netbsd man page for __builtin_constant_p

__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