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__builtin_constant_p(3) [netbsd man page]

__BUILTIN_CONSTANT_P(3) 				   BSD Library Functions Manual 				   __BUILTIN_CONSTANT_P(3)

__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

Check Out this Related Man Page

__BUILTIN_OBJECT_SIZE(3)				   BSD Library Functions Manual 				  __BUILTIN_OBJECT_SIZE(3)

__builtin_object_size -- return the size of the given object SYNOPSIS
size_t __builtin_object_size(void *ptr, int type); DESCRIPTION
The __builtin_object_size() function is a gcc(1) built-in function that returns the size of the ptr object if known at compile time and the object does not have any side effects. RETURN VALUES
If the size of the object is not known or it has side effects the __builtin_object_size() function returns: (size_t)-1 for type 0 and 1. (size_t)0 for type 2 and 3. If the size of the object is known, then the __builtin_object_size() function returns the maximum size of all the objects that the compiler knows that they can be pointed to by ptr when type & 2 == 0, and the minimum size when type & 2 != 0. SEE ALSO
gcc(1), __builtin_return_address(3), attribute(3), ssp(3) HISTORY
The __builtin_object_size() appeared in GCC 4.1. CAVEATS
This is a non-standard, compiler-specific extension. BSD
December 19, 2010 BSD

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