SIGALTSTACK(2) System Calls SIGALTSTACK(2)
NAME
sigaltstack - set or get signal alternate stack context
SYNOPSIS
#include <signal.h>
int sigaltstack(
const stack_t *restrict ss,
stack_t *restrict oss);
DESCRIPTION
The
sigaltstack() function allows a thread to define and examine the
state of an alternate stack area on which signals are processed. If
ss 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
sigaction(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.
The
stack_t structure includes the following members:
int *ss_sp
long ss_size
int ss_flags
If
ss is not
NULL, it points to a structure specifying the alternate
signal stack that will take effect upon successful return from
sigaltstack(). The
ss_sp and
ss_size members specify the new base and
size of the stack, which is automatically adjusted for direction of
growth and alignment. The
ss_flags member specifies the new stack
state and may be set to the following:
SS_DISABLE The stack is to be disabled and
ss_sp and
ss_size are
ignored. If
SS_DISABLE is not set, the stack will be
enabled.
If
oss is not
NULL, it points to a structure specifying the alternate
signal stack that was in effect prior to the call to
sigaltstack().
The
ss_sp and
ss_size members specify the base and size of that
stack. The
ss_flags member specifies the stack's state, and may
contain the following values:
SS_ONSTACK 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.
SS_DISABLE The alternate signal stack is currently disabled.
RETURN VALUES
Upon successful completion,
0 is return. Otherwise,
-1 is returned
and
errno is set to indicate the error.
ERRORS
The
sigaltstack() function will fail if:
EFAULT The
ss or
oss argument points to an illegal address.
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 The size of the alternate stack area is less than
MINSIGSTKSZ.
EPERM An attempt was made to modify an active stack.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Standard |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
SEE ALSO
getcontext(2),
mmap(2),
sigaction(2),
ucontext.h(3HEAD),
attributes(7),
standards(7)NOTES
The value
SIGSTKSZ 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
MINSIGSTKSZ 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.
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:
#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");
November 1, 2003 SIGALTSTACK(2)
