NAME

__builtin_constant_p — GNU extension to determine compile time constants

SYNOPSIS

int
__builtin_constant_p(value);

DESCRIPTION

The __builtin_constant_p() is a GNU extension for determining whether a value is known to be constant at compile time. The function is closely related to the concept of “constant folding” used by modern optimizing compilers.

If the value is known to be a compile-time constant, a value 1 is returned. If __builtin_constant_p() returns 0, the value is not a compile-time constant in the sense that gcc(1) was unable to determine whether the value is constant or not.

EXAMPLES

A typical example of the use of __builtin_constant_p() involves a situation where it may be desirable to fold the computation if it involves a constant, but a function call is needed otherwise. For instance, bswap16(3) is defined in NetBSD as:

#define bswap16(x) \ 
        (__builtin_constant_p((x)) ? \ 
         __byte_swap_u16_constant(x) : __BYTE_SWAP_U16_VARIABLE(x))

SEE ALSO

gcc(1), __builtin_object_size(3), __builtin_return_address(3)

CAVEATS

This is a non-standard, compiler-specific extension.