NAME
sigaltstack —
set and/or get signal stack
context
LIBRARY
library “libc”
SYNOPSIS
#include
<signal.h>
typedef struct {
char *ss_sp;
size_t ss_size;
int ss_flags;
} stack_t;
int
sigaltstack(const
stack_t * restrict ss,
stack_t * restrict oss);
DESCRIPTION
Thesigaltstack()
system call allows defining an alternate stack on which signals are to be
processed for the current thread. If ss is non-zero, it
specifies a pointer to and the size of a
signal
stack on which to deliver signals. When a signal's action indicates
its handler should execute on the signal stack (specified with a
sigaction(2) system 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.
An active stack cannot be modified.
If SS_DISABLE is set in
ss_flags, ss_sp and
ss_size are ignored and the signal stack will be
disabled. 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 thread 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, NULL) < 0)
perror("sigaltstack");
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
The sigaltstack() system call 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.
- [
EPERM] - An attempt was made to modify an active stack.
- [
EINVAL] - The ss_flags field was invalid.
- [
ENOMEM] - Size of alternate stack area is less than or equal to
MINSIGSTKSZ.
SEE ALSO
HISTORY
The predecessor to sigaltstack(), the
sigstack() system call, appeared in
4.2BSD.