Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi>
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.Dd December 19, 2010 .Dt __BUILTIN_CONSTANT_P 3 .Os .Sh NAME .Nm __builtin_constant_p .Nd GNU extension to determine compile time constants .Sh SYNOPSIS .Ft int .Fn __builtin_constant_p "value" .Sh DESCRIPTION The .Fn __builtin_constant_p is a .Tn GNU extension for determining whether a value is known to be constant at compile time. The function is closely related to the concept of .Dq constant folding used by modern optimizing compilers.
p If the .Fa value is known to be a compile-time constant, a value 1 is returned. If .Fn __builtin_constant_p returns 0, the .Fa value is not a compile-time constant in the sense that .Xr gcc 1 was unable to determine whether the value is constant or not. .Sh EXAMPLES A typical example of the use of .Fn __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, .Xr bswap16 3 is defined in .Nx as: d -literal -offset indent #define bswap16(x) \\ (__builtin_constant_p((x)) ? \\ __byte_swap_u16_constant(x) : __BYTE_SWAP_U16_VARIABLE(x)) .Ed .Sh SEE ALSO .Xr gcc 1 , .Xr __builtin_object_size 3 , .Xr __builtin_return_address 3 .Sh CAVEATS This is a non-standard, compiler-specific extension.