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SIGALTSTACK

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

NAME

sigaltstack - set and/or get signal stack context

SYNOPSIS

#include <signal.h>

int sigaltstack(const stack_t *ss, stack_t *old_ss);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

sigaltstack():

: _XOPEN_SOURCE >= 500
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE

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 old_ss 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.

The stack_t type used to type the arguments of this function is defined as follows:

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, the fields of this structure are set as follows:

ss.ss_flags: This field is set to zero.
ss.ss_sp: This field specifies the starting address of the stack. 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.
ss.ss_size: This field specifies the 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.

To disable an existing stack, specify ss.ss_flags as SS_DISABLE. In this case, the remaining fields in ss are ignored.

If old_ss 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 old_ss.ss_sp and old_ss.ss_size fields return the starting address and size of that stack. The old_ss.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.

By specifying ss as NULL, and old_ss as a non-NULL value, one can obtain the current settings for the alternate signal stack without changing them.

RETURN VALUE

sigaltstack() returns 0 on success, or -1 on failure with errno set to indicate the error.

ERRORS

EFAULT: Either ss or old_ss 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 an invalid flag.
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).

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
sigaltstack()	Thread safety	MT-Safe

CONFORMING TO

POSIX.1-2001, POSIX.1-2009, SUSv2, SVr4.

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 */;

BUGS

In the lead up to the development of the Linux 2.4 kernel, someone got confused and allowed the kernel to accept SS_ONSTACK in ss.ss_flags, which results in behavior that is the same as when ss_flags is 0. On other implementations, and according to POSIX.1, SS_ONSTACK appears only as a reported flag in old_ss.ss_flags. There is no need ever to specify this flag in ss.ss_flags (and indeed, doing so decreases portability, since some implementations give an error if SS_ONSTACK is specified in ss.ss_flags).

COLOPHON

This page is part of release 4.13 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 https://www.kernel.org/doc/man-pages/.

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