SIGALTSTACK(3P)           POSIX Programmer's Manual          SIGALTSTACK(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       sigaltstack — set and get signal alternate stack context

SYNOPSIS         top

       #include <signal.h>

       int sigaltstack(const stack_t *restrict ss, stack_t *restrict oss);

DESCRIPTION         top

       The sigaltstack() function allows a process to define and examine the
       state of an alternate stack for signal handlers for the current
       thread. Signals that have been explicitly declared to execute on the
       alternate stack shall be delivered on the alternate stack.

       If ss is not a null pointer, it points to a stack_t structure that
       specifies the alternate signal stack that shall take effect upon
       return from sigaltstack().  The ss_flags member specifies the new
       stack state. If it is set to SS_DISABLE, the stack is disabled and
       ss_sp and ss_size are ignored. Otherwise, the stack shall be enabled,
       and the ss_sp and ss_size members specify the new address and size of
       the stack.

       The range of addresses starting at ss_sp up to but not including
       ss_sp+ss_size is available to the implementation for use as the
       stack. This function makes no assumptions regarding which end is the
       stack base and in which direction the stack grows as items are

       If oss is not a null pointer, upon successful completion it shall
       point to a stack_t structure that specifies the alternate signal
       stack that was in effect prior to the call to sigaltstack().  The
       ss_sp and ss_size members specify the address and size of that stack.
       The ss_flags member specifies the stack's state, and may contain one
       of the following values:

       SS_ONSTACK  The process is currently executing on the alternate
                   signal stack.  Attempts to modify the alternate signal
                   stack while the process is executing on it fail. This
                   flag shall not be modified by processes.

       SS_DISABLE  The alternate signal stack is currently disabled.

       The value SIGSTKSZ is a system default specifying the number of bytes
       that would be used to cover the usual case when manually 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 system implementation overhead. The
       defined in <signal.h>.

       After a successful call to one of the exec functions, there are no
       alternate signal stacks in the new process image.

       In some implementations, a signal (whether or not indicated to
       execute on the alternate stack) shall always execute on the alternate
       stack if it is delivered while another signal is being caught using
       the alternate stack.

       Use of this function by library threads that are not bound to kernel-
       scheduled entities results in undefined behavior.

RETURN VALUE         top

       Upon successful completion, sigaltstack() shall return 0; otherwise,
       it shall return −1 and set errno to indicate the error.

ERRORS         top

       The sigaltstack() function shall fail if:

       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.

       The following sections are informative.

EXAMPLES         top

   Allocating Memory for an Alternate Stack
       The following example illustrates a method for allocating memory for
       an alternate stack.

           #include <signal.h>
           if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
               /* Error return. */
           sigstk.ss_size = SIGSTKSZ;
           sigstk.ss_flags = 0;
           if (sigaltstack(&sigstk,(stack_t *)0) < 0)


       On some implementations, stack space is automatically extended as
       needed. On those implementations, automatic extension is typically
       not available for an alternate stack. If the stack overflows, the
       behavior is undefined.

RATIONALE         top




SEE ALSO         top

       Section 2.4, Signal Concepts, exec(1p), sigaction(3p), sigsetjmp(3p)

       The Base Definitions volume of POSIX.1‐2008, signal.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see .

IEEE/The Open Group                 2013                     SIGALTSTACK(3P)