SIGALTSTACK


HOME

SIGALTSTACK

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
EXAMPLE
SEE ALSO
COLOPHON

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 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.

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_DISABLE.

ENOMEM

The specified size of the new alternate signal stack ss.ss_size as 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).

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 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().

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 3.69 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/.







Opportunity


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.





Free Software


Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.


Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.





Free Books


The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.


Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.





Education


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.


Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.