5. Extensions to the C Language Family

GNU C provides several language features not found in ISO standard C. (The `-pedantic' option directs GCC to print a warning message if any of these features is used.) To test for the availability of these features in conditional compilation, check for a predefined macro __GNUC__, which is always defined under GCC.

These extensions are available in C and Objective C. Most of them are also available in C++. See section Extensions to the C++ Language, for extensions that apply only to C++.

Some features that are in ISO C99 but not C89 or C++ are also, as extensions, accepted by GCC in C89 mode and in C++.

5.1 Statements and Declarations in Expressions		Putting statements and declarations inside expressions.
5.2 Locally Declared Labels		Labels local to a statement-expression.
5.3 Labels as Values		Getting pointers to labels, and computed gotos.
5.4 Nested Functions		As in Algol and Pascal, lexical scoping of functions.
5.5 Constructing Function Calls		Dispatching a call to another function.
5.6 Naming an Expression's Type		Giving a name to the type of some expression.
5.7 Referring to a Type with typeof		typeof: referring to the type of an expression.
5.8 Generalized Lvalues		Using `?:', `,' and casts in lvalues.
5.9 Conditionals with Omitted Operands		Omitting the middle operand of a `?:' expression.
5.10 Double-Word Integers		Double-word integers---long long int.
5.11 Complex Numbers		Data types for complex numbers.
5.12 Hex Floats		Hexadecimal floating-point constants.
5.13 Arrays of Length Zero		Zero-length arrays.
5.14 Arrays of Variable Length		Arrays whose length is computed at run time.
5.15 Macros with a Variable Number of Arguments.		Macros with a variable number of arguments.
5.16 Slightly Looser Rules for Escaped Newlines		Slightly looser rules for escaped newlines.
5.17 String Literals with Embedded Newlines		String literals with embedded newlines.
5.18 Non-Lvalue Arrays May Have Subscripts		Any array can be subscripted, even if not an lvalue.
5.19 Arithmetic on void- and Function-Pointers		Arithmetic on void-pointers and function pointers.
5.20 Non-Constant Initializers		Non-constant initializers.
5.21 Compound Literals		Compound literals give structures, unions or arrays as values.
5.22 Designated Initializers		Labeling elements of initializers.
5.24 Cast to a Union Type		Casting to union type from any member of the union.
5.23 Case Ranges		`case 1 ... 9' and such.
5.25 Mixed Declarations and Code		Mixing declarations and code.
5.26 Declaring Attributes of Functions		Declaring that functions have no side effects, or that they can never return.
5.27 Attribute Syntax		Formal syntax for attributes.
5.28 Prototypes and Old-Style Function Definitions		Prototype declarations and old-style definitions.
5.29 C++ Style Comments		C++ comments are recognized.
5.30 Dollar Signs in Identifier Names		Dollar sign is allowed in identifiers.
5.31 The Character ESC in Constants		`\e' stands for the character ESC.
5.33 Specifying Attributes of Variables		Specifying attributes of variables.
5.34 Specifying Attributes of Types		Specifying attributes of types.
5.32 Inquiring on Alignment of Types or Variables		Inquiring about the alignment of a type or variable.
5.35 An Inline Function is As Fast As a Macro		Defining inline functions (as fast as macros).
5.36 Assembler Instructions with C Expression Operands		Assembler instructions with C expressions as operands.

                         (With them you can define "built-in" functions.)

5.37 Controlling Names Used in Assembler Code		Specifying the assembler name to use for a C symbol.
5.38 Variables in Specified Registers		Defining variables residing in specified registers.
5.39 Alternate Keywords		__const__, __asm__, etc., for header files.
5.40 Incomplete enum Types		enum foo;, with details to follow.
5.41 Function Names as Strings		Printable strings which are the name of the current function.
5.42 Getting the Return or Frame Address of a Function		Getting the return or frame address of a function.
5.43 Other built-in functions provided by GCC		Other built-in functions.

5.1 Statements and Declarations in Expressions

A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.

Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:

({ int y = foo (); int z; if (y > 0) z = y; else z = - y; z; })

is a valid (though slightly more complex than necessary) expression for the absolute value of foo ().

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. (If you use some other kind of statement last within the braces, the construct has type void, and thus effectively no value.)

This feature is especially useful in making macro definitions "safe" (so that they evaluate each operand exactly once). For example, the "maximum" function is commonly defined as a macro in standard C as follows:

#define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either a or b twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here let's assume int), you can define the macro safely as follows:

#define maxint(a,b) \ ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.

If you don't know the type of the operand, you can still do this, but you must use typeof (see section 5.7 Referring to a Type with typeof) or type naming (see section 5.6 Naming an Expression's Type).

Statement expressions are not supported fully in G++, and their fate there is unclear. (It is possible that they will become fully supported at some point, or that they will be deprecated, or that the bugs that are present will continue to exist indefinitely.) Presently, statement expressions do not work well as default arguments.

