In addition to C11, several GNU extensions are implemented and more may be implemented in the future.



__typeof__(E) is a type specifier for the type of expression E. Arrays and function designator expressions do not decay into pointers, just like when used with sizeof.

#__asm__ labels

A declarator can be followed by __asm__("somelabel") to specify the assembler name of the object or function. This name is taken literally, so the resulting symbol will not be mangled according to the target's usual rules. The name may contain characters not allowed in regular identifiers.

#Built-in functions and types
  • **__builtin_alloca**: Allocate memory on the stack.
  • **__builtin_constant_p**: Test whether the argument is a constant expression.
  • **__builtin_inff**: float positive infinity value.
  • **__builtin_nanf**: float quiet NaN value.
  • __builtin_offsetof: Return the offset of a member in a struct or union.
  • **__builtin_types_compatible_p**: Test whether the two types are compatible.
  • **__builtin_va_arg**: Built-in suitable for implementing the va_arg macro.
  • **__builtin_va_copy**: Built-in suitible for implementing the va_copy macro.
  • **__builtin_va_end**: Built-in suitible for implementing the va_end macro.
  • **__builtin_va_list**: Built-in suitable for implementing the va_list type.
  • **__builtin_va_start**: Built-in suitable for implementing the va_start macro.
#Enumerator values outside the range of int

ISO C requires that enumerator values be in the range of int. GNU C allows any integer value, at the cost of enumerator constants possibly having different types inside and outside the enum specifier.

In the following example, A has type unsigned inside the enum specifier, and long outside, so both of the assertions fail.

enum E {
    A = 0x80000000,
    B = sizeof(A),
    C = A < -1,
    D = -1,
_Static_assert(B == sizeof(A), "sizeof(A) changed");
_Static_assert(C == A < -1, "signedness of typeof(A) changed");

As a compromise, we allow enumerator values larger than INT_MAX and less than or equal to UINT_MAX, but only if no other enumerators have a negative value. This way, enumerator constants have a fixed type.

#Statement expressions

In GNU C, you may use a compound statement as expressions when they are enclosed in parentheses. The last statement in the compound statement must be an expression statement, whose value is used as the result of the statement expression.

#Empty declarations

GNU C allows empty top-level declarations (i.e. ;).

#Empty initializer list

GNU C allows empty initializer lists when initializing an object, for example

struct {
	int a, b;
} s = {};

This behaves exactly the same as {0}.

#Missing operand in conditional expression

GNU C allows you to omit the second operand in conditional expressions, in which case the first operand is used. So E1 ? : E2 behaves the same as E1 ? E1 : E2, except that E1 is evaluated only once.

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commit 4a4d1233b0d284023ed0d0001c15b41641880eba
Author: Michael Forney <>
Date:   2020-09-12T02:47:48-07:00

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