NAME

       getcontext, setcontext - get and set current user context

SYNOPSIS

       #include <ucontext.h>

       int getcontext(ucontext_t *ucp);
       int setcontext(const ucontext_t *ucp);

DESCRIPTION

       The  getcontext() function shall initialize the structure pointed to by ucp to the current
       user context of the calling thread. The ucontext_t type that ucp  points  to  defines  the
       user  context  and  includes  the  contents of the calling thread's machine registers, the
       signal mask, and the current execution stack.

       The setcontext() function shall restore the user context pointed to by ucp.  A  successful
       call to setcontext() shall not return; program execution resumes at the point specified by
       the ucp argument passed to setcontext(). The ucp argument should be created  either  by  a
       prior call to getcontext() or makecontext(), or by being passed as an argument to a signal
       handler. If the ucp argument was created with getcontext(), program execution continues as
       if  the  corresponding  call  of  getcontext()  had just returned. If the ucp argument was
       created with makecontext(), program  execution  continues  with  the  function  passed  to
       makecontext().   When  that function returns, the thread shall continue as if after a call
       to setcontext() with the ucp argument that was input  to  makecontext().  If  the  uc_link
       member of the ucontext_t structure pointed to by the ucp argument is equal to 0, then this
       context is the main context, and the thread shall exit  when  this  context  returns.  The
       effects of passing a ucp argument obtained from any other source are unspecified.

RETURN VALUE

       Upon successful completion, setcontext() shall not return and getcontext() shall return 0;
       otherwise, a value of -1 shall be returned.

ERRORS

       No errors are defined.

       The following sections are informative.

EXAMPLES

       Refer to makecontext() .

APPLICATION USAGE

       When a signal handler is executed, the current user context is saved and a new context  is
       created.  If  the  thread  leaves the signal handler via longjmp(), then it is unspecified
       whether the context at the time of the corresponding setjmp() call is  restored  and  thus
       whether  future  calls  to  getcontext() provide an accurate representation of the current
       context, since the context restored by longjmp() does  not  necessarily  contain  all  the
       information  that  setcontext()  requires.  Signal  handlers  should  use  siglongjmp() or
       setcontext() instead.

       Conforming applications should not modify or access the uc_mcontext member of  ucontext_t.
       A conforming application cannot assume that context includes any process-wide static data,
       possibly including errno. Users manipulating contexts should take  care  to  handle  these
       explicitly when required.

       Use   of   contexts  to  create  alternate  stacks  is  not  defined  by  this  volume  of
       IEEE Std 1003.1-2001.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       bsd_signal() , makecontext() , setcontext() , setjmp() ,  sigaction()  ,  sigaltstack()  ,
       siglongjmp()   ,   sigprocmask()   ,   sigsetjmp()   ,  the  Base  Definitions  volume  of
       IEEE Std 1003.1-2001, <ucontext.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 .