.\" Copyright (c) 1983, 1991, 1992, 1993.\"	The Regents of the University of California.  All rights reserved..\".\" 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..\" 3. All advertising materials mentioning features or use of this software.\"    must display the following acknowledgement:.\"	This product includes software developed by the University of.\"	California, Berkeley and its contributors..\" 4. Neither the name of the University nor the names of its contributors.\"    may be used to endorse or promote products derived from this software.\"    without specific prior written permission..\".\" THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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..\".\"     @(#)sigaltstack.2	8.1 (Berkeley) 6/4/93.\".Dd June 4, 1993.Dt SIGALTSTACK 2.Os BSD 4.2.Sh NAME.Nm sigaltstack.Nd set and/or get signal stack context.Sh SYNOPSIS.Fd #include <sys/types.h>.Fd #include <signal.h>.Bd -literalstruct sigaltstack {	caddr_t ss_sp;	long	ss_size;	int     ss_flags;};.Ed.Ft int.Fn sigaltstack "const struct sigaltstack *ss" "struct sigaltstack *oss".Sh DESCRIPTION.Fn Sigaltstackallows users to define an alternate stack on which signalsare to be processed.If.Fa ssis non-zero,it specifies a pointer to and the size of a.Em "signal stack"on which to deliver signals,and tells the system if the process is currently executingon that stack.When a signal's action indicates its handlershould execute on the signal stack (specified with a.Xr sigaction 2call), the system checks to seeif the process is currently executing on that stack.If the process is not currently executing on the signal stack,the system arranges a switch to the signal stack for theduration of the signal handler's execution. .PpIf.Dv SA_DISABLEis set in.Fa ss_flags ,.Fa ss_spand.Fa ss_sizeare ignored and the signal stack will be disabled.Trying to disable an active stack will cause.Nmto return -1 with.Va errnoset to.Dv EINVAL .A disabled stack will cause all signals to betaken on the regular user stack.If the stack is later re-enabled then all signals that were specifiedto be processed on an alternate stack will resume doing so..PpIf.Fa ossis non-zero, the current signal stack state is returned.The.Fa ss_flagsfield will contain the value.Dv SA_ONSTACKif the process is currently on a signal stack and.Dv SA_DISABLEif the signal stack is currently disabled..Sh NOTESThe value.Dv SIGSTKSZis defined to be the number of bytes/chars that would be used to coverthe usual case when allocating an alternate stack area.The following code fragment is typically used to allocate an alternate stack..Bd -literal -offset indentif ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)	/* error return */sigstk.ss_size = SIGSTKSZ;sigstk.ss_flags = 0;if (sigaltstack(&sigstk,0) < 0)	perror("sigaltstack");.EdAn alternative approach is provided for programs with signal handlersthat require a specific amount of stack space other than the default size.The value.Dv MINSIGSTKSZis defined to be the number of bytes/chars that is required bythe operating system to implement the alternate stack feature.In computing an alternate stack size,programs should add.Dv MINSIGSTKSZto their stack requirements to allow for the operating system overhead..PpSignal stacks are automatically adjusted for the direction of stackgrowth and alignment requirements.Signal stacks may or may not be protected by the hardware andare not ``grown'' automatically as is done for the normal stack.If the stack overflows and this space is not protectedunpredictable results may occur..Sh RETURN VALUESUpon successful completion, a value of 0 is returned.Otherwise, a value of -1 is returned and .Va errnois set to indicate the error..Sh ERRORS.Fn Sigstackwill fail and the signal stack context will remain unchangedif one of the following occurs..Bl -tag -width [ENOMEM].It Bq Er EFAULTEither.Fa ssor.Fa osspoints to memory that is not a valid part of the processaddress space..It Bq Er EINVALAn attempt was made to disable an active stack..It Bq Er ENOMEMSize of alternate stack area is less than or equal to.Dv MINSIGSTKSZ ..El.Sh SEE ALSO.Xr sigaction 2 ,.Xr setjmp 3.Sh HISTORYThe predecessor to.Nm sigaltstack ,the.Fn sigstacksystem call, appeared in.Bx 4.2 .