.\" $NetBSD: sigaltstack.2,v 1.3 1995/02/27 10:41:52 cgd Exp $ .\" .\" Copyright (c) 1983, 1991, 1992, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)sigaltstack.2 8.1 (Berkeley) 6/4/93 .\" .Dd June 4, 1993 .Dt SIGALTSTACK 2 .Os BSD 4.2 .Sh NAME .Nm sigaltstack .Nd set and/or get signal stack context .Sh SYNOPSIS .Fd #include .Ft int .Fo sigaltstack .Fa "const stack_t *restrict ss" .Fa "stack_t *restrict oss" .Fc .Sh DESCRIPTION .Fn Sigaltstack allows users to define an alternate stack on which signals are to be processed. If .Fa ss is non-zero, it specifies a pointer to and the size of a .Em "signal stack" on which to deliver signals, and tells the system if the process is currently executing on that stack. When a signal's action indicates its handler should execute on the signal stack (specified with a .Xr sigaction 2 call), the system checks to see if the process is currently executing on that stack. If the process is not currently executing on the signal stack, the system arranges a switch to the signal stack for the duration of the signal handler's execution. .Pp If .Dv SS_DISABLE is set in .Fa ss_flags , .Fa ss_sp and .Fa ss_size are ignored and the signal stack will be disabled. Trying to disable an active stack will cause .Nm to return -1 with .Va errno set to .Dv EINVAL . A disabled stack will cause all signals to be taken on the regular user stack. If the stack is later re-enabled then all signals that were specified to be processed on an alternate stack will resume doing so. .Pp If .Fa oss is non-zero, the current signal stack state is returned. The .Fa ss_flags field will contain the value .Dv SA_ONSTACK if the process is currently on a signal stack and .Dv SS_DISABLE if the signal stack is currently disabled. .Sh NOTES The value .Dv SIGSTKSZ is defined to be the number of bytes/chars that would be used to cover the usual case when allocating an alternate stack area. The following code fragment is typically used to allocate an alternate stack. .Bd -literal -offset indent if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) /* error return */ sigstk.ss_size = SIGSTKSZ; sigstk.ss_flags = 0; if (sigaltstack(&sigstk,0) < 0) perror("sigaltstack"); .Ed An alternative approach is provided for programs with signal handlers that require a specific amount of stack space other than the default size. The value .Dv MINSIGSTKSZ is defined to be the number of bytes/chars that is required by the operating system to implement the alternate stack feature. In computing an alternate stack size, programs should add .Dv MINSIGSTKSZ to their stack requirements to allow for the operating system overhead. .Pp Signal stacks are automatically adjusted for the direction of stack growth and alignment requirements. Signal stacks may or may not be protected by the hardware and are not ``grown'' automatically as is done for the normal stack. If the stack overflows and this space is not protected unpredictable results may occur. .Sh RETURN VALUES Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and .Va errno is set to indicate the error. .Sh ERRORS .Fn Sigaltstack will fail and the signal stack context will remain unchanged if one of the following occurs. .Bl -tag -width [ENOMEM] .\" =========== .It Bq Er EFAULT Either .Fa ss or .Fa oss points to memory that is not a valid part of the process address space. .\" =========== .It Bq Er EINVAL An attempt is made to disable an active stack. .\" =========== .It Bq Er EINVAL The .Fa ss argument is not a null pointer, and the ss_flags member pointed to by .Fa ss contains flags other than SS_DISABLE. .\" =========== .It Bq Er ENOMEM The size of the alternate stack area is less than or equal to .Dv MINSIGSTKSZ . .\" =========== .It Bq Er EPERM An attempt was made to modify an active stack. .El .Sh LEGACY SYNOPSIS .Fd #include .Fd #include .Pp The include file .In sys/types.h is necessary. .Pp .Bd -literal struct sigaltstack { char *ss_sp; int ss_size; int ss_flags; }; .Ed .Pp .Ft int .br .Fo sigaltstack .Fa "const struct sigaltstack *ss" .Fa "struct sigaltstack *oss" .Fc ; .Pp The variable types have changed. Specifically, the .Vt sigaltstack struct is no longer used. .Sh COMPATIBILITY Use of the (obsolete) .Vt sigaltstack struct will cause compiler diagnostics. Use .Vt stack_t , defined in .In signal.h . .Sh SEE ALSO .Xr sigaction 2 , .Xr setjmp 3 , .Xr compat 5 .Sh HISTORY The predecessor to .Nm sigaltstack , the .Fn sigstack system call, appeared in .Bx 4.2 .