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MAKECONTEXT(3P) 		    POSIX Programmer's Manual			  MAKECONTEXT(3P)

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

       makecontext, swapcontext - manipulate user contexts

       #include <ucontext.h>

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

       The  makecontext() function shall modify the context specified by ucp, which has been ini-
       tialized using getcontext().  When this context is resumed using swapcontext() or  setcon-
       text(),	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  behav-
       ior 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.

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

       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.

       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");

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

		  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);

		  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;




       exit(),	 getcontext(),	 sigaction(),  sigprocmask(),  the  Base  Definitions  volume  of
       IEEE Std 1003.1-2001, <ucontext.h>

       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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				  MAKECONTEXT(3P)
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