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



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

NAME
       sigaltstack -- set and get signal alternate stack context

SYNOPSIS
       #include <signal.h>

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

DESCRIPTION
       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
       SS_ONSTACK, SS_DISABLE, SIGSTKSZ, and MINSIGSTKSZ are 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
       Upon successful completion, sigaltstack() shall return 0; otherwise, it
       shall return -1 and set errno to indicate the error.

ERRORS
       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
   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)
               perror("sigaltstack");

APPLICATION USAGE
       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
       None.

FUTURE DIRECTIONS
       None.

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

       The Base Definitions volume of POSIX.1-2017, <signal.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  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 http://www.opengroup.org/unix/online.html .

       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                       SIGALTSTACK(3)