In addition, there are semantic issues with statement-expressions in C++. If you try to use statement-expressions instead of inline functions in C++, you may be surprised at the way object destruction is handled. For example:

#define foo(a) ({int b = (a); b + 3; })

does not work the same way as:

inline int foo(int a) { int b = a; return b + 3; }

In particular, if the expression passed into foo involves the creation of temporaries, the destructors for those temporaries will be run earlier in the case of the macro than in the case of the function.

These considerations mean that it is probably a bad idea to use statement-expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement-expression that lead to precisely this bug.)

5.2 Locally Declared Labels

Each statement expression is a scope in which local labels can be declared. A local label is simply an identifier; you can jump to it with an ordinary goto statement, but only from within the statement expression it belongs to.

A local label declaration looks like this:

__label__ label;

or

__label__ label1, label2, ...;

Local label declarations must come at the beginning of the statement expression, right after the `({', before any ordinary declarations.

The label declaration defines the label name, but does not define the label itself. You must do this in the usual way, with label:, within the statements of the statement expression.

The local label feature is useful because statement expressions are often used in macros. If the macro contains nested loops, a goto can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function. A local label avoids this problem. For example:

#define SEARCH(array, target) \ ({ \ __label__ found; \ typeof (target) _SEARCH_target = (target); \ typeof (*(array)) *_SEARCH_array = (array); \ int i, j; \ int value; \ for (i = 0; i < max; i++) \ for (j = 0; j < max; j++) \ if (_SEARCH_array[i][j] == _SEARCH_target) \ { value = i; goto found; } \ value = -1; \ found: \ value; \ })

5.3 Labels as Values

You can get the address of a label defined in the current function (or a containing function) with the unary operator `&&'. The value has type void *. This value is a constant and can be used wherever a constant of that type is valid. For example:

void *ptr; ... ptr = &&foo;

To use these values, you need to be able to jump to one. This is done with the computed goto statement(2), goto *exp;. For example,

goto *ptr;

Any expression of type void * is allowed.

One way of using these constants is in initializing a static array that will serve as a jump table:

static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

goto *array[i];

Note that this does not check whether the subscript is in bounds--array indexing in C never does that.

Such an array of label values serves a purpose much like that of the switch statement. The switch statement is cleaner, so use that rather than an array unless the problem does not fit a switch statement very well.

Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.

You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.

An alternate way to write the above example is

static const int array[] = { &&foo - &&foo, &&bar - &&foo, &&hack - &&foo }; goto *(&&foo + array[i]);

This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.

5.4 Nested Functions

A nested function is a function defined inside another function. (Nested functions are not supported for GNU C++.) The nested function's name is local to the block where it is defined. For example, here we define a nested function named square, and call it twice:

foo (double a, double b) { double square (double z) { return z * z; } return square (a) + square (b); }

The nested function can access all the variables of the containing function that are visible at the point of its definition. This is called lexical scoping. For example, here we show a nested function which uses an inherited variable named offset:

bar (int *array, int offset, int size) { int access (int *array, int index) { return array[index + offset]; } int i; ... for (i = 0; i < size; i++) ... access (array, i) ... }

Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.

It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:

hack (int *array, int size) { void store (int index, int value) { array[index] = value; } intermediate (store, size); }

Here, the function intermediate receives the address of store as an argument. If intermediate calls store, the arguments given to store are used to store into array. But this technique works only so long as the containing function (hack, in this example) does not exit.

If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.

GCC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as http://people.debian.org/~karlheg/Usenix88-lexic.pdf.

A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (see section 5.2 Locally Declared Labels). Such a jump returns instantly to the containing function, exiting the nested function which did the goto and any intermediate functions as well. Here is an example:

bar (int *array, int offset, int size) { __label__ failure; int access (int *array, int index) { if (index > size) goto failure; return array[index + offset]; } int i; ... for (i = 0; i < size; i++) ... access (array, i) ... ... return 0; /* Control comes here from access if it detects an error. */ failure: return -1; }

A nested function always has internal linkage. Declaring one with extern is erroneous. If you need to declare the nested function before its definition, use auto (which is otherwise meaningless for function declarations).

bar (int *array, int offset, int size) { __label__ failure; auto int access (int *, int); ... int access (int *array, int index) { if (index > size) goto failure; return array[index + offset]; } ... }

5.5 Constructing Function Calls

Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.

You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).

Built-in Function: void * __builtin_apply_args ()

This built-in function returns a pointer to data describing how to perform a call with the same arguments as were passed to the current function.

The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.

Built-in Function: void * __builtin_apply (void (*function)(), void *arguments, size_t size)

This built-in function invokes function with a copy of the parameters described by arguments and size.

The value of arguments should be the value returned by __builtin_apply_args. The argument size specifies the size of the stack argument data, in bytes.

This function returns a pointer to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack.

It is not always simple to compute the proper value for size. The value is used by __builtin_apply to compute the amount of data that should be pushed on the stack and copied from the incoming argument area.

Built-in Function: void __builtin_return (void *result)

This built-in function returns the value described by result from the containing function. You should specify, for result, a value returned by __builtin_apply.

