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sigstack(3) - phpMan
SIGALTSTACK(2) Linux Programmer's Manual SIGALTSTACK(2)
NAME
sigaltstack - set and/or get signal stack context
SYNOPSIS
#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
DESCRIPTION
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 sig‐
nal 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 exe‐
cuting on it.)
SS_DISABLE
The alternate signal stack is currently disabled.
RETURN VALUE
sigaltstack() returns 0 on success, or -1 on failure with errno set to indicate the error.
ERRORS
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_DIS‐
ABLE.
ENOMEM The specified size of the new alternate signal stack ss.ss_size was less than MIN‐
STKSZ.
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).
CONFORMING TO
SUSv2, SVr4, POSIX.1-2001.
NOTES
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 setr‐
limit(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 sig‐
nals 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 set‐
tings.
sigaltstack() supersedes the older sigstack() call. For backward compatibility, glibc
also provides sigstack(). All new applications should be written using sigaltstack().
History
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.
EXAMPLE
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 */;
SEE ALSO
execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3), signal(7)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-26 SIGALTSTACK(2)
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