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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)sigaltstack.2 8.2 (Berkeley) 5/1/95 29.\" $FreeBSD: src/lib/libc/sys/sigaltstack.2,v 1.11.2.7 2002/09/15 00:32:41 archie Exp $ 30.\" $DragonFly: src/lib/libc/sys/sigaltstack.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 31.\" 32.Dd May 1, 1995 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 char *ss_sp; 45 size_t ss_size; 46 int ss_flags; 47} stack_t; 48.Ed 49.Ft int 50.Fn sigaltstack "const stack_t *ss" "stack_t *oss" 51.Sh DESCRIPTION 52.Fn Sigaltstack 53allows users to define an alternate stack on which signals 54are to be processed. 55If 56.Fa ss 57is non-zero, 58it specifies a pointer to and the size of a 59.Em "signal stack" 60on which to deliver signals, 61and tells the system if the process is currently executing 62on that stack. 63When a signal's action indicates its handler 64should execute on the signal stack (specified with a 65.Xr sigaction 2 66call), the system checks to see 67if the process is currently executing on that stack. 68If the process is not currently executing on the signal stack, 69the system arranges a switch to the signal stack for the 70duration of the signal handler's execution. 71.Pp 72If 73.Dv SS_DISABLE 74is set in 75.Fa ss_flags , 76.Fa ss_sp 77and 78.Fa ss_size 79are ignored and the signal stack will be disabled. 80Trying to disable an active stack will cause 81.Fn sigaltstack 82to return -1 with 83.Va errno 84set to 85.Er EINVAL . 86A disabled stack will cause all signals to be 87taken on the regular user stack. 88If the stack is later re-enabled then all signals that were specified 89to be processed on an alternate stack will resume doing so. 90.Pp 91If 92.Fa oss 93is non-zero, the current signal stack state is returned. 94The 95.Fa ss_flags 96field will contain the value 97.Dv SS_ONSTACK 98if the process is currently on a signal stack and 99.Dv SS_DISABLE 100if the signal stack is currently disabled. 101.Sh NOTES 102The value 103.Dv SIGSTKSZ 104is defined to be the number of bytes/chars that would be used to cover 105the usual case when allocating an alternate stack area. 106The following code fragment is typically used to allocate an alternate stack. 107.Bd -literal -offset indent 108if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 109 /* error return */ 110sigstk.ss_size = SIGSTKSZ; 111sigstk.ss_flags = 0; 112if (sigaltstack(&sigstk,0) < 0) 113 perror("sigaltstack"); 114.Ed 115An alternative approach is provided for programs with signal handlers 116that require a specific amount of stack space other than the default size. 117The value 118.Dv MINSIGSTKSZ 119is defined to be the number of bytes/chars that is required by 120the operating system to implement the alternate stack feature. 121In computing an alternate stack size, 122programs should add 123.Dv MINSIGSTKSZ 124to their stack requirements to allow for the operating system overhead. 125.Pp 126Signal stacks are automatically adjusted for the direction of stack 127growth and alignment requirements. 128Signal stacks may or may not be protected by the hardware and 129are not ``grown'' automatically as is done for the normal stack. 130If the stack overflows and this space is not protected 131unpredictable results may occur. 132.Sh RETURN VALUES 133.Rv -std sigaltstack 134.Sh ERRORS 135.Fn Sigaltstack 136will fail and the signal stack context will remain unchanged 137if one of the following occurs. 138.Bl -tag -width Er 139.It Bq Er EFAULT 140Either 141.Fa ss 142or 143.Fa oss 144points to memory that is not a valid part of the process 145address space. 146.It Bq Er EINVAL 147An attempt was made to disable an active stack. 148.It Bq Er ENOMEM 149Size of alternate stack area is less than or equal to 150.Dv MINSIGSTKSZ . 151.El 152.Sh SEE ALSO 153.Xr sigaction 2 , 154.Xr setjmp 3 155.Sh HISTORY 156The predecessor to 157.Fn sigaltstack , 158the 159.Fn sigstack 160system call, appeared in 161.Bx 4.2 . 162