1.\" $OpenBSD: sigaltstack.2,v 1.14 2009/07/09 10:14:41 eric Exp $ 2.\" $NetBSD: sigaltstack.2,v 1.3 1995/02/27 10:41:52 cgd Exp $ 3.\" 4.\" Copyright (c) 1983, 1991, 1992, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)sigaltstack.2 8.1 (Berkeley) 6/4/93 32.\" 33.Dd $Mdocdate: July 9 2009 $ 34.Dt SIGALTSTACK 2 35.Os 36.Sh NAME 37.Nm sigaltstack 38.Nd set and/or get signal stack context 39.Sh SYNOPSIS 40.Fd #include <sys/types.h> 41.Fd #include <signal.h> 42.Bd -literal 43 44struct sigaltstack { 45 void *ss_sp; 46 size_t ss_size; 47 int ss_flags; 48}; 49.Ed 50.Ft int 51.Fn sigaltstack "const struct sigaltstack *ss" "struct sigaltstack *oss" 52.Sh DESCRIPTION 53.Fn sigaltstack 54allows users to define an alternate stack on which signals 55are to be processed. 56If 57.Fa ss 58is non-zero, 59it specifies a pointer to and the size of a 60.Em "signal stack" 61on which to deliver signals, 62and tells the system if the process is currently executing 63on that stack. 64When a signal's action indicates its handler 65should execute on the signal stack (specified with a 66.Xr sigaction 2 67call), the system checks to see 68if the process is currently executing on that stack. 69If the process is not currently executing on the signal stack, 70the system arranges a switch to the signal stack for the 71duration of the signal handler's execution. 72.Pp 73If 74.Dv SS_DISABLE 75is set in 76.Fa ss_flags , 77.Fa ss_sp 78and 79.Fa ss_size 80are ignored and the signal stack will be disabled. 81Trying to disable an active stack will cause 82.Nm 83to return \-1 with 84.Va errno 85set to 86.Er EINVAL . 87A disabled stack will cause all signals to be 88taken on the regular user stack. 89If the stack is later re-enabled then all signals that were specified 90to be processed on an alternate stack will resume doing so. 91.Pp 92If 93.Fa oss 94is non-zero, the current signal stack state is returned. 95The 96.Fa ss_flags 97field will contain the value 98.Dv SS_ONSTACK 99if the process is currently on a signal stack and 100.Dv SS_DISABLE 101if the signal stack is currently disabled. 102.Sh NOTES 103The value 104.Dv SIGSTKSZ 105is defined to be the number of bytes/chars that would be used to cover 106the usual case when allocating an alternate stack area. 107The following code fragment is typically used to allocate an alternate stack. 108.Bd -literal -offset indent 109if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 110 /* error return */ 111sigstk.ss_size = SIGSTKSZ; 112sigstk.ss_flags = 0; 113if (sigaltstack(&sigstk, 0) == -1) 114 perror("sigaltstack"); 115.Ed 116.Pp 117An alternative approach is provided for programs with signal handlers 118that require a specific amount of stack space other than the default size. 119The value 120.Dv MINSIGSTKSZ 121is defined to be the number of bytes/chars that is required by 122the operating system to implement the alternate stack feature. 123In computing an alternate stack size, 124programs should add 125.Dv MINSIGSTKSZ 126to their stack requirements to allow for the operating system overhead. 127.Pp 128Signal stacks are automatically adjusted for the direction of stack 129growth and alignment requirements. 130Signal stacks may or may not be protected by the hardware and 131are not ``grown'' automatically as is done for the normal stack. 132If the stack overflows and this space is not protected 133unpredictable results may occur. 134.Sh RETURN VALUES 135Upon successful completion, a value of 0 is returned. 136Otherwise, a value of \-1 is returned and 137.Va errno 138is set to indicate the error. 139.Sh ERRORS 140.Fn sigaltstack 141will fail and the signal stack context will remain unchanged 142if one of the following occurs. 143.Bl -tag -width [ENOMEM] 144.It Bq Er EFAULT 145Either 146.Fa ss 147or 148.Fa oss 149points to memory that is not a valid part of the process 150address space. 151.It Bq Er EINVAL 152An attempt was made to disable an active stack. 153.It Bq Er ENOMEM 154Size of alternate stack area is less than or equal to 155.Dv MINSIGSTKSZ . 156.El 157.Sh SEE ALSO 158.Xr sigaction 2 , 159.Xr setjmp 3 160.Sh HISTORY 161The predecessor to 162.Nm sigaltstack , 163the 164.Fn sigstack 165system call, appeared in 166.Bx 4.2 . 167