Go to the first, previous, next, last section, table of contents.

# Extensions to the C Language Family

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.

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++.

## 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 Referring to a Type with `typeof`) or type naming (see section Naming an Expression's Type).

## 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;                                          \
    })

## 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 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.

## 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.

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 directory ``pub/tmb'`, file ``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];
        }
      ...
    }

## 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).

`__builtin_apply_args ()`
  This built-in function returns a pointer of type `void *` 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.
`__builtin_apply (function, arguments, size)`
  This built-in function invokes function (type `void (*)()`) with a copy of the parameters described by arguments (type `void *`) and size (type `int`). 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 of type `void *` 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.
`__builtin_return (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`.

## 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.

## 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`.

## 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.

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`.

## 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.

## 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. 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 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.

## Complex Numbers

GNU C supports complex data types. You can declare both complex integer types and complex floating types, using the keyword `__complex__`.

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.

To extract the real part of a complex-valued expression exp, write `__real__ exp`. Likewise, use `__imag__` to extract the imaginary part.

The operator ``~'` performs complex conjugation when used on a value with a complex type.

GNU CC 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 GNU CC 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.

## 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`.

## 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.

## 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 this expansion:

    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: ``##'` before a rest argument that is empty discards the preceding sequence of non-whitespace characters from the macro definition. (If another macro argument precedes, none of it is discarded.)

It might be better to discard the last preprocessor token instead of the last preceding sequence of non-whitespace characters; in fact, we may someday change this feature to do so. We advise you to write the macro definition so that the preceding sequence of non-whitespace characters is just a single token, so that the meaning will not change if we change the definition of this feature.

## 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];
    }

## 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.

## Non-Constant Initializers

As in standard C++, 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 };
      ...
    }

## 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.

## 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. This extension is not implemented in GNU C++.

To specify an array index, write ``[index]'` or ``[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.

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

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

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 is ``.fieldname ='`., as shown here:

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

You can also use an element label (with either the colon syntax or the period-equal 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 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 };

## 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:

## 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 Constructor Expressions.)

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);

## 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. Eight attributes, `noreturn`, `const`, `format`, `section`, `constructor`, `destructor`, `unused` and `weak` are currently defined for functions. Other attributes, including `section` are supported for variables declarations (see section Specifying Attributes of Variables) and for types (see section 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`.

`noreturn`
  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 `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 GNU C 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;

`const`
  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 with the attribute `const`. For example,

      int square (int) __attribute__ ((const));

  says that the hypothetical function `square` is safe to call fewer times than the program says. The attribute `const` is not implemented in GNU C 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. 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`.
`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'`.
`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. GNU CC will not produce a warning for this function.
`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.
`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.
`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.
`cdecl`
  On the Intel 386, the `cdecl` 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. This is useful to override the effects of the ``-mrtd'` switch.

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 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. There is no telling what the same `#pragma` might mean 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.

## 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;
    }

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

## C++ Style Comments

In GNU C, you may use C++ style comments, which start with ``//'` and continue until the end of the line. Many other C implementations allow such comments, and they are likely to be in a future C standard. However, C++ style comments are not recognized if you specify ``-ansi'` or ``-traditional'`, since they are incompatible with traditional constructs like `dividend//*comment*/divisor`.

## 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)

## The Character `ESC` in Constants

You can use the sequence ``\e'` in a string or character constant to stand for the ASCII character `ESC`.

## 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.

## 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. Eight attributes are currently defined for variables: `aligned`, `mode`, `nocommon`, `packed`, `section`, `transparent_union`, `unused`, and `weak`. Other attributes are available for functions (see section Declaring Attributes of Functions) and for types (see section 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 `__aligned__` instead of `aligned`.

`aligned (alignment)`
  This attribute specifies a minimum alignment for 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. As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write:

      short array[3] __attribute__ ((aligned));

  Whenever you leave out the alignment factor in an `aligned` attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way. The `aligned` attribute can only increase the alignment; but you can decrease it by specifying `packed` as well. See below. Note that the effectiveness of `aligned` attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying `aligned(16)` in an `__attribute__` will still only provide you with 8 byte alignment. See your linker documentation for further information.
`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. You may also specify a mode of ``byte'` or ``__byte__'` to indicate the mode corresponding to a one-byte integer, ``word'` or ``__word__'` for the mode of a one-word integer, and ``pointer'` or ``__pointer__'` for the mode used to represent pointers.
`nocommon`
  This attribute specifies requests GNU CC not to place a variable "common" but instead to allocate space for it directly. If you specify the ``-fno-common'` flag, GNU CC will do this for all variables. Specifying the `nocommon` attribute for a variable provides an initialization of zeros. A variable may only be initialized in one source file.
`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. Here is a structure in which the field `x` is packed, so that it immediately follows `a`:

      struct foo
      {
        char a;
        int x[2] __attribute__ ((packed));
      };

`section ("section-name")`
  Normally, the compiler places the objects it generates in sections like `data` and `bss`. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The `section` attribute specifies that a variable (or function) lives in a particular section. For example, this small program uses several specific section names:

      struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
      struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
      char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
      int init_data_copy __attribute__ ((section ("INITDATACOPY"))) = 0;

      main()
      {
        /* Initialize stack pointer */
        init_sp (stack + sizeof (stack));

        /* Initialize initialized data */
        memcpy (&init_data_copy, &data, &edata - &data);

        /* Turn on the serial ports */
        init_duart (&a);
        init_duart (&b);
      }

  Use the `section` attribute with an initialized definition of a global variable, as shown in the example. GNU CC issues a warning and otherwise ignores the `section` attribute in uninitialized variable declarations. You may only use the `section` attribute with a fully initialized global definition because of the way linkers work. The linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the `common` (or `bss`) section and can be multiply "defined". You can force a variable to be initialized with the ``-fno-common'` flag or the `nocommon` attribute. 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.
`transparent_union`
  This attribute, attached to a function argument variable which is a union, means to pass the argument in the same way that the first union member would be passed. You can also use this attribute on a `typedef` for a union data type; then it applies to all function arguments with that type.
`unused`
  This attribute, attached to a variable, means that the variable is meant to be possibly unused. GNU CC will not produce a warning for this variable.
`weak`
  The `weak` attribute is described in See section Declaring Attributes of Functions.

