swapcontext

MAKECONTEXT(P)             POSIX Programmer's Manual            MAKECONTEXT(P)



NAME
       makecontext, swapcontext - manipulate user contexts

SYNOPSIS
       #include <ucontext.h>

       void makecontext(ucontext_t *ucp, void (*func)(void),
              int argc, ...);
       int swapcontext(ucontext_t *restrict oucp,
              const ucontext_t *restrict ucp);


DESCRIPTION
       The makecontext() function shall modify the context specified by ucp,
       which has been initialized using getcontext().  When this context is
       resumed using swapcontext() or setcontext(), program execution shall
       continue by calling func, passing it the arguments that follow argc in
       the makecontext() call.

       Before a call is made to makecontext(), the application shall ensure
       that the context being modified has a stack allocated for it. The
       application shall ensure that the value of argc matches the number of
       arguments of type int passed to func; otherwise, the behavior is
       undefined.

       The uc_link member is used to determine the context that shall be
       resumed when the context being modified by makecontext() returns. The
       application shall ensure that the uc_link member is initialized prior
       to the call to makecontext().

       The swapcontext() function shall save the current context in the
       context structure pointed to by oucp and shall set the context to the
       context structure pointed to by ucp.

RETURN VALUE
       Upon successful completion, swapcontext() shall return 0. Otherwise, -1
       shall be returned and errno set to indicate the error.

ERRORS
       The swapcontext() function shall fail if:

       ENOMEM The ucp argument does not have enough stack left to complete the
              operation.


       The following sections are informative.

EXAMPLES
       The following example illustrates the use of makecontext():


              #include <stdio.h>
              #include <ucontext.h>


              static ucontext_t ctx[3];


              static void
              f1 (void)
              {
                  puts("start f1");
                  swapcontext(&ctx[1], &ctx[2]);
                  puts("finish f1");
              }


              static void
              f2 (void)
              {
                  puts("start f2");
                  swapcontext(&ctx[2], &ctx[1]);
                  puts("finish f2");
              }


              int
              main (void)
              {
                  char st1[8192];
                  char st2[8192];


                  getcontext(&ctx[1]);
                  ctx[1].uc_stack.ss_sp = st1;
                  ctx[1].uc_stack.ss_size = sizeof st1;
                  ctx[1].uc_link = &ctx[0];
                  makecontext(&ctx[1], f1, 0);


                  getcontext(&ctx[2]);
                  ctx[2].uc_stack.ss_sp = st2;
                  ctx[2].uc_stack.ss_size = sizeof st2;
                  ctx[2].uc_link = &ctx[1];
                  makecontext(&ctx[2], f2, 0);


                  swapcontext(&ctx[0], &ctx[2]);
                  return 0;
              }

APPLICATION USAGE
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       exit() , getcontext() , sigaction() , sigprocmask() , 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 .




IEEE/The Open Group                  2003                       MAKECONTEXT(P)