1.\" Copyright (c) 1983, 1991, 1992, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)sigaltstack.2 8.2 (Berkeley) 5/1/95 33.\" $FreeBSD$ 34.\" 35.Dd May 1, 1995 36.Dt SIGALTSTACK 2 37.Os 38.Sh NAME 39.Nm sigaltstack 40.Nd set and/or get signal stack context 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In signal.h 45.Bd -literal 46typedef struct sigaltstack { 47 char *ss_sp; 48 size_t ss_size; 49 int ss_flags; 50} stack_t; 51.Ed 52.Ft int 53.Fn sigaltstack "const stack_t * restrict ss" "stack_t * restrict oss" 54.Sh DESCRIPTION 55The 56.Fn sigaltstack 57system call 58allows users to define an alternate stack on which signals 59are to be processed. 60If 61.Fa ss 62is non-zero, 63it specifies a pointer to and the size of a 64.Em "signal stack" 65on which to deliver signals, 66and tells the system if the process is currently executing 67on that stack. 68When a signal's action indicates its handler 69should execute on the signal stack (specified with a 70.Xr sigaction 2 71system call), the system checks to see 72if the process is currently executing on that stack. 73If the process is not currently executing on the signal stack, 74the system arranges a switch to the signal stack for the 75duration of the signal handler's execution. 76.Pp 77If 78.Dv SS_DISABLE 79is set in 80.Fa ss_flags , 81.Fa ss_sp 82and 83.Fa ss_size 84are ignored and the signal stack will be disabled. 85Trying to disable an active stack will cause 86.Fn sigaltstack 87to return -1 with 88.Va errno 89set to 90.Er EINVAL . 91A disabled stack will cause all signals to be 92taken on the regular user stack. 93If the stack is later re-enabled then all signals that were specified 94to be processed on an alternate stack will resume doing so. 95.Pp 96If 97.Fa oss 98is non-zero, the current signal stack state is returned. 99The 100.Fa ss_flags 101field will contain the value 102.Dv SS_ONSTACK 103if the process is currently on a signal stack and 104.Dv SS_DISABLE 105if the signal stack is currently disabled. 106.Sh NOTES 107The value 108.Dv SIGSTKSZ 109is defined to be the number of bytes/chars that would be used to cover 110the usual case when allocating an alternate stack area. 111The following code fragment is typically used to allocate an alternate stack. 112.Bd -literal -offset indent 113if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 114 /* error return */ 115sigstk.ss_size = SIGSTKSZ; 116sigstk.ss_flags = 0; 117if (sigaltstack(&sigstk,0) < 0) 118 perror("sigaltstack"); 119.Ed 120An alternative approach is provided for programs with signal handlers 121that require a specific amount of stack space other than the default size. 122The value 123.Dv MINSIGSTKSZ 124is defined to be the number of bytes/chars that is required by 125the operating system to implement the alternate stack feature. 126In computing an alternate stack size, 127programs should add 128.Dv MINSIGSTKSZ 129to their stack requirements to allow for the operating system overhead. 130.Pp 131Signal stacks are automatically adjusted for the direction of stack 132growth and alignment requirements. 133Signal stacks may or may not be protected by the hardware and 134are not ``grown'' automatically as is done for the normal stack. 135If the stack overflows and this space is not protected 136unpredictable results may occur. 137.Sh RETURN VALUES 138.Rv -std sigaltstack 139.Sh ERRORS 140The 141.Fn sigaltstack 142system call 143will fail and the signal stack context will remain unchanged 144if one of the following occurs. 145.Bl -tag -width Er 146.It Bq Er EFAULT 147Either 148.Fa ss 149or 150.Fa oss 151points to memory that is not a valid part of the process 152address space. 153.It Bq Er EINVAL 154An attempt was made to disable an active stack. 155.It Bq Er ENOMEM 156Size of alternate stack area is less than or equal to 157.Dv MINSIGSTKSZ . 158.El 159.Sh SEE ALSO 160.Xr sigaction 2 , 161.Xr setjmp 3 162.Sh HISTORY 163The predecessor to 164.Fn sigaltstack , 165the 166.Fn sigstack 167system call, appeared in 168.Bx 4.2 . 169