5.6 Naming an Expression's Type

You can give a name to the type of an expression using a typedef declaration with an initializer. Here is how to define name as a type name for the type of exp:

typedef name = exp;

This is useful in conjunction with the statements-within-expressions feature. Here is how the two together can be used to define a safe "maximum" macro that operates on any arithmetic type:

#define max(a,b) \ ({typedef _ta = (a), _tb = (b); \ _ta _a = (a); _tb _b = (b); \ _a > _b ? _a : _b; })

The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.

5.7 Referring to a Type with typeof

Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef.

There are two ways of writing the argument to typeof: with an expression or with a type. Here is an example with an expression:

typeof (x[0](1))

This assumes that x is an array of pointers to functions; the type described is that of the values of the functions.

Here is an example with a typename as the argument:

typeof (int *)

Here the type described is that of pointers to int.

If you are writing a header file that must work when included in ISO C programs, write __typeof__ instead of typeof. See section 5.39 Alternate Keywords.

A typeof-construct can be used anywhere a typedef name could be used. For example, you can use it in a declaration, in a cast, or inside of sizeof or typeof.

• This declares y with the type of what x points to.

  typeof (*x) y;

• This declares y as an array of such values.

  typeof (*x) y[4];

• This declares y as an array of pointers to characters:

  typeof (typeof (char *)[4]) y;

  It is equivalent to the following traditional C declaration:

  char *y[4];

  To see the meaning of the declaration using typeof, and why it might be a useful way to write, let's rewrite it with these macros:

  #define pointer(T) typeof(T *) #define array(T, N) typeof(T [N])

  Now the declaration can be rewritten this way:

  array (pointer (char), 4) y;

  Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char.

5.8 Generalized Lvalues

Standard C++ allows compound expressions and conditional expressions as lvalues, and permits casts to reference type, so use of this extension is deprecated for C++ code.

For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent:

(a, b) += 5 a, (b += 5)

Similarly, the address of the compound expression can be taken. These two expressions are equivalent:

&(a, b) a, &b

A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent:

(a ? b : c) = 5 (a ? b = 5 : (c = 5))

A cast is a valid lvalue if its operand is an lvalue. A simple assignment whose left-hand side is a cast works by converting the right-hand side first to the specified type, then to the type of the inner left-hand side expression. After this is stored, the value is converted back to the specified type to become the value of the assignment. Thus, if a has type char *, the following two expressions are equivalent:

(int)a = 5 (int)(a = (char *)(int)5)

An assignment-with-arithmetic operation such as `+=' applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent:

(int)a += 5 (int)(a = (char *)(int) ((int)a + 5))

You cannot take the address of an lvalue cast, because the use of its address would not work out coherently. Suppose that &(int)f were permitted, where f has type float. Then the following statement would try to store an integer bit-pattern where a floating point number belongs:

*&(int)f = 1;

This is quite different from what (int)f = 1 would do--that would convert 1 to floating point and store it. Rather than cause this inconsistency, we think it is better to prohibit use of `&' on a cast.

If you really do want an int * pointer with the address of f, you can simply write (int *)&f.

5.9 Conditionals with Omitted Operands

The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

x ? : y

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

x ? x : y

In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.

5.10 Double-Word Integers

ISO C99 supports data types for integers that are at least 64 bits wide, and as an extension GCC supports them in C89 mode and in C++. Simply write long long int for a signed integer, or unsigned long long int for an unsigned integer. To make an integer constant of type long long int, add the suffix `LL' to the integer. To make an integer constant of type unsigned long long int, add the suffix `ULL' to the integer.

You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC.

There may be pitfalls when you use long long types for function arguments, unless you declare function prototypes. If a function expects type int for its argument, and you pass a value of type long long int, confusion will result because the caller and the subroutine will disagree about the number of bytes for the argument. Likewise, if the function expects long long int and you pass int. The best way to avoid such problems is to use prototypes.

5.11 Complex Numbers

ISO C99 supports complex floating data types, and as an extension GCC supports them in C89 mode and in C++, and supports complex integer data types which are not part of ISO C99. You can declare complex types using the keyword _Complex. As an extension, the older GNU keyword __complex__ is also supported.

For example, `_Complex double x;' declares x as a variable whose real part and imaginary part are both of type double. `_Complex short int y;' declares y to have real and imaginary parts of type short int; this is not likely to be useful, but it shows that the set of complex types is complete.

To write a constant with a complex data type, use the suffix `i' or `j' (either one; they are equivalent). For example, 2.5fi has type _Complex float and 3i has type _Complex int. Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant. This is a GNU extension; if you have an ISO C99 conforming C library (such as GNU libc), and want to construct complex constants of floating type, you should include <complex.h> and use the macros I or _Complex_I instead.

To extract the real part of a complex-valued expression exp, write __real__ exp. Likewise, use __imag__ to extract the imaginary part. This is a GNU extension; for values of floating type, you should use the ISO C99 functions crealf, creal, creall, cimagf, cimag and cimagl, declared in <complex.h> and also provided as built-in functions by GCC.

The operator `~' performs complex conjugation when used on a value with a complex type. This is a GNU extension; for values of floating type, you should use the ISO C99 functions conjf, conj and conjl, declared in <complex.h> and also provided as built-in functions by GCC.

