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SIGALTSTACK(2) Linux Programmer's Manual SIGALTSTACK(2)
sigaltstack - set and/or get signal stack context
#include <signal.h>
int sigaltstack(const stack_t *ss, stack_t *oss);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sigaltstack():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
sigaltstack() allows a process to define a new alternate signal stack and/or
retrieve the state of an existing alternate signal stack. An alternate signal
stack is used during the execution of a signal handler if the establishment of
that handler (see sigaction(2)) requested it.
The normal sequence of events for using an alternate signal stack is the
following:
1. Allocate an area of memory to be used for the alternate signal stack.
2. Use sigaltstack() to inform the system of the existence and location of the
alternate signal stack.
3. When establishing a signal handler using sigaction(2), inform the system
that the signal handler should be executed on the alternate signal stack by
specifying the SA_ONSTACK flag.
The ss argument is used to specify a new alternate signal stack, while the oss
argument is used to retrieve information about the currently established
signal stack. If we are interested in performing just one of these tasks then
the other argument can be specified as NULL. Each of these arguments is a
structure of the following type:
typedef struct {
void *ss_sp; /* Base address of stack */
int ss_flags; /* Flags */
size_t ss_size; /* Number of bytes in stack */
} stack_t;
To establish a new alternate signal stack, ss.ss_flags is set to zero, and
ss.ss_sp and ss.ss_size specify the starting address and size of the stack.
The constant SIGSTKSZ is defined to be large enough to cover the usual size
requirements for an alternate signal stack, and the constant MINSIGSTKSZ
defines the minimum size required to execute a signal handler.
When a signal handler is invoked on the alternate stack, the kernel
automatically aligns the address given in ss.ss_sp to a suitable address
boundary for the underlying hardware architecture.
To disable an existing stack, specify ss.ss_flags as SS_DISABLE. In this
case, the remaining fields in ss are ignored.
If oss is not NULL, then it is used to return information about the alternate
signal stack which was in effect prior to the call to sigaltstack(). The
oss.ss_sp and oss.ss_size fields return the starting address and size of that
stack. The oss.ss_flags may return either of the following values:
SS_ONSTACK
The process is currently executing on the alternate signal stack.
(Note that it is not possible to change the alternate signal stack if
the process is currently executing on it.)
SS_DISABLE
The alternate signal stack is currently disabled.
sigaltstack() returns 0 on success, or -1 on failure with errno set to
indicate the error.
EFAULT Either ss or oss is not NULL and points to an area outside of the
process's address space.
EINVAL ss is not NULL and the ss_flags field contains a nonzero value other
than SS_DISABLE.
ENOMEM The specified size of the new alternate signal stack (ss.ss_size) was
less than MINSTKSZ.
EPERM An attempt was made to change the alternate signal stack while it was
active (i.e., the process was already executing on the current
alternate signal stack).
SUSv2, SVr4, POSIX.1-2001.
The most common usage of an alternate signal stack is to handle the SIGSEGV
signal that is generated if the space available for the normal process stack
is exhausted: in this case, a signal handler for SIGSEGV cannot be invoked on
the process stack; if we wish to handle it, we must use an alternate signal
stack.
Establishing an alternate signal stack is useful if a process expects that it
may exhaust its standard stack. This may occur, for example, because the
stack grows so large that it encounters the upwardly growing heap, or it
reaches a limit established by a call to setrlimit(RLIMIT_STACK, &rlim). If
the standard stack is exhausted, the kernel sends the process a SIGSEGV
signal. In these circumstances the only way to catch this signal is on an
alternate signal stack.
On most hardware architectures supported by Linux, stacks grow downward.
sigaltstack() automatically takes account of the direction of stack growth.
Functions called from a signal handler executing on an alternate signal stack
will also use the alternate signal stack. (This also applies to any handlers
invoked for other signals while the process is executing on the alternate
signal stack.) Unlike the standard stack, the system does not automatically
extend the alternate signal stack. Exceeding the allocated size of the
alternate signal stack will lead to unpredictable results.
A successful call to execve(2) removes any existing alternate signal stack. A
child process created via fork(2) inherits a copy of its parent's alternate
signal stack settings.
sigaltstack() supersedes the older sigstack() call. For backward
compatibility, glibc also provides sigstack(). All new applications should be
written using sigaltstack().
4.2BSD had a sigstack() system call. It used a slightly different struct, and
had the major disadvantage that the caller had to know the direction of stack
growth.
The following code segment demonstrates the use of sigaltstack():
stack_t ss;
ss.ss_sp = malloc(SIGSTKSZ);
if (ss.ss_sp == NULL)
/* Handle error */;
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
if (sigaltstack(&ss, NULL) == -1)
/* Handle error */;
execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3), signal(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-26 SIGALTSTACK(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface