the reduced set, all occurrences of the following trigraph sequences are replaced by the corresponding single character. This replacement occurs before any other processing.

??=   #             ??(   [               ??<   {
??/   \             ??(   ]               ??>   }
??'   ^             ??!   |               ??-   ~

No other such replacements occur.

Trigraph sequences are new with the ANSI standard.

A12.2 - Line Splicing

Lines that end with the backslash character \ are folded by deleting the backslash and the following newline character. This occurs before division into tokens.

A12.3 - Macro Definition and Expansion

A control line of the form

# define identifier token-sequence

causes the preprocessor to replace subsequent instances of the identifier with the given sequence of tokens; leading and trailing white space around the token sequence is discarded. A second #define for the same identifier is erroneous unless the second token sequence is identical to the first, where all white space separations are taken to be equivalent.

A line of the form

# define identifier ( identifier-list ) token-sequence

where there is no space between the first identifier and the (, is a macro definition with parameters given by the identifier list. As with the first form, leading and trailing white space around the token sequence is discarded, and the macro may be redefined only with a definition in which the number and spelling of parameters, and the token sequence, is identical.

A control line of the form

# undef identifier

causes the identifier's preprocessor definition to be forgotten. It is not erroneous to apply #undef to an unknown identifier.

When a macro has been defined in the second form, subsequent textual instances of the macro identifier followed by optional white space, and then by (, a sequence of tokens separated by commas, and a ) constitute a call of the macro. The arguments of the call are the comma-separated token sequences; commas that are quoted or protected by nested parentheses do not separate arguments. During collection, arguments are not macro-expanded. The number of arguments in the call must match the number of parameters in the definition. After the arguments are isolated, leading and trailing white space is removed from them. Then the token sequence resulting from each argument is substituted for each unquoted occurrence of the corresponding parameter's identifier in the replacement token sequence of the macro. Unless the parameter in the replacement sequence is preceded by #, or preceded or followed by ##, the argument tokens are examined for macro calls, and expanded as necessary, just before insertion.

Two special operators influence the replacement process. First, if an occurrence of a parameter in the replacement token sequence is immediately preceded by #, string quotes (") are placed around the corresponding parameter, and then both the # and the parameter identifier are replaced by the quoted argument. A \ character is inserted before each " or \ character that appears surrounding, or inside, a string literal or character constant in the argument.

Second, if the definition token sequence for either kind of macro contains a ## operator, then just after replacement of the parameters, each ## is deleted, together with any white space on either side, so as to concatenate the adjacent tokens and form a new token. The effect is undefined if invalid tokens are produced, or if the result depends on the order of processing of the ## operators. Also, ## may not appear at the beginning or end of a replacement token sequence.

In both kinds of macro, the replacement token sequence is repeatedly rescanned for more defined identifiers. However, once a given identifier has been replaced in a given expansion, it is not replaced if it turns up again during rescanning; instead it is left unchanged.

Even if the final value of a macro expansion begins with #, it is not taken to be a preprocessing directive.

The details of the macro-expansion process are described more precisely in the ANSI standard than in the first edition. The most important change is the addition of the # and ## operators, which make quotation and concatenation admissible. Some of the new rules, especially those involving concatenation, are bizarre. (See example below.)

For example, this facility may be used for "manifest constants," as in

#define TABSIZE 100
int table[TABSIZE];

The definition

#define ABSDIFF(a, b) ((a) > (b) ? (a) - (b) : (b) - (a))

defines a macro to return the absolute value of the difference between its arguments. Unlike a function to do the same thing, the arguments and returned value may have any arithmetic type or even be pointers. Also, the arguments, which might have side effects, are evaluated twice, once for the test and once to produce the value.