GCC can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice-versa). None of the supported debugging info formats has a way to represent noncontiguous allocation like this, so GCC describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is foo, the two fictitious variables are named foo$real and foo$imag. You can examine and set these two fictitious variables with your debugger.

A future version of GDB will know how to recognize such pairs and treat them as a single variable with a complex type.

5.12 Hex Floats

ISO C99 supports floating-point numbers written not only in the usual decimal notation, such as 1.55e1, but also numbers such as 0x1.fp3 written in hexadecimal format. As a GNU extension, GCC supports this in C89 mode (except in some cases when strictly conforming) and in C++. In that format the `0x' hex introducer and the `p' or `P' exponent field are mandatory. The exponent is a decimal number that indicates the power of 2 by which the significant part will be multiplied. Thus `0x1.f' is 1 15/16, `p3' multiplies it by 8, and the value of 0x1.fp3 is the same as 1.55e1.

Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation. Otherwise the compiler would not be able to resolve the ambiguity of, e.g., 0x1.f. This could mean 1.0f or 1.9375 since `f' is also the extension for floating-point constants of type float.

5.13 Arrays of Length Zero

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:

struct line { int length; char contents[0]; }; struct line *thisline = (struct line *) malloc (sizeof (struct line) + this_length); thisline->length = this_length;

In ISO C89, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:

• Flexible array members are written as contents[] without the 0.

• Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero.

• Flexible array members may only appear as the last member of a struct that is otherwise non-empty. GCC currently allows zero-length arrays anywhere. You may encounter problems, however, defining structures containing only a zero-length array. Such usage is deprecated, and we recommend using zero-length arrays only in places in which flexible array members would be allowed.

GCC versions before 3.0 allowed zero-length arrays to be statically initialized. In addition to those cases that were useful, it also allowed initializations in situations that would corrupt later data. Non-empty initialization of zero-length arrays is now deprecated.

Instead GCC allows static initialization of flexible array members. This is equivalent to defining a new structure containing the original structure followed by an array of sufficient size to contain the data. I.e. in the following, f1 is constructed as if it were declared like f2.

struct f1 { int x; int y[]; } f1 = { 1, { 2, 3, 4 } }; struct f2 { struct f1 f1; int data[3]; } f2 = { { 1 }, { 2, 3, 4 } };

The convenience of this extension is that f1 has the desired type, eliminating the need to consistently refer to f2.f1.

This has symmetry with normal static arrays, in that an array of unknown size is also written with [].

Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets. To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object. For example:

struct foo { int x; int y[]; }; struct bar { struct foo z; }; struct foo a = { 1, { 2, 3, 4 } }; // Valid. struct bar b = { { 1, { 2, 3, 4 } } }; // Invalid. struct bar c = { { 1, { } } }; // Valid. struct foo d[1] = { { 1 { 2, 3, 4 } } }; // Invalid.

5.14 Arrays of Variable Length

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in C++. (However, GCC's implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.) These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:

FILE * concat_fopen (char *s1, char *s2, char *mode) { char str[strlen (s1) + strlen (s2) + 1]; strcpy (str, s1); strcat (str, s2); return fopen (str, mode); }

Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.

You can use the function alloca to get an effect much like variable-length arrays. The function alloca is available in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant.

There are other differences between these two methods. Space allocated with alloca exists until the containing function returns. The space for a variable-length array is deallocated as soon as the array name's scope ends. (If you use both variable-length arrays and alloca in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with alloca.)

You can also use variable-length arrays as arguments to functions:

struct entry tester (int len, char data[len][len]) { ... }

The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with sizeof.

If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list--another GNU extension.

struct entry tester (int len; char data[len][len], int len) { ... }

The `int len' before the semicolon is a parameter forward declaration, and it serves the purpose of making the name len known when the declaration of data is parsed.

You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the "real" parameter declarations. Each forward declaration must match a "real" declaration in parameter name and data type. ISO C99 does not support parameter forward declarations.

5.15 Macros with a Variable Number of Arguments.

In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:

#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)

Here `...' is a variable argument. In the invocation of such a macro, it represents the zero or more tokens until the closing parenthesis that ends the invocation, including any commas. This set of tokens replaces the identifier __VA_ARGS__ in the macro body wherever it appears. See the CPP manual for more information.

GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. Here is an example:

#define debug(format, args...) fprintf (stderr, format, args)

This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive.

GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition.

In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument. For example, this invocation is invalid in ISO C, because there is no comma after the string:

debug ("A message")

GNU CPP permits you to completely omit the variable arguments in this way. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string.

To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, `##'. If instead you write

#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)

and if the variable arguments are omitted or empty, the `##' operator causes the preprocessor to remove the comma before it. If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma. Just like any other pasted macro argument, these arguments are not macro expanded.

5.16 Slightly Looser Rules for Escaped Newlines

Recently, the non-traditional preprocessor has relaxed its treatment of escaped newlines. Previously, the newline had to immediately follow a backslash. The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, including multi-line strings, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.

5.17 String Literals with Embedded Newlines

As an extension, GNU CPP permits string literals to cross multiple lines without escaping the embedded newlines. Each embedded newline is replaced with a single `\n' character in the resulting string literal, regardless of what form the newline took originally.

