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.TH SIGSTACK 3C "Feb 28, 1996"
.SH NAME
sigstack \- set and/or get alternate signal stack context
.SH SYNOPSIS
.LP
.nf
#include <signal.h>

\fBint\fR \fBsigstack\fR(\fBstruct sigstack *\fR\fIss\fR, \fBstruct sigstack *\fR\fIoss\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBsigstack()\fR function allows the calling process to indicate to the
system an area of its address space to be used for processing signals received
by the process.
.sp
.LP
If the \fIss\fR argument is not a null pointer, it must point to a
\fBsigstack\fR structure. The length of the application-supplied stack must be
at least \fBSIGSTKSZ\fR bytes. If the alternate signal stack overflows, the
resulting behavior is undefined.  (See \fBUSAGE\fR below.)
.RS +4
.TP
.ie t \(bu
.el o
The value of the \fBss_onstack\fR member indicates whether the process wants
the system to use an alternate signal stack when delivering signals.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The value of the \fBss_sp\fR member indicates the desired location of the
alternate signal stack area in the process' address space.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If the \fIss\fR argument is a null pointer, the current alternate signal stack
context is not changed.
.RE
.sp
.LP
If the \fIoss\fR argument is not a null pointer, it points to a \fBsigstack\fR
structure in which the current alternate signal stack context is placed.  The
value stored in the \fBss_onstack\fR member of \fIoss\fR will be non-zero if
the process is currently executing on the alternate signal stack.  If the
\fIoss\fR argument is a null pointer, the current alternate signal stack
context is not returned.
.sp
.LP
When a signal's action indicates its handler should execute on the alternate
signal stack (specified by calling \fBsigaction\fR(2)), \fBsigstack()\fR checks
to see if the process is currently executing on that stack.  If the process is
not currently executing on the alternate signal stack, the system arranges a
switch to the alternate signal stack for the duration of the signal handler's
execution.
.sp
.LP
After a successful call to one of the \fIexec\fR functions, there are no
alternate signal stacks in the new process image.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBsigstack()\fR returns \fB0\fR.  Otherwise, it
returns \fB\(mi1\fR and sets \fBerrno\fR to indicate the error.
.SH ERRORS
.sp
.LP
The \fBsigstack()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEPERM\fR\fR
.ad
.RS 9n
An attempt was made to modify an active stack.
.RE

.SH USAGE
.sp
.LP
A portable application, when being written or rewritten, should use
\fBsigaltstack\fR(2) instead of \fBsigstack()\fR.
.sp
.LP
The direction of stack growth is not indicated in the historical definition of
\fBstruct sigstack\fR. The only way to portably establish a stack pointer is
for the application to determine stack growth direction, or to allocate a block
of storage and set the stack pointer to the middle. \fBsigstack()\fR may assume
that the size of the signal stack is \fBSIGSTKSZ\fR as found in
<\fBsignal.h\fR>. An application that would like to specify a signal stack size
other than \fBSIGSTKSZ\fR should use \fBsigaltstack\fR(2).
.sp
.LP
Applications should not use \fBlongjmp\fR(3C) to leave a signal handler that is
running on a stack established with \fBsigstack()\fR. Doing so may disable
future use of the signal stack.  For abnormal exit from a signal handler,
\fBsiglongjmp\fR(3C), \fBsetcontext\fR(2), or \fBswapcontext\fR(3C) may be
used. These functions fully support switching from one stack to another.
.sp
.LP
The \fBsigstack()\fR function requires the application to have knowledge of the
underlying system's stack architecture.  For this reason, \fBsigaltstack\fR(2)
is recommended over this function.
.SH SEE ALSO
.sp
.LP
\fBfork\fR(2), \fB_longjmp\fR(3C), \fBlongjmp\fR(3C), \fBsetjmp\fR(3C),
\fBsigaltstack\fR(2), \fBsiglongjmp\fR(3C), \fBsigsetjmp\fR(3C)