__BUILTIN_CONSTANT_P(3) BSD Library Functions Manual __BUILTIN_CONSTANT_P(3)
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.
BSD
December 19, 2010 BSD