CPP currently allows such strings in directives as well (other than the `#include' family). This is deprecated and will eventually be removed.

5.18 Non-Lvalue Arrays May Have Subscripts

Subscripting is allowed on arrays that are not lvalues, even though the unary `&' operator is not. (In ISO C99, both are allowed (though the array may not be used after the next sequence point), but this ISO C99 feature is not yet fully supported in GCC.) For example, this is valid in GNU C though not valid in C89:

struct foo {int a[4];}; struct foo f(); bar (int index) { return f().a[index]; }

5.19 Arithmetic on void- and Function-Pointers

In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1.

A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.

The option `-Wpointer-arith' requests a warning if these extensions are used.

5.20 Non-Constant Initializers

As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:

foo (float f, float g) { float beat_freqs[2] = { f-g, f+g }; ... }

5.21 Compound Literals

ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer. (GCC does not yet implement the full ISO C99 semantics for compound literals.) As an extension, GCC supports compound literals in C89 mode and in C++.

Usually, the specified type is a structure. Assume that struct foo and structure are declared as shown:

struct foo {int a; char b[2];} structure;

Here is an example of constructing a struct foo with a compound literal:

structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

{ struct foo temp = {x + y, 'a', 0}; structure = temp; }

You can also construct an array. If all the elements of the compound literal are (made up of) simple constant expressions, suitable for use in initializers, then the compound literal is an lvalue and can be coerced to a pointer to its first element, as shown here:

char **foo = (char *[]) { "x", "y", "z" };

Array compound literals whose elements are not simple constants are not very useful, because the compound literal is not an lvalue; ISO C99 specifies that it is, being a temporary object with automatic storage duration associated with the enclosing block, but GCC does not yet implement this. There are currently only two valid ways to use it with GCC: to subscript it, or initialize an array variable with it. The former is probably slower than a switch statement, while the latter does the same thing an ordinary C initializer would do. Here is an example of subscripting an array compound literal:

output = ((int[]) { 2, x, 28 }) [input];

Compound literals for scalar types and union types are is also allowed, but then the compound literal is equivalent to a cast.

5.22 Designated Initializers

Standard C89 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C89 mode as well. This extension is not implemented in GNU C++.

To specify an array index, write `[index] =' before the element value. For example,

int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write `[index]' before the element value, with no `='.

To initialize a range of elements to the same value, write `[first ... last] = value'. This is a GNU extension. For example,

int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.

Note that the length of the array is the highest value specified plus one.

In a structure initializer, specify the name of a field to initialize with `.fieldname =' before the element value. For example, given the following structure,

struct point { int x, y; };

the following initialization

struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

struct point p = { xvalue, yvalue };

Another syntax which has the same meaning, obsolete since GCC 2.5, is `fieldname:', as shown here:

struct point p = { y: yvalue, x: xvalue };

The `[index]' or `.fieldname' is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,

union foo { int i; double d; }; union foo f = { .d = 4 };

will convert 4 to a double to store it in the union using the second element. By contrast, casting 4 to type union foo would store it into the union as the integer i, since it is an integer. (See section 5.24 Cast to a Union Type.)

You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,

int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an enum type. For example:

int whitespace[256] = { [' '] = 1, ['\t'] = 1, ['\h'] = 1, ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

You can also write a series of `.fieldname' and `[index]' designators before an `=' to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the `struct point' declaration above:

struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

If the same field is initialized multiple times, it will have value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, gcc will discard them and issue a warning.

5.23 Case Ranges

You can specify a range of consecutive values in a single case label, like this:

case low ... high:

This has the same effect as the proper number of individual case labels, one for each integer value from low to high, inclusive.

This feature is especially useful for ranges of ASCII character codes:

case 'A' ... 'Z':

Be careful: Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values. For example, write this:

case 1 ... 5:

rather than this:

case 1...5:

5.24 Cast to a Union Type

A cast to union type is similar to other casts, except that the type specified is a union type. You can specify the type either with union tag or with a typedef name. A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts. (See section 5.21 Compound Literals.)

The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:

union foo { int i; double d; }; int x; double y;

both x and y can be cast to type union foo.

Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:

union foo u; ... u = (union foo) x == u.i = x u = (union foo) y == u.d = y

You can also use the union cast as a function argument:

void hack (union foo); ... hack ((union foo) x);

5.25 Mixed Declarations and Code

ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GCC also allows this in C89 mode. For example, you could do:

int i; ... i++; int j = i + 2;

Each identifier is visible from where it is declared until the end of the enclosing block.

5.26 Declaring Attributes of Functions

In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.

The keyword __attribute__ allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. Fourteen attributes, noreturn, pure, const, format, format_arg, no_instrument_function, section, constructor, destructor, unused, weak, malloc, alias and no_check_memory_usage are currently defined for functions. Several other attributes are defined for functions on particular target systems. Other attributes, including section are supported for variables declarations (see section 5.33 Specifying Attributes of Variables) and for types (see section 5.34 Specifying Attributes of Types).

You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use __noreturn__ instead of noreturn.

