focal (3) sigaltstack.3posix.gz

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PROLOG
       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
       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.1‐2008, <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
       http://www.unix.org/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 .