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.\" $OpenBSD: sigaltstack.2,v 1.24 2021/11/21 23:44:55 jan Exp $
.\" $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. 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 $Mdocdate: November 21 2021 $
.Dt SIGALTSTACK 2
.Os
.Sh NAME
.Nm sigaltstack
.Nd set and/or get signal stack context
.Sh SYNOPSIS
.In signal.h
.Bd -literal
typedef struct sigaltstack {
void *ss_sp;
size_t ss_size;
int ss_flags;
} stack_t;
.Ed
.Ft int
.Fn sigaltstack "const stack_t *ss" "stack_t *oss"
.Sh DESCRIPTION
.Fn sigaltstack
allows users to define an alternate stack on which signals
delivered to this thread
are to be processed.
If
.Fa ss
is non-zero and
.Dv SS_DISABLE
is set in
.Fa ss_flags ,
the signal stack will be disabled.
A disabled stack will cause all signals to be
taken on the regular user stack.
Trying to disable an active stack will cause
.Fn sigaltstack
to return \-1 with
.Va errno
set to
.Er EPERM .
.Pp
Otherwise,
.Fa ss_sp
specifies a pointer to a space to be used as the signal stack and
.Fa ss_size
specifies the size of that space.
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 thread is currently executing on that stack.
If the thread 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
.Fa oss
is non-zero, the current signal stack state is returned.
The
.Fa ss_flags
field will contain the value
.Dv SS_ONSTACK
if the thread is currently on a signal stack and
.Dv SS_DISABLE
if the signal stack is currently disabled.
.Pp
To permit the space to operate as a stack, a page-aligned
inner region will be zeroed and have
.Dv MAP_STACK
(see
.Xr mmap 2 )
enabled.
Once the sigaltstack is disabled,
.Dv MAP_STACK
remains on the memory, so it is best to deallocate the memory
via a method that results in
.Xr munmap 2 .
.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, NULL) == -1)
perror("sigaltstack");
.Ed
.Pp
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
.Dq 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
.Rv -std
.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
The
.Fa ss_flags
member pointed to by the
.Fa ss
argument contains flags other than
.Dv SS_DISABLE .
.It Bq Er ENOMEM
Size of alternate stack area is less than or equal to
.Dv MINSIGSTKSZ .
.It Bq Er EPERM
An attempt was made to disable an active stack.
.El
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr setjmp 3
.Sh STANDARDS
The
.Fn sigaltstack
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The predecessor to
.Fn sigaltstack ,
the
.Fn sigstack
system call, appeared in
.Bx 4.2 .
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