1 GNU Extensions to the C Language

GNU C provides several language features not found in ANSI standard C. (The ‘-pedantic’ option directs GNU CC 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 GNU CC.

1.1 Statements and Declarations within 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 Referring to a Type with typeof) or type naming (see section Naming an Expression’s Type).

1.2 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;                                          \
})

1.3 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(1), 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 can 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.

1.4 Nested Functions

A nested function is a function defined inside another function. 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) …
}

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.

GNU CC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available from ‘maya.idiap.ch’ in the file ‘pub/tmb/usenix88-lexic.ps.Z’.

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 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];
    }
  …
}

1.5 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.

1.6 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 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 ANSI C programs, write __typeof__ instead of typeof. See section 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.

1.7 Generalized Lvalues

Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them.

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.

1.8 Conditional Expressions 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.

1.9 Double-Word Integers

GNU C supports data types for integers that are twice as long as long int. Simply write long long int for a signed integer, or unsigned long long int for an unsigned 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 GNU CC.

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.

1.10 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 standard C, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

1.11 Arrays of Variable Length

Variable-length automatic arrays are allowed in GNU C. 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.

1.12 Macros with Variable Numbers of Arguments

In GNU C, a macro can accept a variable number of arguments, much as a function can. The syntax for defining the macro looks much like that used for a function. Here is an example:

#define eprintf(format, args...)  \
 fprintf (stderr, format, ## args)

Here args is a rest argument: it takes in zero or more arguments, as many as the call contains. All of them plus the commas between them form the value of args, which is substituted into the macro body where args is used. Thus, we have these expansions:

eprintf ("%s:%d: ", input_file_name, line_number)
→
fprintf (stderr, "%s:%d: ", input_file_name, line_number)

Note that the comma after the string constant comes from the definition of eprintf, whereas the last comma comes from the value of args.

The reason for using ‘##’ is to handle the case when args matches no arguments at all. In this case, args has an empty value. In this case, the second comma in the definition becomes an embarrassment: if it got through to the expansion of the macro, we would get something like this:

fprintf (stderr, "success!\n", )

which is invalid C syntax. ‘##’ gets rid of the comma, so we get the following instead:

fprintf (stderr, "success!\n")

This is a special feature of the GNU C preprocessor: ‘##’ adjacent to a rest argument discards the token on the other side of the ‘##’, if the rest argument value is empty.

1.13 Non-Lvalue Arrays May Have Subscripts

Subscripting is allowed on arrays that are not lvalues, even though the unary ‘&’ operator is not. For example, this is valid in GNU C though not valid in other C dialects:

struct foo {int a[4];};

struct foo f();

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

1.14 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.

1.15 Non-Constant Initializers

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 };
  …
}

1.16 Constructor Expressions

GNU C supports constructor expressions. A constructor 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.

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 constructor:

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 constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here:

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

Array constructors whose elements are not simple constants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: 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 constructor:

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

Constructor expressions for scalar types and union types are is also allowed, but then the constructor expression is equivalent to a cast.

1.17 Labeled Elements in Initializers

Standard C 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 GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to.

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.

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 };

You can also use an element label 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 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 label 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 };

1.18 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:

1.19 Cast to a Union Type

A cast to union type is like any other cast, except that the type specified is a union type. You can specify the type either with union tag or with a typedef name.

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);

1.20 Declaring Attributes of Functions

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

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

extern void volatile fatal ();

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

The volatile keyword tells the compiler to assume that fatal cannot return. This makes slightly better code, but more importantly it helps avoid spurious warnings of uninitialized variables.

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

Many functions do not examine any values except their arguments, and have no effects except the return value. 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 const. For example,

extern int const square ();

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

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.

We recommend placing the keyword const after the function’s return type. It makes no difference in the example above, but when the return type is a pointer, it is the only way to make the function itself const. For example,

const char *mincp (int);

says that mincp returns const char *—a pointer to a const object. To declare mincp const, you must write this:

char * const mincp (int);

Some people object to this feature, suggesting that ANSI C’s #pragma should be used instead. There are two reasons for not doing this.

1. It is impossible to generate #pragma commands from a macro.
2. The #pragma command is just as likely as these keywords to mean something else in another compiler.

These two reasons apply to almost any application that might be proposed for #pragma. It is basically a mistake to use #pragma for anything.

The keyword __attribute__ allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. One attribute, format, is currently defined for functions. Others are implemented for variables and structure fields (see section Declaring Attributes of Functions).

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

The format attribute specifies that a function takes printf or scanf 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 either printf or scanf. 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.

