NAME
sigaltstack —
set and/or get signal stack
context
LIBRARY
library “libc”
SYNOPSIS
#include
<signal.h>
typedef struct sigaltstack {
char *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 alternate 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
EPERM. 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 alternate 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 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,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 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
The sigaltstack() function returns the
value 0 if successful; 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 argument is not a null pointer, and the
ss_flags member pointed to by
ss contains flags other than
SS_DISABLE. - [
ENOMEM] - Size of alternate stack area is less than or equal to
- [
EPERM] - An attempt was made to modify an active stack.
MINSIGSTKSZ.
SEE ALSO
HISTORY
The predecessor to sigaltstack(), the
sigstack system call, appeared in
4.2BSD.