1.\" $NetBSD: __builtin_constant_p.3,v 1.2 2010/12/19 10:08:44 jruoho Exp $ 2.\" 3.\" Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi> 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 16.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 17.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 18.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 19.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 20.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 21.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 22.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 23.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 24.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 25.\" POSSIBILITY OF SUCH DAMAGE. 26.\" 27.Dd December 19, 2010 28.Dt __BUILTIN_CONSTANT_P 3 29.Os 30.Sh NAME 31.Nm __builtin_constant_p 32.Nd GNU extension to determine compile time constants 33.Sh SYNOPSIS 34.Ft int 35.Fn __builtin_constant_p "value" 36.Sh DESCRIPTION 37The 38.Fn __builtin_constant_p 39is a 40.Tn GNU 41extension for determining whether a value 42is known to be constant at compile time. 43The function is closely related to the concept of 44.Dq constant folding 45used by modern optimizing compilers. 46.Pp 47If the 48.Fa value 49is known to be a compile-time constant, a value 1 is returned. 50If 51.Fn __builtin_constant_p 52returns 0, the 53.Fa value 54is not a compile-time constant in the sense that 55.Xr gcc 1 56was unable to determine whether the value is constant or not. 57.Sh EXAMPLES 58A typical example of the use of 59.Fn __builtin_constant_p 60involves a situation where it may be desirable to 61fold the computation if it involves a constant, 62but a function call is needed otherwise. 63For instance, 64.Xr bswap16 3 65is defined in 66.Nx 67as: 68.Bd -literal -offset indent 69#define bswap16(x) \\ 70 (__builtin_constant_p((x)) ? \\ 71 __byte_swap_u16_constant(x) : __BYTE_SWAP_U16_VARIABLE(x)) 72.Ed 73.Sh SEE ALSO 74.Xr gcc 1 , 75.Xr __builtin_object_size 3 , 76.Xr __builtin_return_address 3 77.Sh CAVEATS 78This is a non-standard, compiler-specific extension. 79