See section 5.27 Attribute Syntax, for details of the exact syntax for using attributes.

noreturn

A few standard library functions, such as abort and exit, cannot return. GCC knows this automatically. Some programs define their own functions that never return. You can declare them noreturn to tell the compiler this fact. For example,

void fatal () __attribute__ ((noreturn)); void fatal (...) { ... /* Print error message. */ ... exit (1); }

The noreturn keyword tells the compiler to assume that fatal cannot return. It can then optimize without regard to what would happen if fatal ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables.

Do not assume that registers saved by the calling function are restored before calling the noreturn function.

It does not make sense for a noreturn function to have a return type other than void.

The attribute noreturn is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows:

typedef void voidfn (); volatile voidfn fatal;

pure

Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute pure. For example,

int square (int) __attribute__ ((pure));

says that the hypothetical function square is safe to call fewer times than the program says.

Some of common examples of pure functions are strlen or memcmp. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as feof in a multithreading environment).

The attribute pure is not implemented in GCC versions earlier than 2.96.

const

Many functions do not examine any values except their arguments, and have no effects except the return value. Basically this is just slightly more strict class than the pure attribute above, since function is not allowed to read global memory.

Note that a function that has pointer arguments and examines the data pointed to must not be declared const. Likewise, a function that calls a non-const function usually must not be const. It does not make sense for a const function to return void.

The attribute const is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows:

typedef int intfn (); extern const intfn square;

This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value.

format (archetype, string-index, first-to-check)

The format attribute specifies that a function takes printf, scanf, strftime or strfmon style arguments which should be type-checked against a format string. For example, the declaration:

extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3)));

causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format.

The parameter archetype determines how the format string is interpreted, and should be printf, scanf, strftime or strfmon. (You can also use __printf__, __scanf__, __strftime__ or __strfmon__.) The parameter string-index specifies which argument is the format string argument (starting from 1), while first-to-check is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as vprintf), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. For strftime formats, the third parameter is required to be zero.

In the example above, the format string (my_format) is the second argument of the function my_print, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3.

The format attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors. The compiler always (unless `-ffreestanding' is used) checks formats for the standard library functions printf, fprintf, sprintf, scanf, fscanf, sscanf, strftime, vprintf, vfprintf and vsprintf whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'. In C99 mode, the functions snprintf, vsnprintf, vscanf, vfscanf and vsscanf are also checked. Except in strictly conforming C standard modes, the X/Open function strfmon is also checked. See section Options Controlling C Dialect.

format_arg (string-index)

The format_arg attribute specifies that a function takes a format string for a printf, scanf, strftime or strfmon style function and modifies it (for example, to translate it into another language), so the result can be passed to a printf, scanf, strftime or strfmon style function (with the remaining arguments to the format function the same as they would have been for the unmodified string). For example, the declaration:

extern char * my_dgettext (char *my_domain, const char *my_format) __attribute__ ((format_arg (2)));

causes the compiler to check the arguments in calls to a printf, scanf, strftime or strfmon type function, whose format string argument is a call to the my_dgettext function, for consistency with the format string argument my_format. If the format_arg attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when `-Wformat-nonliteral' is used, but the calls could not be checked without the attribute.

The parameter string-index specifies which argument is the format string argument (starting from 1).

The format-arg attribute allows you to identify your own functions which modify format strings, so that GCC can check the calls to printf, scanf, strftime or strfmon type function whose operands are a call to one of your own function. The compiler always treats gettext, dgettext, and dcgettext in this manner except when strict ISO C support is requested by `-ansi' or an appropriate `-std' option, or `-ffreestanding' is used. See section Options Controlling C Dialect.

no_instrument_function

If `-finstrument-functions' is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented.

section ("section-name")

Normally, the compiler places the code it generates in the text section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The section attribute specifies that a function lives in a particular section. For example, the declaration:

extern void foobar (void) __attribute__ ((section ("bar")));

puts the function foobar in the bar section.

Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

constructor

destructor

The constructor attribute causes the function to be called automatically before execution enters main (). Similarly, the destructor attribute causes the function to be called automatically after main () has completed or exit () has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program.

These attributes are not currently implemented for Objective C.

unused

This attribute, attached to a function, means that the function is meant to be possibly unused. GCC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++.

weak

The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.

malloc

The malloc attribute is used to tell the compiler that a function may be treated as if it were the malloc function. The compiler assumes that calls to malloc result in a pointers that cannot alias anything. This will often improve optimization.

alias ("target")

The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,

void __f () { /* do something */; } void f () __attribute__ ((weak, alias ("__f")));

declares `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used.

Not all target machines support this attribute.

no_check_memory_usage

The no_check_memory_usage attribute causes GCC to omit checks of memory references when it generates code for that function. Normally if you specify `-fcheck-memory-usage' (see see section 3.18 Options for Code Generation Conventions), GCC generates calls to support routines before most memory accesses to permit support code to record usage and detect uses of uninitialized or unallocated storage. Since GCC cannot handle asm statements properly they are not allowed in such functions. If you declare a function with this attribute, GCC will not generate memory checking code for that function, permitting the use of asm statements without having to compile that function with different options. This also allows you to write support routines of your own if you wish, without getting infinite recursion if they get compiled with `-fcheck-memory-usage'.

regparm (number)

On the Intel 386, the regparm attribute causes the compiler to pass up to number integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack.

stdcall

On the Intel 386, the stdcall attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments.