To specify multiple attributes, separate them by commas within the double parentheses: for example, ``__attribute__ ((aligned (16), packed))'`.

## Specifying Attributes of Types

The keyword `__attribute__` allows you to specify special attributes of `struct` and `union` types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Three attributes are currently defined for types: `aligned`, `packed`, and `transparent_union`. Other attributes are defined for functions (see section Declaring Attributes of Functions) and for variables (see section Specifying Attributes of Variables).

You may also specify any one of these attributes with ``__'` preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use `__aligned__` instead of `aligned`.

You may specify the `aligned` and `transparent_union` attributes either in a `typedef` declaration or just past the closing curly brace of a complete enum, struct or union type definition and the `packed` attribute only past the closing brace of a definition.

`aligned (alignment)`
  This attribute specifies a minimum alignment (in bytes) for variables of the specified type. For example, the declarations:

      struct S { short f[3]; } __attribute__ ((aligned (8));
      typedef int more_aligned_int __attribute__ ((aligned (8));

  force the compiler to insure (as fas as it can) that each variable whose type is `struct S` or `more_aligned_int` will be allocated and aligned at least on a 8-byte boundary. On a Sparc, having all variables of type `struct S` aligned to 8-byte boundaries allows the compiler to use the `ldd` and `std` (doubleword load and store) instructions when copying one variable of type `struct S` to another, thus improving run-time efficiency. Note that the alignment of any given `struct` or `union` type is required by the ANSI C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the `struct` or `union` in question. This means that you can effectively adjust the alignment of a `struct` or `union` type by attaching an `aligned` attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire `struct` or `union` type. As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given `struct` or `union` type. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. For example, you could write:

      struct S { short f[3]; } __attribute__ ((aligned));

  Whenever you leave out the alignment factor in an `aligned` attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables which have types that you have aligned this way. In the example above, if the size of each `short` is 2 bytes, then the size of the entire `struct S` type is 6 bytes. The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire `struct S` type to 8 bytes. Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types. The `aligned` attribute can only increase the alignment; but you can decrease it by specifying `packed` as well. See below. Note that the effectiveness of `aligned` attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying `aligned(16)` in an `__attribute__` will still only provide you with 8 byte alignment. See your linker documentation for further information.
`packed`
  This attribute, attached to an `enum`, `struct`, or `union` type definition, specified that the minimum required memory be used to represent the type. Specifying this attribute for `struct` and `union` types is equivalent to specifying the `packed` attribute on each of the structure or union members. Specifying the ``-fshort-enums'` flag on the line is equivalent to specifying the `packed` attribute on all `enum` definitions. You may only specify this attribute after a closing curly brace on an `enum` definition, not in a `typedef` declaration.
`transparent_union`
  This attribute, attached to a `union` type definition, indicates that any variable having that union type should, if passed to a function, be passed in the same way that the first union member would be passed. For example:

      union foo
      {
        char a;
        int x[2];
      } __attribute__ ((transparent_union));

To specify multiple attributes, separate them by commas within the double parentheses: for example, ``__attribute__ ((aligned (16), packed))'`.

## 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. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. 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.

Note that in C and Objective C, unlike C++, the `inline` keyword does not affect the linkage of the function.

GNU CC automatically inlines member functions defined within the class body of C++ programs even if they are not explicitly declared `inline`. (You can override this with ``-fno-default-inline'`; see section Options Controlling C++ Dialect.)

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.

## 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 section Operand Constraints).

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. If the output expression cannot be directly addressed (for example, it is a bit field), your constraint must allow a register. In that case, GNU CC will use the register as the output of the `asm`, and then store that register into the output.

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 unpredictable 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 section Constraint Modifier Characters.

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.

Speaking of labels, jumps from one `asm` to another are not supported. The compiler's optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize.

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.

## 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.

## 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`.)

### 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.

### 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.

## 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.

## 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.

This extension is not supported by GNU C++.

## Function Names as Strings

GNU CC predefines two string variables to be the name of the current function. The variable `__FUNCTION__` is the name of the function as it appears in the source. The variable `__PRETTY_FUNCTION__` is the name of the function pretty printed in a language specific fashion.

These names are always the same in a C function, but in a C++ function they may be different. For example, this program:

    extern "C" {
    extern int printf (char *, ...);
    }

    class a {
     public:
      sub (int i)
        {
          printf ("__FUNCTION__ = %s\n", __FUNCTION__);
          printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
        }
    };

    int
    main (void)
    {
      a ax;
      ax.sub (0);
      return 0;
    }

gives this output:

    __FUNCTION__ = sub
    __PRETTY_FUNCTION__ = int  a::sub (int)

These names are not macros: they are predefined string variables. For example, ``#ifdef __FUNCTION__'` does not have any special meaning inside a function, since the preprocessor does not do anything special with the identifier `__FUNCTION__`.

Go to the first, previous, next, last section, table of contents.