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 GNU CC can check the calls to these functions for errors. The compiler always checks formats for the ANSI library functions printf, fprintf, sprintf, scanf, fscanf, sscanf, vprintf, vfprintf and vsprintf whenever such warnings are requested (using ‘-Wformat’), so there is no need to modify the header file ‘stdio.h’.

1.21 Prototypes and Old-Style Function Definitions

GNU C extends ANSI 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.  */
#if __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. ANSI 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 ANSI 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;
}

1.22 Dollar Signs in Identifier Names

In GNU C, you may use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers.

On some machines, dollar signs are allowed in identifiers if you specify ‘-traditional’. On a few systems they are allowed by default, even if you do not use ‘-traditional’. But they are never allowed if you specify ‘-ansi’.

There are certain ANSI C programs (obscure, to be sure) that would compile incorrectly if dollar signs were permitted in identifiers. For example:

#define foo(a) #a
#define lose(b) foo (b)
#define test$
lose (test)

1.23 The Character <ESC> in Constants

You can use the sequence ‘\e’ in a string or character constant to stand for the ASCII character <ESC>.

1.24 Inquiring on Alignment of Types or Variables

The keyword __alignof__ allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like sizeof.

For example, if the target machine requires a double value to be aligned on an 8-byte boundary, then __alignof__ (double) is 8. This is true on many RISC machines. On more traditional machine designs, __alignof__ (double) is 4 or even 2.

Some machines never actually require alignment; they allow reference to any data type even at an odd addresses. For these machines, __alignof__ reports the recommended alignment of a type.

When the operand of __alignof__ is an lvalue rather than a type, the value is the largest alignment that the lvalue is known to have. It may have this alignment as a result of its data type, or because it is part of a structure and inherits alignment from that structure. For example, after this declaration:

struct foo { int x; char y; } foo1;

the value of __alignof__ (foo1.y) is probably 2 or 4, the same as __alignof__ (int), even though the data type of foo1.y does not itself demand any alignment.

A related feature which lets you specify the alignment of an object is __attribute__ ((aligned (alignment))); see the following section.

1.25 Specifying Attributes of Variables

The keyword __attribute__ allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Four attributes are currently defined: aligned, format, mode and packed. format is used for functions, and thus not documented here; see Declaring Attributes of Functions.

aligned (alignment)

This attribute specifies the alignment of the variable or structure field, measured in bytes. For example, the declaration:

int x __attribute__ ((aligned (16))) = 0;

causes the compiler to allocate the global variable x on a 16-byte boundary. On a 68040, this could be used in conjunction with an asm expression to access the move16 instruction which requires 16-byte aligned operands.

You can also specify the alignment of structure fields. For example, to create a double-word aligned int pair, you could write:

struct foo { int x[2] __attribute__ ((aligned (8))); };

This is an alternative to creating a union with a double member that forces the union to be double-word aligned.

It is not possible to specify the alignment of functions; the alignment of functions is determined by the machine’s requirements and cannot be changed. You cannot specify alignment for a typedef name because such a name is just an alias, not a distinct type.

The linker of your operating system imposes a maximum alignment. If the linker aligns each object file on a four byte boundary, then it is beyond the compiler’s power to cause anything to be aligned to a larger boundary than that. For example, if the linker happens to put this object file at address 136 (eight more than a multiple of 64), then the compiler cannot guarantee an alignment of more than 8 just by aligning variables in the object file.

mode (mode)

This attribute specifies the data type for the declaration—whichever type corresponds to the mode mode. This in effect lets you request an integer or floating point type according to its width.

packed

The packed attribute specifies that a variable or structure field should have the smallest possible alignment—one byte for a variable, and one bit for a field, unless you specify a larger value with the aligned attribute.

1.26 An Inline Function is As Fast As a Macro

By declaring a function inline, you can direct GNU CC to integrate that function’s code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function’s code needs to be included. Inlining of functions is an optimization and it really “works” only in optimizing compilation. If you don’t use ‘-O’, no function is really inline.

To declare a function inline, use the inline keyword in its declaration, like this:

inline int
inc (int *a)
{
  (*a)++;
}

(If you are writing a header file to be included in ANSI C programs, write __inline__ instead of inline. See section Alternate Keywords.)

You can also make all “simple enough” functions inline with the option ‘-finline-functions’. Note that certain usages in a function definition can make it unsuitable for inline substitution.

When a function is both inline and static, if all calls to the function are integrated into the caller, and the function’s address is never used, then the function’s own assembler code is never referenced. In this case, GNU CC does not actually output assembler code for the function, unless you specify the option ‘-fkeep-inline-functions’. Some calls cannot be integrated for various reasons (in particular, calls that precede the function’s definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can’t be inlined.

When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion.

If you specify both inline and extern in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it.

