NAME
sigaltstack
—
set and/or get signal stack
context
SYNOPSIS
#include
<signal.h>
typedef struct sigaltstack { void *ss_sp; size_t ss_size; int ss_flags; } stack_t;
int
sigaltstack
(const
stack_t *ss, stack_t
*oss);
DESCRIPTION
sigaltstack
()
allows users to define an alternate stack on which signals delivered to this
thread are to be processed. If ss is non-zero and
SS_DISABLE
is set in 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 sigaltstack
() to return -1 with
errno set to EPERM
.
Otherwise, ss_sp specifies a pointer to a space to be used as the signal stack and 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 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.
If oss is non-zero, the current signal stack
state is returned. The ss_flags field will contain the
value SS_ONSTACK
if the thread is currently on a
signal stack and SS_DISABLE
if the signal stack is
currently disabled.
To permit the space to operate as a stack, a page-aligned inner
region will be zeroed and have MAP_STACK
(see
mmap(2)) enabled. Once the sigaltstack is disabled,
MAP_STACK
remains on the memory, so it is best to
deallocate the memory via a method that results in
munmap(2).
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 alternate stack area. The following code fragment is typically
used to allocate an alternate stack.
if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) /* error return */ sigstk.ss_size = SIGSTKSZ; sigstk.ss_flags = 0; if (sigaltstack(&sigstk, NULL) == -1) 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 alternate stack feature. In computing an alternate 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
] - The ss_flags member pointed to by the
ss argument contains flags other than
SS_DISABLE
. - [
ENOMEM
] - Size of alternate stack area is less than or equal to
MINSIGSTKSZ
. - [
EPERM
] - An attempt was made to disable an active stack.
SEE ALSO
STANDARDS
The sigaltstack
() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
The predecessor to sigaltstack
(), the
sigstack
() system call, appeared in
4.2BSD.