Given the definition

#define tempfile(dir) #dir "/%s"

the macro call tempfile(/usr/tmp) yields

"/usr/tmp" "/%s"

which will subsequently be catenated into a single string. After

#define cat(x, y) x ## y

the call cat(var, 123) yields var123. However, the call cat(cat(1,2),3)) is undefined: the presence of ## prevents the arguments of the outer call from being expanded. Thus it produces the token string

cat ( 1 , 2)3

and )3 (the catenation of the last token of the first argument with the first token of the second) is not a legal token. If a second level of macro definition is introduced,

#define xcat(x,y) cat(x,y)

things work more smoothly; xcat(xcat( 1 , 2), 3) does produce 123, because the expansion of xcat itself does not involve the ## operator.

Likewise, ABSDIFF(ABSDIFF(a,b),c) produces the expected, fully-expanded result.

A12.4 - File Inclusion

A control line of the form

# include <filename>

causes the replacement of that line by the entire contents of the file filename. The characters in the name filename must not include > or newline, and the effect is undefined if it contains any of ", ', \, or /*· The named file is searched for in a sequence of implementation-dependent places.

Similarly, a control line of the form

# include "filename"

searches first in association with the original source file (a deliberately implementation-dependent phrase), and if that search fails, then as if in the first form. The effect of using ', \, or /* in the filename remains undefined, but > is permitted.

Finally, a directive of the form

# include token-sequence

not matching one of the previous forms is interpreted by expanding the token sequence as for normal text; one of the two forms with < ... > or " ... " must result, and it is then treated as previously described.

#include files may be nested.

A12.5 - Conditional Compilation

Parts of a program may be compiled conditionally, according to the following schematic syntax.

preprocessor-conditional:
    if-line text elif-parts else-part_opt #endif
if-line:
    # if constant-expression
    # ifdef identifier
    # ifndef identifier

elif-parts:
    elif-line text
    elif-parts_opt

elif-line:
    # elif constant-expression

else-part:
    else-line text

else-line:
    # else

Each of the directives (if-line, elif-line, else-line, and #endif) appears alone on a line. The constant expressions in #if and subsequent #elif lines are evaluated in order until an expression with a non-zero value is found; text following a line with a zero value is discarded. The text following the successful directive line is treated normally. "Text" here refers to any material, including preprocessor lines, that is not part of the conditional structure; it may be empty. Once a successful #if or #elif line has been found and its text processed, succeeding #elif and #else lines, together with their text, are discarded. If all the expressions are zero, and there is an #else, the text following the #else is treated normally. Text controlled by inactive arms of the conditional is ignored except for checking the nesting of conditionals.

The constant expression in #if and #elif is subject to ordinary macro replacement. Moreover, any expressions of the form

defined identifier

or

defined ( identifier )

are replaced, before scanning for macros, by 1L if the identifier is defined in the preprocessor, and by 0L if not. Any identifiers remaining after macro expansion are replaced by 0L. Finally, each integer constant is considered to be suffixed with L, so that all arithmetic is taken to be long or unsigned long.

The resulting constant expression (§A7.19) is restricted: it must be integral, and may not contain sizeof, a cast, or an enumeration constant.

The control lines

#ifdef identifier
#ifndef identifier

are equivalent to

# if defined identifier
# if ! defined identifier

respectively.

#elif is new since the first edition, although it has been available in some preprocessors. The defined preprocessor operator is also new.

A12.6 - Line Control

For the benefit of other preprocessors that generate C programs, a line in one of the forms

# line constant "filename"
# line constant

causes the compiler to believe, for purposes of error diagnostics, that the line number of the next source line is given by the decimal integer constant and the current input file is named by the identifier. If the quoted filename is absent, the remembered name does not change. Macros in the line are expanded before it is interpreted.

A12.7 - Error Generation

A preprocessor line of the form

# error token-sequence_opt

causes the processor to write a diagnostic message that includes the token sequence.

A12.8 - Pragmas

A control line of the form

# pragma token-sequence_opt

causes the processor to perform an implementation-dependent action. An unrecognized pragma is ignored.

A12.9 - Null Directive

A preprocessor line of the form

#

has no effect.

A12.10 - Predefined Names

Several identifiers are predefined, and expand to produce special information. They, and also the preprocessor expression operator defined, may not be undefined or redefined.