The PowerPC compiler for Windows NT currently ignores the stdcall attribute.

cdecl

On the Intel 386, the cdecl attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the `-mrtd' switch.

The PowerPC compiler for Windows NT currently ignores the cdecl attribute.

longcall

On the RS/6000 and PowerPC, the longcall attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called.

long_call/short_call

This attribute allows to specify how to call a particular function on ARM. Both attributes override the `-mlong-calls' (see section 3.17.7 ARM Options) command line switch and #pragma long_calls settings. The long_call attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register. The short_call attribute always places the offset to the function from the call site into the `BL' instruction directly.

dllimport

On the PowerPC running Windows NT, the dllimport attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library. The pointer name is formed by combining __imp_ and the function name.

dllexport

On the PowerPC running Windows NT, the dllexport attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with the dllimport attribute. The pointer name is formed by combining __imp_ and the function name.

exception (except-func [, except-arg])

On the PowerPC running Windows NT, the exception attribute causes the compiler to modify the structured exception table entry it emits for the declared function. The string or identifier except-func is placed in the third entry of the structured exception table. It represents a function, which is called by the exception handling mechanism if an exception occurs. If it was specified, the string or identifier except-arg is placed in the fourth entry of the structured exception table.

function_vector

Use this option on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector.

You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly.

interrupt

Use this option on the ARM, AVR and M32R/D ports to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

Note, interrupt handlers for the H8/300, H8/300H and SH processors can be specified via the interrupt_handler attribute.

Note, on the AVR interrupts will be enabled inside the function.

Note, for the ARM you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:

void f () __attribute__ ((interrupt ("IRQ")));

Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.

interrupt_handler

Use this option on the H8/300, H8/300H and SH to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

sp_switch

Use this option on the SH to indicate an interrupt_handler function should switch to an alternate stack. It expects a string argument that names a global variable holding the address of the alternate stack.

void *alt_stack; void f () __attribute__ ((interrupt_handler, sp_switch ("alt_stack")));

trap_exit

Use this option on the SH for an interrupt_handle to return using trapa instead of rte. This attribute expects an integer argument specifying the trap number to be used.

eightbit_data

Use this option on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data.

You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly.

tiny_data

Use this option on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data.

signal

Use this option on the AVR to indicate that the specified function is an signal handler. The compiler will generate function entry and exit sequences suitable for use in an signal handler when this attribute is present. Interrupts will be disabled inside function.

naked

Use this option on the ARM or AVR ports to indicate that the specified function do not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences.

model (model-name)

Use this attribute on the M32R/D to set the addressability of an object, and the code generated for a function. The identifier model-name is one of small, medium, or large, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and are callable with the bl instruction.

Medium model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and are callable with the bl instruction.

Large model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).

You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.

Some people object to the __attribute__ feature, suggesting that ISO C's #pragma should be used instead. At the time __attribute__ was designed, there were two reasons for not doing this.

1. It is impossible to generate #pragma commands from a macro.

2. There is no telling what the same #pragma might mean in another compiler.

These two reasons applied to almost any application that might have been proposed for #pragma. It was basically a mistake to use #pragma for anything.

The ISO C99 standard includes _Pragma, which now allows pragmas to be generated from macros. In addition, a #pragma GCC namespace is now in use for GCC-specific pragmas. However, it has been found convenient to use __attribute__ to achieve a natural attachment of attributes to their corresponding declarations, whereas #pragma GCC is of use for constructs that do not naturally form part of the grammar. See section `Miscellaneous Preprocessing Directives' in The C Preprocessor.

5.27 Attribute Syntax

This section describes the syntax with which __attribute__ may be used, and the constructs to which attribute specifiers bind, for the C language. Some details may vary for C++ and Objective C. Because of infelicities in the grammar for attributes, some forms described here may not be successfully parsed in all cases.

See section 5.26 Declaring Attributes of Functions, for details of the semantics of attributes applying to functions. See section 5.33 Specifying Attributes of Variables, for details of the semantics of attributes applying to variables. See section 5.34 Specifying Attributes of Types, for details of the semantics of attributes applying to structure, union and enumerated types.

An attribute specifier is of the form __attribute__ ((attribute-list)). An attribute list is a possibly empty comma-separated sequence of attributes, where each attribute is one of the following:

• Empty. Empty attributes are ignored.

• A word (which may be an identifier such as unused, or a reserved word such as const).

• A word, followed by, in parentheses, parameters for the attribute. These parameters take one of the following forms:
  • An identifier. For example, mode attributes use this form.

  • An identifier followed by a comma and a non-empty comma-separated list of expressions. For example, format attributes use this form.

  • A possibly empty comma-separated list of expressions. For example, format_arg attributes use this form with the list being a single integer constant expression, and alias attributes use this form with the list being a single string constant.

An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.

