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 con-
       text 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 Technol-
       ogy -- 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)