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.TH sigaltstack 2 "18 July 2016" "Oracle Solaris 11.4" "System Calls"
.SH NAME
sigaltstack \- set or get signal alternate stack context
.SH SYNOPSIS

.LP
.nf
#include <signal.h>

\fBint\fR \fBsigaltstack\fR(\fBconst stack_t *restrict\fR \fIss\fR, \fBstack_t *restrict\fR \fIoss\fR);
.fi
.SH DESCRIPTION
.sp
.LP
The \fBsigaltstack()\fR function allows a thread to define and examine the state of an alternate stack area on which signals are processed. If \fIss\fR is non-zero, it specifies a pointer to and the size of a stack area on which to deliver signals, and informs the system whether the thread is currently executing on that stack. When a signal's action indicates its handler should execute on the alternate signal stack (specified with a \fBsigaction\fR(2) call), the system checks whether the thread chosen to execute the signal handler is currently executing on that stack. If the thread is not currently executing on the signal stack, the system arranges a switch to the alternate signal stack for the duration of the signal handler's execution.
.sp
.LP
The \fBstack_t\fR structure includes the following members:
.sp
.in +2
.nf
int   *ss_sp
long  ss_size
int   ss_flags
.fi
.in -2
.sp
.sp
.LP
If \fIss\fR is not \fINULL\fR, it points to a structure specifying the alternate signal stack that will take effect upon successful return from \fBsigaltstack()\fR. The \fBss_sp\fR and \fBss_size\fR members specify the new base and size of the stack, which is automatically adjusted for direction of growth and alignment. The \fBss_flags\fR member specifies the new stack state and may be set to the following:
.sp
.ne 2
.mk
.na
\fB\fBSS_DISABLE\fR\fR
.ad
.RS 14n
.rt
The stack is to be disabled and \fBss_sp\fR and \fBss_size\fR are ignored. If \fBSS_DISABLE\fR is not set, the stack will be enabled.
.RE

.sp
.LP
If \fIoss\fR is not \fINULL\fR, it points to a structure specifying the alternate signal stack that was in effect prior to the call to \fBsigaltstack()\fR. The \fBss_sp\fR and \fBss_size\fR members specify the base and size of that stack. The \fBss_flags\fR member specifies the stack's state, and may contain the following values:
.sp
.ne 2
.mk
.na
\fB\fBSS_ONSTACK\fR\fR
.ad
.RS 14n
.rt
The thread is currently executing on the alternate signal stack. Attempts to modify the alternate signal stack while the thread is executing on it will fail.
.RE

.sp
.ne 2
.mk
.na
\fB\fBSS_DISABLE\fR\fR
.ad
.RS 14n
.rt
The alternate signal stack is currently disabled.
.RE

.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fB0\fR is return. Otherwise, \fB\(mi1\fR is returned and \fBerrno\fR is set to indicate the error.
.SH ERRORS
.sp
.LP
The \fBsigaltstack()\fR function will fail if:
.sp
.ne 2
.mk
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
.rt
The \fIss\fR or \fIoss\fR argument points to an illegal address.
.RE

.sp
.ne 2
.mk
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
.rt
The \fIss\fR argument is not a null pointer, and the \fBss_flags\fR member pointed to by \fIss\fR contains flags other than \fBSS_DISABLE\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
.rt
The size of the alternate stack area is less than \fBMINSIGSTKSZ\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fBEPERM\fR\fR
.ad
.RS 10n
.rt
An attempt was made to modify an active stack.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp
.TS
tab(
) box;
cw(2.75i) |cw(2.75i) 
lw(2.75i) |lw(2.75i) 
.
ATTRIBUTE TYPE
ATTRIBUTE VALUE
_
Interface Stability
Committed
_
MT-Level
Async-Signal-Safe
_
Standard
See \fBstandards\fR(7).
.TE
.sp
.SH SEE ALSO
.sp
.LP
\fBsigaction\fR(2), \fBgetcontext\fR(2), \fBmmap\fR(2), \fBucontext.h\fR(3HEAD), \fBattributes\fR(7), \fBstandards\fR(7), \fBsxadm\fR(8)
.SH NOTES
.sp
.LP
The value \fBSIGSTKSZ\fR is defined to be the number of bytes that would be used to cover the usual case when allocating an alternate stack area. The value \fBMINSIGSTKSZ\fR is defined to be the minimum stack size for a signal handler. In computing an alternate stack size, a program should add that amount to its stack requirements to allow for the operating system overhead.
.sp
.LP
The following code fragment is typically used to allocate an alternate stack with an adjacent red zone (an unmapped page) to guard against stack overflow, as with default stacks:
.sp
.in +2
.nf
#include <signal.h>
#include <sys/mman.h>

stack_t sigstk;
sigstk.ss_sp = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
        MAP_PRIVATE | MAP_ANON, -1, 0);
if (sigstk.ss_sp == MAP_FAILED)
        /* error return */;
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, NULL) < 0)
        perror("sigaltstack");
.fi
.in -2
.sp
.sp
.LP
If the \fBADISTACK\fR security extension is enabled for the process, \fBADI\fR must be enabled on the specified alternate stack area in order to extend coverage to the stack. See \fBmmap\fR(2) and \fBsxadm\fR(8).