__LINE__      A decimal constant containing the current source line number.

__FILE__      A string literal containing the name of the file being compiled.

__DATE__      A string literal containing the date of compilation, in the form
              "Mmm dd yyyy".

__TIME__      A string literal containing the time of compilation, in the form
              "hh:mm:ss".

__STDC__      The constant 1. It is intended that this identifier be defined to be
              only in standard-conforming implementations.

#error and #pragma are new with the ANSI standard; the predefined preprocessor macros are new, but some of them have been available in some implementations.

A13 - Grammar

Below is a recapitulation of the grammar that was given throughout the earlier part of this appendix. It has exactly the same content, but is in a different order.

The grammar has undefined terminal symbols integer-constant, character-constant, floating-constant, identifier, string, and enumeration-constant; the typewriter style words and symbols are terminals given literally. This grammar can be transformed mechanically into input acceptable to an automatic parser-generator. Besides adding whatever syntactic marking is used to indicate alternatives in productions, it is necessary to expand the "one or• constructions, and (depending on the rules of the parsergenerator) to duplicate each production with an opt symbol, once with the symbol and once without. With one further change, namely deleting the production typedef-name: identifier and making typedef-name a terminal symbol, this grammar is acceptable to the YACC parser-generator. It has only one conflict, generated by the if-else ambiguity.

translation-unit:
    external-declaration
    translation-unit external-declaration

external-declaration:
    function-definition
    declaration

function-definition:
    declaration-specifiers_opt, declarator declaration-list_opt, compound-statement

declaration:
    declaration-specifiers init-declarator-list_opt ;

declaration-list:
    declaration
    declaration-list declaration

declaration-specifiers:
    storage-class-specifier declaration-specifiers_opt
    type-specifier declaration-specifiers_opt
    type-qualifier declaration-specifiers_opt

storage-class-specifier: one of
    auto   register   static   extern   typedef

type-specifier: one of
    void   char   short   int   long   float   double   signed
    unsigned   struct-or-union-specifier   enum-specifier   typedef-name

type-qualifier: one of
    const   volatile

struct-or-union-specifier:
    struct-or-union identifier_opt { struct-declaration-list }
    struct-or-union identifier

struct-or-union: one of
    struct   union

struct-declaration-list:
    struct-declaration
    struct-declaration-list struct-declaration

init-declarator-list:
    init-declarator
    init-declarator-list , init-declarator

init-declarator:
    declarator
    declarator = initializer

struct-declaration:
    specifier-qualifier-list struct-declarator-list ;

specifier-qualifier-list:
    type-specifier specifier-qualifier-list_opt
    type-qualifier specifier-qualifier-list_opt

struct-declarator-list:
    struct-declarator
    struct-declarator-list , struct-declarator

struct-declarator:
    declarator
    declarator_opt : constant-expression

enum-specifier:
    enum identifier_opt { enumerator-list }
    enum identifier

enumerator-list:
    enumerator
    enumerator-list , enumerator

enumerator:
    identifier
    identifier = constant-expression

declarator:
    pointer_opt direct-declarator

direct-declarator:
    identifier
    ( declarator )
    direct-declarator [ constant-expression_opt ]
    direct-declarator ( parameter-type-list )
    direct-declarator ( identifier-list_opt )

pointer:
    * type-qualifier-list_opt
    * type-qualifier-list_opt pointer

type-qualifier-list:
    type-qualifier
    type-qualifier-list type-qualifier

parameter-type-list:
    parameter-list
    parameter-list , ...

parameter-list:
    parameter-declaration
    parameter-list , parameter-declaration

parameter-declaration:
    declaration-specifiers declarator
    declaration-specifiers abstract-declarator_opt

identifier-list:
    identifier
    identifier-list , identifier

initializer:
    assignment-expression
    { initializer-list }
    { initializer-list , }

initializer-list:
    initializer
    initializer-list , initializer

type-name:
    specifier-qualifier-list abstract-declarator_opt

abstract-declarator:
    pointer
    pointer_opt direct-abstract-declarator

direct-abstract-declarator:
    ( abstract-declarator )
    direct-abstract-declarator_opt [ constant-expression_opt ]
    direct-abstract-declarator_opt ( parameter-type-list_opt )

