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Revision 1.10, Thu Jul 19 06:44:12 2012 UTC (8 years, 4 months ago) by wiz
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.\" $NetBSD: __builtin_object_size.3,v 1.10 2012/07/19 06:44:12 wiz Exp $ .\" .\" Copyright (c) 2007 The NetBSD Foundation, Inc. .\" All rights reserved. .\" .\" This code is derived from software contributed to The NetBSD Foundation .\" by Christos Zoulas. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS .\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED .\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS .\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR .\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF .\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS .\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN .\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE .\" POSSIBILITY OF SUCH DAMAGE. .\" .\" .Dd July 18, 2012 .Dt __BUILTIN_OBJECT_SIZE 3 .Os .Sh NAME .Nm __builtin_object_size .Nd return the size of the given object .Sh SYNOPSIS .Ft size_t .Fn __builtin_object_size "void *ptr" "int type" .Sh DESCRIPTION The .Fn __builtin_object_size function is a .Xr gcc 1 built-in function that returns the size of the .Fa ptr object if known at compile time and the object does not have any side effects. .Sh RETURN VALUES If the size of the object is not known or it has side effects the .Fn __builtin_object_size function returns: .Bl -tag -width (size_t)\-1 -offset indent .It Dv (size_t)\-1 for .Fa type .Dv 0 and .Dv 1 . .It Dv (size_t)0 for .Fa type .Dv 2 and .Dv 3 . .El .Pp If the size of the object is known, then the .Fn __builtin_object_size function returns the maximum size of all the objects that the compiler knows that they can be pointed to by .Fa ptr when .Fa type .Dv \*[Am] 2 == 0 , and the minimum size when .Fa type .Dv \*[Am] 2 != 0 . .Sh SEE ALSO .Xr gcc 1 , .Xr __builtin_return_address 3 , .Xr attribute 3 , .Xr ssp 3 .Sh HISTORY The .Fn __builtin_object_size appeared in .Tn GCC 4.1 . .Sh CAVEATS This is a non-standard, compiler-specific extension. .Pp Note that currently the object size calculation pass is only done at -O1 or above, meaning that this function always returns \-1 when the optimizer is off. .Pp There are some discussions about always doing the object size pass, but the issue is that without the optimization pass data sizes are not going to be correct. .Pp For that reason currently code fortification (size-checked replacement functions) is disabled when optimization is off.