This combination of inline and extern has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking inline and extern) in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library.

GNU C does not inline any functions when not optimizing. It is not clear whether it is better to inline or not, in this case, but we found that a correct implementation when not optimizing was difficult. So we did the easy thing, and turned it off.

1.27 Assembler Instructions with C Expression Operands

In an assembler instruction using asm, you can now specify the operands of the instruction using C expressions. This means no more guessing which registers or memory locations will contain the data you want to use.

You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.

For example, here is how to use the 68881’s fsinx instruction:

asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here angle is the C expression for the input operand while result is that of the output operand. Each has ‘"f"’ as its operand constraint, saying that a floating point register is required. The ‘=’ in ‘=f’ indicates that the operand is an output; all output operands’ constraints must use ‘=’. The constraints use the same language used in the machine description (see Operand Constraints in Using and Porting GCC).

Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand, and another separates the last output operand from the first input, if any. Commas separate output operands and separate inputs. The total number of operands is limited to ten or to the maximum number of operands in any instruction pattern in the machine description, whichever is greater.

If there are no output operands, and there are input operands, then there must be two consecutive colons surrounding the place where the output operands would go.

Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means, or whether it is valid assembler input. The extended asm feature is most often used for machine instructions that the compiler itself does not know exist.

The output operands must be write-only; GNU CC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm does not support input-output or read-write operands. For this reason, the constraint character ‘+’, which indicates such an operand, may not be used.

When the assembler instruction has a read-write operand, or an operand in which only some of the bits are to be changed, you must logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) ‘combine’ instruction with bar as its read-only source operand and foo as its read-write destination:

asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint ‘"0"’ for operand 1 says that it must occupy the same location as operand 0. A digit in constraint is allowed only in an input operand, and it must refer to an output operand.

Only a digit in the constraint can guarantee that one operand will be in the same place as another. The mere fact that foo is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code. The following would not work:

asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GNU CC knows no reason not to do so. For example, the compiler might find a copy of the value of foo in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to foo’s own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GNU CC can’t tell that.

Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the Vax:

asm volatile ("movc3 %0,%1,%2"
              : /* no outputs */
              : "g" (from), "g" (to), "g" (count)
              : "r0", "r1", "r2", "r3", "r4", "r5");

If you refer to a particular hardware register from the assembler code, then you will probably have to list the register after the third colon to tell the compiler that the register’s value is modified. In many assemblers, the register names begin with ‘%’; to produce one ‘%’ in the assembler code, you must write ‘%%’ in the input.

If your assembler instruction can alter the condition code register, add ‘cc’ to the list of clobbered registers. GNU CC on some machines represents the condition codes as a specific hardware register; ‘cc’ serves to name this register. On other machines, the condition code is handled differently, and specifying ‘cc’ has no effect. But it is valid no matter what the machine.

If your assembler instruction modifies memory in an unpredicable fashion, add ‘memory’ to the list of clobbered registers. This will cause GNU CC to not keep memory values cached in registers across the assembler instruction.

You can put multiple assembler instructions together in a single asm template, separated either with newlines (written as ‘\n’) or with semicolons if the assembler allows such semicolons. The GNU assembler allows semicolons and all Unix assemblers seem to do so. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands’ addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes that the subroutine _foo accepts arguments in registers 9 and 10:

asm ("movl %0,r9;movl %1,r10;call _foo"
     : /* no outputs */
     : "g" (from), "g" (to)
     : "r9", "r10");

Unless an output operand has the ‘&’ constraint modifier, GNU CC may allocate it in the same register as an unrelated input operand, on the assumption that the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use ‘&’ for each output operand that may not overlap an input. See Constraint Modifier Characters in Using and Porting GCC.

If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the asm construct, as follows:

asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:"
     : "g" (result)
     : "g" (input));

This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.

Usually the most convenient way to use these asm instructions is to encapsulate them in macros that look like functions. For example,

#define sin(x)       \
({ double __value, __arg = (x);   \
   asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
   __value; })

Here the variable __arg is used to make sure that the instruction operates on a proper double value, and to accept only those arguments x which can convert automatically to a double.

Another way to make sure the instruction operates on the correct data type is to use a cast in the asm. This is different from using a variable __arg in that it converts more different types. For example, if the desired type were int, casting the argument to int would accept a pointer with no complaint, while assigning the argument to an int variable named __arg would warn about using a pointer unless the caller explicitly casts it.

If an asm has output operands, GNU CC assumes for optimization purposes that the instruction has no side effects except to change the output operands. This does not mean that instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren’t used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register.

You can prevent an asm instruction from being deleted, moved significantly, or combined, by writing the keyword volatile after the asm. For example:

#define set_priority(x)  \
asm volatile ("set_priority %0": /* no outputs */ : "g" (x))

An instruction without output operands will not be deleted or moved significantly, regardless, unless it is unreachable.

