__builtin_constant_p(3) [netbsd man page]

__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

__BUILTIN_RETURN_ADDRESS(3)				   BSD Library Functions Manual 			       __BUILTIN_RETURN_ADDRESS(3)

NAME

     __builtin_return_address -- the return address of a function

SYNOPSIS

     void *
     __builtin_return_address(unsigned int level);

     void *
     __builtin_frame_address(unsigned int level);

DESCRIPTION

     The __builtin_return_address() is a GNU extension for obtaining the return address of the current function or one of the callers of the cur-
     rent function.

     The parameter level specifies the number of frames that should be scanned up in the call stack.  A value 0 returns the address of the current
     function, a value 1 requests the address of the caller of the current function, a value 2 asks for the address of the caller's caller, and so
     forth.  If the top of the call stack has been reached, the function will return 0.  Note also that on some architectures it is only possible
     to determine the address of the current function.	In such cases a value 0 is returned.  Thus, it is usually safe to only use the value 0 for
     level.

     The __builtin_frame_address() behaves similarly, but returns the address of the function frame rather than the return address of the func-
     tion.

SEE ALSO

     gcc(1), __builtin_object_size(3)

CAVEATS

     These are non-standard, compiler-specific extensions.

BSD
								 December 19, 2010							       BSD