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



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

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, on 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  <sig-
       nal.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_DIS-
              ABLE.

       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
       Signal Concepts, sigaction(), sigsetjmp(), the Base Definitions volume of IEEE Std 1003.1-2001, <signal.h>

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
       Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                               SIGALTSTACK(3P)