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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.\" $FreeBSD$ 30.\" 31.Dd May 1, 1995 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 * restrict ss" "stack_t * restrict oss" 50.Sh DESCRIPTION 51The 52.Fn sigaltstack 53system call 54allows users to define an alternate stack on which signals 55are to be processed. 56If 57.Fa ss 58is non-zero, 59it specifies a pointer to and the size of a 60.Em "signal stack" 61on which to deliver signals, 62and tells the system if the process is currently executing 63on that stack. 64When a signal's action indicates its handler 65should execute on the signal stack (specified with a 66.Xr sigaction 2 67system call), the system checks to see 68if the process is currently executing on that stack. 69If the process is not currently executing on the signal stack, 70the system arranges a switch to the signal stack for the 71duration of the signal handler's execution. 72.Pp 73If 74.Dv SS_DISABLE 75is set in 76.Fa ss_flags , 77.Fa ss_sp 78and 79.Fa ss_size 80are ignored and the signal stack will be disabled. 81Trying to disable an active stack will cause 82.Fn sigaltstack 83to return -1 with 84.Va errno 85set to 86.Er EINVAL . 87A disabled stack will cause all signals to be 88taken on the regular user stack. 89If the stack is later re-enabled then all signals that were specified 90to be processed on an alternate stack will resume doing so. 91.Pp 92If 93.Fa oss 94is non-zero, the current signal stack state is returned. 95The 96.Fa ss_flags 97field will contain the value 98.Dv SS_ONSTACK 99if the process is currently on a signal stack and 100.Dv SS_DISABLE 101if the signal stack is currently disabled. 102.Sh NOTES 103The value 104.Dv SIGSTKSZ 105is defined to be the number of bytes/chars that would be used to cover 106the usual case when allocating an alternate stack area. 107The following code fragment is typically used to allocate an alternate stack. 108.Bd -literal -offset indent 109if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 110 /* error return */ 111sigstk.ss_size = SIGSTKSZ; 112sigstk.ss_flags = 0; 113if (sigaltstack(&sigstk,0) < 0) 114 perror("sigaltstack"); 115.Ed 116An alternative approach is provided for programs with signal handlers 117that require a specific amount of stack space other than the default size. 118The value 119.Dv MINSIGSTKSZ 120is defined to be the number of bytes/chars that is required by 121the operating system to implement the alternate stack feature. 122In computing an alternate stack size, 123programs should add 124.Dv MINSIGSTKSZ 125to their stack requirements to allow for the operating system overhead. 126.Pp 127Signal stacks are automatically adjusted for the direction of stack 128growth and alignment requirements. 129Signal stacks may or may not be protected by the hardware and 130are not ``grown'' automatically as is done for the normal stack. 131If the stack overflows and this space is not protected 132unpredictable results may occur. 133.Sh RETURN VALUES 134.Rv -std sigaltstack 135.Sh ERRORS 136The 137.Fn sigaltstack 138system call 139will fail and the signal stack context will remain unchanged 140if one of the following occurs. 141.Bl -tag -width Er 142.It Bq Er EFAULT 143Either 144.Fa ss 145or 146.Fa oss 147points to memory that is not a valid part of the process 148address space. 149.It Bq Er EINVAL 150An attempt was made to disable an active stack. 151.It Bq Er ENOMEM 152Size of alternate stack area is less than or equal to 153.Dv MINSIGSTKSZ . 154.El 155.Sh SEE ALSO 156.Xr sigaction 2 , 157.Xr setjmp 3 158.Sh HISTORY 159The predecessor to 160.Fn sigaltstack , 161the 162.Fn sigstack 163system call, appeared in 164.Bx 4.2 . 165