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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 .