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

       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.

       sigaltstack — set and get signal alternate stack context

       #include <signal.h>

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

       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 pushed.

       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 constants

       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.

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

       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.

   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.



       Section 2.4, Signal Concepts, exec, sigaction(), sigsetjmp()

       The Base Definitions volume of POSIX.1‐2008, <signal.h>

       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(3POSIX)