gcc_extensions [Doku Wiki]

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
===== Overview =====

Most useful GCC extensions to C language; avoid by using ''-pedantic''.

===== Extensions =====

==== 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.

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

==== Local labels ====

GCC allows you to declare local labels in any nested block scope. A local label is just like an ordinary label, but you can only reference it (with a goto statement, or by taking its address) within the block in which it is declared. 

<code>
__label__ label;
</code>

==== 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.

<code>
void *ptr;
/* … */
ptr = &&foo;
...
goto *ptr;
</code>

==== Nested functions ====

<code>
foo (double a, double b)
{
  double square (double z) { return z * z; }

  return square (a) + square (b);
}
</code>

See "trampolines."

==== Nonlocal gotos ====

Use ''%%__builtin_setjmp%%'' and ''%%__builtin_longjmp%%''.

==== Constructing function calls ====

==== typeof ====

<code>
#define max(a,b) \
  ({ typeof (a) _a = (a); \
      typeof (b) _b = (b); \
    _a > _b ? _a : _b; })
</code>

==== Conditionals ====

Equivalent:

<code>
x ? x : y
x ? : y
</code>

==== Non-constant initializers ====

==== Designated initializers ====

==== Case ranges ====

<code>
case low ... high:
</code>

Don't forget spaces.