NAME
sigaltstack —
set and/or get signal
stack context
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <signal.h>
typedef struct sigaltstack {
void *ss_sp;
size_t ss_size;
int ss_flags;
} stack_t;
int
sigaltstack(
const
stack_t * restrict ss,
stack_t * restrict oss);
DESCRIPTION
sigaltstack() allows users to define an alternative stack on
which signals are to be processed. If
ss is non-zero, it
specifies a pointer to and the size of a
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
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.
If
SS_DISABLE
is set in
ss_flags,
ss_sp and
ss_size are ignored and
the signal stack will be disabled. Trying to disable an active stack will
cause
sigaltstack to return -1 with
errno set to
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 alternative stack will resume doing so.
If
oss is non-zero, the current signal stack state is
returned. The
ss_flags field will contain the value
SS_ONSTACK
if the process is currently on a signal
stack and
SS_DISABLE
if the signal stack is currently
disabled.
NOTES
The value
SIGSTKSZ
is defined to be the number of
bytes/chars that would be used to cover the usual case when allocating an
alternative stack area. The following code fragment is typically used to
allocate an alternative stack.
if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
/* error return */
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk,0) < 0)
perror("sigaltstack");
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
MINSIGSTKSZ
is defined to be the number of
bytes/chars that is required by the operating system to implement the
alternative stack feature. In computing an alternative stack size, programs
should add
MINSIGSTKSZ
to their stack requirements to
allow for the operating system overhead.
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.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable
errno
is set to indicate the error.
ERRORS
sigaltstack() will fail and the signal stack context will
remain unchanged if one of the following occurs.
-
-
- [
EFAULT
]
- Either ss or
oss points to memory that is not a valid part of the
process address space.
-
-
- [
EINVAL
]
- An attempt was made to disable an active stack.
-
-
- [
ENOMEM
]
- Size of alternative stack area is less than
MINSIGSTKSZ
.
SEE ALSO
sigaction(2),
setjmp(3),
signal(7)
STANDARDS
The
sigaltstack() function conforms to
X/Open
Portability Guide Issue 4, Version 2 (“XPG4.2”).
HISTORY
The predecessor to
sigaltstack, the
sigstack() system call, appeared in
4.2BSD.