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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.\" 31.Dd January 27, 2015 32.Dt SIGALTSTACK 2 33.Os 34.Sh NAME 35.Nm sigaltstack 36.Nd set and/or get signal stack context 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In signal.h 41.Bd -literal 42typedef struct sigaltstack { 43 char *ss_sp; 44 size_t ss_size; 45 int ss_flags; 46} stack_t; 47.Ed 48.Ft int 49.Fn sigaltstack "const stack_t *ss" "stack_t *oss" 50.Sh DESCRIPTION 51.Fn Sigaltstack 52allows users to define an alternate stack on which signals 53are to be processed. 54If 55.Fa ss 56is non-zero, 57it specifies a pointer to and the size of a 58.Em "signal stack" 59on which to deliver signals, 60and tells the system if the process is currently executing 61on that stack. 62When a signal's action indicates its handler 63should execute on the signal stack (specified with a 64.Xr sigaction 2 65call), the system checks to see 66if the process is currently executing on that stack. 67If the process is not currently executing on the signal stack, 68the system arranges a switch to the signal stack for the 69duration of the signal handler's execution. 70.Pp 71If 72.Dv SS_DISABLE 73is set in 74.Fa ss_flags , 75.Fa ss_sp 76and 77.Fa ss_size 78are ignored and the signal stack will be disabled. 79Trying to disable an active stack will cause 80.Fn sigaltstack 81to return -1 with 82.Va errno 83set to 84.Er EPERM . 85A disabled stack will cause all signals to be 86taken on the regular user stack. 87If the stack is later re-enabled then all signals that were specified 88to be processed on an alternate stack will resume doing so. 89.Pp 90If 91.Fa oss 92is non-zero, the current signal stack state is returned. 93The 94.Fa ss_flags 95field will contain the value 96.Dv SS_ONSTACK 97if the process is currently on a signal stack and 98.Dv SS_DISABLE 99if the signal stack is currently disabled. 100.Sh NOTES 101The value 102.Dv SIGSTKSZ 103is defined to be the number of bytes/chars that would be used to cover 104the usual case when allocating an alternate stack area. 105The following code fragment is typically used to allocate an alternate stack. 106.Bd -literal -offset indent 107if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 108 /* error return */ 109sigstk.ss_size = SIGSTKSZ; 110sigstk.ss_flags = 0; 111if (sigaltstack(&sigstk,0) < 0) 112 perror("sigaltstack"); 113.Ed 114.Pp 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 147The 148.Fa ss 149argument is not a null pointer, and the 150.Fa ss_flags 151member pointed to by 152.Fa ss 153contains flags other than 154.Dv SS_DISABLE . 155.It Bq Er ENOMEM 156Size of alternate stack area is less than or equal to 157.It Bq Er EPERM 158An attempt was made to modify an active stack. 159.Dv MINSIGSTKSZ . 160.El 161.Sh SEE ALSO 162.Xr sigaction 2 , 163.Xr setjmp 3 164.Sh HISTORY 165The predecessor to 166.Fn sigaltstack , 167the 168.Fn sigstack 169system call, appeared in 170.Bx 4.2 . 171