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 .