1.\" $NetBSD: sigaltstack.2,v 1.24 2014/09/27 12:11:13 njoly Exp $ 2.\" 3.\" Copyright (c) 1983, 1991, 1992, 1993 4.\" The Regents of the University of California. 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.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)sigaltstack.2 8.2 (Berkeley) 5/1/95 31.\" 32.Dd March 2, 2012 33.Dt SIGALTSTACK 2 34.Os 35.Sh NAME 36.Nm sigaltstack 37.Nd set and/or get signal stack context 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In signal.h 42.Bd -literal 43typedef struct sigaltstack { 44 void *ss_sp; 45 size_t ss_size; 46 int ss_flags; 47} stack_t; 48.Ed 49.Pp 50.Ft int 51.Fn sigaltstack "const stack_t * restrict ss" "stack_t * restrict oss" 52.Sh DESCRIPTION 53.Fn sigaltstack 54allows users to define an alternative 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 alternative 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 alternative stack area. 107The following code fragment is typically used to allocate an alternative 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(\*[Am]sigstk,0) \*[Lt] 0) 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 alternative stack feature. 123In computing an alternative 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 135.Rv -std 136.Sh ERRORS 137.Fn sigaltstack 138will fail and the signal stack context will remain unchanged 139if one of the following occurs. 140.Bl -tag -width Er 141.It Bq Er EFAULT 142Either 143.Fa ss 144or 145.Fa oss 146points to memory that is not a valid part of the process 147address space. 148.It Bq Er EINVAL 149An attempt was made to disable an active stack. 150.It Bq Er ENOMEM 151Size of alternative stack area is less than 152.Dv MINSIGSTKSZ . 153.El 154.Sh SEE ALSO 155.Xr sigaction 2 , 156.Xr setjmp 3 , 157.Xr signal 7 158.Sh STANDARDS 159The 160.Fn sigaltstack 161function conforms to 162.St -xpg4.2 . 163.Sh HISTORY 164The predecessor to 165.Nm sigaltstack , 166the 167.Fn sigstack 168system call, appeared in 169.Bx 4.2 . 170