typedef-name:
    identifier

statement:
    labeled-statement
    expression-statement
    compound-statement
    selection-statement
    iteration-statement
    jump-statement

labeled-statement:
    identifier : statement
    case constant-expression : statement
    default : statement

expression-statement:
    expression_opt ;

compound-statement:
    { declaration-list_opt statement-list_opt }

statement-list:
    statement
    statement-list statement

selection-statement:
    if ( expression ) statement
    if ( expression ) statement else statement
    switch ( expression ) statement

iteration-statement:
    while ( expression ) statement
    do statement while ( expression ) ;
    for ( expression_opt, ; expression_opt ; expression_opt ) statement

jump-statement:
    goto identifier ;
    continue ;
    break ;
    return expression_opt ;

expression:
    assignment-expression
    expression , assignment-expression

assignment-expression:
    conditional-expression
    unary-expression assignment-operator assignment-expression

assignment-operator: one of
    =   *=   /=   %=   +=   -=   <<=   >>=   &=   ^=   |=

conditional-expression:
    logical-OR-expression
    logical-OR-expression ? expression : conditional-expression

constant-expression:
    conditional-expression

logical-OR-expression:
    logical-AND-expression
    logical-OR-expression || logical-AND-expression

logical-AND-expression:
    inclusive-OR-expression
    logical-AND-expression && inclusive-OR-expression

inclusive-OR-expression:
    exclusive-OR-expression
    inclusive-OR-expression | exclusive-OR-expression

exclusive-OR-expression:
    AND-expression
    exclusive-OR-expression ^ AND-expression

AND-expression:
    equality-expression
    AND-expression & equality-expression

equality-expression:
    relational-expression
    equality-expression == relational-expression
    equality-expression != relational-expression

relational-expression:
    shift-expression
    relational-expression < shift-expression
    relational-expression > shift-expression
    relational-expression <= shift-expression
    relational-expression >= shift-expression

shift-expression:
    additive-expression
    shift-expression << additive-expression
    shift-expression >> additive-expression

additive-expression:
    multiplicative-expression
    additive-expression + multiplicative-expression
    additive-expression - multiplicative-expression

multiplicative-expression:
    cast-expression
    multiplicative-expression * cast-expression
    multiplicative-expression / cast-expression
    multiplicative-expression % cast-expression

cast-expression:
    unary-expression
    ( type-name ) cast-expression

unary-expression:
    postfix-expression
    ++ unary-expression
    -- unary-expression
    unary-operator cast-expression
    sizeof unary-expression
    sizeof ( type-name )

unary-operator: one of
    &   *   +   -   ~   !

postfix-expression:
    primary-expression
    postfix-expression [ expression ]
    postfix-expression ( argument-expression-list_opt )
    postfix-expression . identifier
    postfix-expression -> identifier
    postfix-expression ++
    postfix-expression --

primary-expression:
    identifier
    constant
    string
    ( expression )

argument-expression-list:
    assignment-expression
    argument-expression-list , assignment-expression

constant:
    integer-constant
    character-constant
    floating-constant
    enumeration-constant

The following grammar for the preprocessor summarizes the structure of control lines, but is not suitable for mechanized parsing. It includes the symbol text, which means ordinary program text, non-conditional preprocessor control lines, or complete preprocessor conditional constructions.

control-line:
    # define identifier token-sequence
    # define identifier( identifier , ... , identifier ) token-sequence
    # undef identifier
    # include <filename>
    # include "filename"
    # include token-sequence
    # line constant "filename"
    # line constant
    # error token-sequence_opt
    # pragma token-sequence_opt
    #
    preprocessor-conditional

preprocessor-conditional:
    if-line text elif-parts else-part_opt # endif

if-line:
    # if constant-expression
    # ifdef identifier
    # ifndef identifier

elif-parts:
    elif-line text
    elif-parts_opt

elif-line:
    # elif constant-expression

else-part:
    else-line text

else-line:
    # else