An attribute specifier list may appear after the colon following a label, other than a case or default label. The only attribute it makes sense to use after a label is unused. This feature is intended for code generated by programs which contains labels that may be unused but which is compiled with `-Wall'. It would not normally be appropriate to use in it human-written code, though it could be useful in cases where the code that jumps to the label is contained within an #ifdef conditional.

An attribute specifier list may appear as part of a struct, union or enum specifier. It may go either immediately after the struct, union or enum keyword, or after the closing brace. It is ignored if the content of the structure, union or enumerated type is not defined in the specifier in which the attribute specifier list is used--that is, in usages such as struct __attribute__((foo)) bar with no following opening brace. Where attribute specifiers follow the closing brace, they are considered to relate to the structure, union or enumerated type defined, not to any enclosing declaration the type specifier appears in, and the type defined is not complete until after the attribute specifiers.

Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration). In future, attribute specifiers in some places may however apply to a particular declarator within a declaration instead; these cases are noted below. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.

Any list of specifiers and qualifiers at the start of a declaration may contain attribute specifiers, whether or not such a list may in that context contain storage class specifiers. (Some attributes, however, are essentially in the nature of storage class specifiers, and only make sense where storage class specifiers may be used; for example, section.) There is one necessary limitation to this syntax: the first old-style parameter declaration in a function definition cannot begin with an attribute specifier, because such an attribute applies to the function instead by syntax described below (which, however, is not yet implemented in this case). In some other cases, attribute specifiers are permitted by this grammar but not yet supported by the compiler. All attribute specifiers in this place relate to the declaration as a whole. In the obsolescent usage where a type of int is implied by the absence of type specifiers, such a list of specifiers and qualifiers may be an attribute specifier list with no other specifiers or qualifiers.

An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. At present, such attribute specifiers apply not only to the identifier before whose declarator they appear, but to all subsequent identifiers declared in that declaration, but in future they may apply only to that single identifier. For example, in __attribute__((noreturn)) void d0 (void), __attribute__((format(printf, 1, 2))) d1 (const char *, ...), d2 (void), the noreturn attribute applies to all the functions declared; the format attribute should only apply to d1, but at present applies to d2 as well (and so causes an error).

An attribute specifier list may appear immediately before the comma, = or semicolon terminating the declaration of an identifier other than a function definition. At present, such attribute specifiers apply to the declared object or function, but in future they may attach to the outermost adjacent declarator. In simple cases there is no difference, but, for example, in void (****f)(void) __attribute__((noreturn));, at present the noreturn attribute applies to f, which causes a warning since f is not a function, but in future it may apply to the function ****f. The precise semantics of what attributes in such cases will apply to are not yet specified. Where an assembler name for an object or function is specified (see section 5.37 Controlling Names Used in Assembler Code), at present the attribute must follow the asm specification; in future, attributes before the asm specification may apply to the adjacent declarator, and those after it to the declared object or function.

An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).

An attribute specifier list may appear at the start of a nested declarator. At present, there are some limitations in this usage: the attributes apply to the identifier declared, and to all subsequent identifiers declared in that declaration (if it includes a comma-separated list of declarators), rather than to a specific declarator. When attribute specifiers follow the * of a pointer declarator, they must presently follow any type qualifiers present, and cannot be mixed with them. The following describes intended future semantics which make this syntax more useful only. It will make the most sense if you are familiar with the formal specification of declarators in the ISO C standard.

Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T D1, where T contains declaration specifiers that specify a type Type (such as int) and D1 is a declarator that contains an identifier ident. The type specified for ident for derived declarators whose type does not include an attribute specifier is as in the ISO C standard.

If D1 has the form ( attribute-specifier-list D ), and the declaration T D specifies the type "derived-declarator-type-list Type" for ident, then T D1 specifies the type "derived-declarator-type-list attribute-specifier-list Type" for ident.

If D1 has the form * type-qualifier-and-attribute-specifier-list D, and the declaration T D specifies the type "derived-declarator-type-list Type" for ident, then T D1 specifies the type "derived-declarator-type-list type-qualifier-and-attribute-specifier-list Type" for ident.

For example, void (__attribute__((noreturn)) ****f)(); specifies the type "pointer to pointer to pointer to pointer to non-returning function returning void". As another example, char *__attribute__((aligned(8))) *f; specifies the type "pointer to 8-byte-aligned pointer to char". Note again that this describes intended future semantics, not current implementation.

5.28 Prototypes and Old-Style Function Definitions

GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:

/* Use prototypes unless the compiler is old-fashioned. */ #ifdef __STDC__ #define P(x) x #else #define P(x) () #endif /* Prototype function declaration. */ int isroot P((uid_t)); /* Old-style function definition. */ int isroot (x) /* ??? lossage here ??? */ uid_t x; { return x == 0; }

Suppose the type uid_t happens to be short. ISO C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted. Therefore in this example the function definition's argument is really an int, which does not match the prototype argument type of short.

This restriction of ISO C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the uid_t type is short, int, or long. Therefore, in cases like these GNU C allows a prototype to override a later old-style definition. More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion. Thus in GNU C the above example is equivalent to the following:

int isroot (uid_t); int isroot (uid_t x) { return x == 0; }

GNU C++ does not support old-style function definitions, so this extension is irrelevant.