1.\" $NetBSD: sigaltstack.2,v 1.6 1997/11/26 16:51:12 kleink 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. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)sigaltstack.2 8.1 (Berkeley) 6/4/93 35.\" 36.Dd June 4, 1993 37.Dt SIGALTSTACK 2 38.Os BSD 4.2 39.Sh NAME 40.Nm sigaltstack 41.Nd set and/or get signal stack context 42.Sh SYNOPSIS 43.Fd #include <signal.h> 44.Pp 45.Bd -literal 46typedef struct { 47 void *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 *ss" "stack_t *oss" 54.Sh DESCRIPTION 55.Fn Sigaltstack 56allows users to define an alternative stack on which signals 57are to be processed. 58If 59.Fa ss 60is non-zero, 61it specifies a pointer to and the size of a 62.Em "signal stack" 63on which to deliver signals, 64and tells the system if the process is currently executing 65on that stack. 66When a signal's action indicates its handler 67should execute on the signal stack (specified with a 68.Xr sigaction 2 69call), the system checks to see 70if the process is currently executing on that stack. 71If the process is not currently executing on the signal stack, 72the system arranges a switch to the signal stack for the 73duration of the signal handler's execution. 74.Pp 75If 76.Dv SS_DISABLE 77is set in 78.Fa ss_flags , 79.Fa ss_sp 80and 81.Fa ss_size 82are ignored and the signal stack will be disabled. 83Trying to disable an active stack will cause 84.Nm 85to return -1 with 86.Va errno 87set to 88.Dv EINVAL . 89A disabled stack will cause all signals to be 90taken on the regular user stack. 91If the stack is later re-enabled then all signals that were specified 92to be processed on an alternative stack will resume doing so. 93.Pp 94If 95.Fa oss 96is non-zero, the current signal stack state is returned. 97The 98.Fa ss_flags 99field will contain the value 100.Dv SS_ONSTACK 101if the process is currently on a signal stack and 102.Dv SS_DISABLE 103if the signal stack is currently disabled. 104.Sh NOTES 105The value 106.Dv SIGSTKSZ 107is defined to be the number of bytes/chars that would be used to cover 108the usual case when allocating an alternative stack area. 109The following code fragment is typically used to allocate an alternative stack. 110.Bd -literal -offset indent 111if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 112 /* error return */ 113sigstk.ss_size = SIGSTKSZ; 114sigstk.ss_flags = 0; 115if (sigaltstack(&sigstk,0) < 0) 116 perror("sigaltstack"); 117.Ed 118.Pp 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 alternative stack feature. 125In computing an alternative 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 137Upon successful completion, a value of 0 is returned. 138Otherwise, a value of -1 is returned and 139.Va errno 140is set to indicate the error. 141.Sh ERRORS 142.Fn Sigstack 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 alternative 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 STANDARDS 163The 164.Fn sigaltstack 165function conforms to 166.St -xpg4.2 . 167.Sh HISTORY 168The predecessor to 169.Nm sigaltstack , 170the 171.Fn sigstack 172system call, appeared in 173.Bx 4.2 . 174