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: src/lib/libc/sys/sigaltstack.2,v 1.11.2.7 2002/09/15 00:32:41 archie Exp $ 34.\" $DragonFly: src/lib/libc/sys/sigaltstack.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 35.\" 36.Dd May 1, 1995 37.Dt SIGALTSTACK 2 38.Os 39.Sh NAME 40.Nm sigaltstack 41.Nd set and/or get signal stack context 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In signal.h 46.Bd -literal 47typedef struct sigaltstack { 48 char *ss_sp; 49 size_t ss_size; 50 int ss_flags; 51} stack_t; 52.Ed 53.Ft int 54.Fn sigaltstack "const stack_t *ss" "stack_t *oss" 55.Sh DESCRIPTION 56.Fn Sigaltstack 57allows users to define an alternate stack on which signals 58are to be processed. 59If 60.Fa ss 61is non-zero, 62it specifies a pointer to and the size of a 63.Em "signal stack" 64on which to deliver signals, 65and tells the system if the process is currently executing 66on that stack. 67When a signal's action indicates its handler 68should execute on the signal stack (specified with a 69.Xr sigaction 2 70call), the system checks to see 71if the process is currently executing on that stack. 72If the process is not currently executing on the signal stack, 73the system arranges a switch to the signal stack for the 74duration of the signal handler's execution. 75.Pp 76If 77.Dv SS_DISABLE 78is set in 79.Fa ss_flags , 80.Fa ss_sp 81and 82.Fa ss_size 83are ignored and the signal stack will be disabled. 84Trying to disable an active stack will cause 85.Fn sigaltstack 86to return -1 with 87.Va errno 88set to 89.Er EINVAL . 90A disabled stack will cause all signals to be 91taken on the regular user stack. 92If the stack is later re-enabled then all signals that were specified 93to be processed on an alternate stack will resume doing so. 94.Pp 95If 96.Fa oss 97is non-zero, the current signal stack state is returned. 98The 99.Fa ss_flags 100field will contain the value 101.Dv SS_ONSTACK 102if the process is currently on a signal stack and 103.Dv SS_DISABLE 104if the signal stack is currently disabled. 105.Sh NOTES 106The value 107.Dv SIGSTKSZ 108is defined to be the number of bytes/chars that would be used to cover 109the usual case when allocating an alternate stack area. 110The following code fragment is typically used to allocate an alternate stack. 111.Bd -literal -offset indent 112if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 113 /* error return */ 114sigstk.ss_size = SIGSTKSZ; 115sigstk.ss_flags = 0; 116if (sigaltstack(&sigstk,0) < 0) 117 perror("sigaltstack"); 118.Ed 119An alternative approach is provided for programs with signal handlers 120that require a specific amount of stack space other than the default size. 121The value 122.Dv MINSIGSTKSZ 123is defined to be the number of bytes/chars that is required by 124the operating system to implement the alternate stack feature. 125In computing an alternate stack size, 126programs should add 127.Dv MINSIGSTKSZ 128to their stack requirements to allow for the operating system overhead. 129.Pp 130Signal stacks are automatically adjusted for the direction of stack 131growth and alignment requirements. 132Signal stacks may or may not be protected by the hardware and 133are not ``grown'' automatically as is done for the normal stack. 134If the stack overflows and this space is not protected 135unpredictable results may occur. 136.Sh RETURN VALUES 137.Rv -std sigaltstack 138.Sh ERRORS 139.Fn Sigaltstack 140will fail and the signal stack context will remain unchanged 141if one of the following occurs. 142.Bl -tag -width Er 143.It Bq Er EFAULT 144Either 145.Fa ss 146or 147.Fa oss 148points to memory that is not a valid part of the process 149address space. 150.It Bq Er EINVAL 151An attempt was made to disable an active stack. 152.It Bq Er ENOMEM 153Size of alternate stack area is less than or equal to 154.Dv MINSIGSTKSZ . 155.El 156.Sh SEE ALSO 157.Xr sigaction 2 , 158.Xr setjmp 3 159.Sh HISTORY 160The predecessor to 161.Fn sigaltstack , 162the 163.Fn sigstack 164system call, appeared in 165.Bx 4.2 . 166