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 <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

       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, Copyright (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.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                        SIGALTSTACK(P)