Note that even a volatile asm instruction can be moved in ways that appear insignificant to the compiler, such as across jump instructions. You can’t expect a sequence of volatile asm instructions to remain perfectly consecutive. If you want consecutive output, use a single asm.

It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following “store” instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn’t arise for ordinary “test” and “compare” instructions because they don’t have any output operands.

If you are writing a header file that should be includable in ANSI C programs, write __asm__ instead of asm. See section Alternate Keywords.

1.28 Controlling Names Used in Assembler Code

You can specify the name to be used in the assembler code for a C function or variable by writing the asm (or __asm__) keyword after the declarator as follows:

int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable foo in the assembler code should be ‘myfoo’ rather than the usual ‘_foo’.

On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.

You cannot use asm in this way in a function definition; but you can get the same effect by writing a declaration for the function before its definition and putting asm there, like this:

extern func () asm ("FUNC");

func (x, y)
     int x, y;
…

It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GNU CC does not as yet have the ability to store static variables in registers. Perhaps that will be added.

1.29 Variables in Specified Registers

GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.

• Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters which have a couple of global variables that are accessed very often.
• Local register variables in specific registers do not reserve the registers. The compiler’s data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses.

  These local variables are sometimes convenient for use with the extended asm feature (see section Assembler Instructions with C Expression Operands), if you want to write one output of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraints specified for that operand in the asm.)

1.29.1 Defining Global Register Variables

You can define a global register variable in GNU C like this:

register int *foo asm ("a5");

Here a5 is the name of the register which should be used. Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it.

Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type. The register a5 would be a good choice on a 68000 for a variable of pointer type. On machines with register windows, be sure to choose a “global” register that is not affected magically by the function call mechanism.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register %a5.

Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.

Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.

It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).

It is not safe for one function that uses a global register variable to call another such function foo by way of a third function lose that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn’t declared). This is because lose might save the register and put some other value there. For example, you can’t expect a global register variable to be available in the comparison-function that you pass to qsort, since qsort might have put something else in that register. (If you are prepared to recompile qsort with the same global register variable, you can solve this problem.)

If you want to recompile qsort or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option ‘-ffixed-reg’. You need not actually add a global register declaration to their source code.

A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.

On most machines, longjmp will restore to each global register variable the value it had at the time of the setjmp. On some machines, however, longjmp will not change the value of global register variables. To be portable, the function that called setjmp should make other arrangements to save the values of the global register variables, and to restore them in a longjmp. This way, the same thing will happen regardless of what longjmp does.

All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.

Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.

On the Sparc, there are reports that g3 … g7 are suitable registers, but certain library functions, such as getwd, as well as the subroutines for division and remainder, modify g3 and g4. g1 and g2 are local temporaries.

On the 68000, a2 … a5 should be suitable, as should d2 … d7. Of course, it will not do to use more than a few of those.

1.29.2 Specifying Registers for Local Variables

You can define a local register variable with a specified register like this:

register int *foo asm ("a5");

Here a5 is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see section Assembler Instructions with C Expression Operands). Both of these things generally require that you conditionalize your program according to cpu type.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register %a5.

Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.

Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable’s value is not live. However, these registers are made unavailable for use in the reload pass. I would not be surprised if excessive use of this feature leaves the compiler too few available registers to compile certain functions.

1.30 Alternate Keywords

The option ‘-traditional’ disables certain keywords; ‘-ansi’ disables certain others. This causes trouble when you want to use GNU C extensions, or ANSI C features, in a general-purpose header file that should be usable by all programs, including ANSI C programs and traditional ones. The keywords asm, typeof and inline cannot be used since they won’t work in a program compiled with ‘-ansi’, while the keywords const, volatile, signed, typeof and inline won’t work in a program compiled with ‘-traditional’.

The way to solve these problems is to put ‘__’ at the beginning and end of each problematical keyword. For example, use __asm__ instead of asm, __const__ instead of const, and __inline__ instead of inline.

Other C compilers won’t accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:

#ifndef __GNUC__
#define __asm__ asm
#endif

‘-pedantic’ causes warnings for many GNU C extensions. You can prevent such warnings within one expression by writing __extension__ before the expression. __extension__ has no effect aside from this.

1.31 Incomplete enum Types

You can define an enum tag without specifying its possible values. This results in an incomplete type, much like what you get if you write struct foo without describing the elements. A later declaration which does specify the possible values completes the type.

You can’t allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.

This extension may not be very useful, but it makes the handling of enum more consistent with the way struct and union are handled.

Footnotes

(1)

The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.

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