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MAKECONTEXT(3)			    Linux Programmer's Manual			   MAKECONTEXT(3)

       makecontext, swapcontext - manipulate user context

       #include <ucontext.h>

       void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);

       int swapcontext(ucontext_t *oucp, ucontext_t *ucp);

       In  a  System  V-like environment, one has the type ucontext_t defined in <ucontext.h> and
       the four functions getcontext(3),  setcontext(3),  makecontext()  and  swapcontext()  that
       allow user-level context switching between multiple threads of control within a process.

       For the type and the first two functions, see getcontext(3).

       The makecontext() function modifies the context pointed to by ucp (which was obtained from
       a call to getcontext(3)).  Before invoking makecontext(), the caller must allocate  a  new
       stack  for  this  context  and assign its address to ucp->uc_stack, and define a successor
       context and assign its address to ucp->uc_link.

       When this context is later activated (using setcontext(3) or swapcontext())  the  function
       func  is  called,  and  passed the series of integer (int) arguments that follow argc; the
       caller must specify the number of these arguments in argc.  When  this  function  returns,
       the  successor context is activated.  If the successor context pointer is NULL, the thread

       The swapcontext() function saves the current context in the structure pointed to by  oucp,
       and then activates the context pointed to by ucp.

       When successful, swapcontext() does not return.	(But we may return later, in case oucp is
       activated, in which case it looks like swapcontext() returns 0.)  On error,  swapcontext()
       returns -1 and sets errno appropriately.

       ENOMEM Insufficient stack space left.

       makecontext() and swapcontext() are provided in glibc since version 2.1.

       SUSv2,  POSIX.1-2001.   POSIX.1-2008 removes the specifications of makecontext() and swap-
       context(), citing portability issues, and recommending that applications be  rewritten  to
       use POSIX threads instead.

       The interpretation of ucp->uc_stack is just as in sigaltstack(2), namely, this struct con-
       tains the start and length of a memory area to be used as the  stack,  regardless  of  the
       direction of growth of the stack.  Thus, it is not necessary for the user program to worry
       about this direction.

       On architectures where int and pointer types are the same size (e.g., x86-32,  where  both
       types  are  32  bits),  you  may be able to get away with passing pointers as arguments to
       makecontext() following argc.  However, doing this is not guaranteed to	be  portable,  is
       undefined  according  to the standards, and won't work on architectures where pointers are
       larger than ints.  Nevertheless, starting with version 2.8, glibc makes	some  changes  to
       makecontext(), to permit this on some 64-bit architectures (e.g., x86-64).

       The  example program below demonstrates the use of getcontext(3), makecontext(), and swap-
       context().  Running the program produces the following output:

	   $ ./a.out
	   main: swapcontext(&uctx_main, &uctx_func2)
	   func2: started
	   func2: swapcontext(&uctx_func2, &uctx_func1)
	   func1: started
	   func1: swapcontext(&uctx_func1, &uctx_func2)
	   func2: returning
	   func1: returning
	   main: exiting

   Program source

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

       static ucontext_t uctx_main, uctx_func1, uctx_func2;

       #define handle_error(msg) \
	   do { perror(msg); exit(EXIT_FAILURE); } while (0)

       static void
	   printf("func1: started\n");
	   printf("func1: swapcontext(&uctx_func1, &uctx_func2)\n");
	   if (swapcontext(&uctx_func1, &uctx_func2) == -1)
	   printf("func1: returning\n");

       static void
	   printf("func2: started\n");
	   printf("func2: swapcontext(&uctx_func2, &uctx_func1)\n");
	   if (swapcontext(&uctx_func2, &uctx_func1) == -1)
	   printf("func2: returning\n");

       main(int argc, char *argv[])
	   char func1_stack[16384];
	   char func2_stack[16384];

	   if (getcontext(&uctx_func1) == -1)
	   uctx_func1.uc_stack.ss_sp = func1_stack;
	   uctx_func1.uc_stack.ss_size = sizeof(func1_stack);
	   uctx_func1.uc_link = &uctx_main;
	   makecontext(&uctx_func1, func1, 0);

	   if (getcontext(&uctx_func2) == -1)
	   uctx_func2.uc_stack.ss_sp = func2_stack;
	   uctx_func2.uc_stack.ss_size = sizeof(func2_stack);
	   /* Successor context is f1(), unless argc > 1 */
	   uctx_func2.uc_link = (argc > 1) ? NULL : &uctx_func1;
	   makecontext(&uctx_func2, func2, 0);

	   printf("main: swapcontext(&uctx_main, &uctx_func2)\n");
	   if (swapcontext(&uctx_main, &uctx_func2) == -1)

	   printf("main: exiting\n");

       sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3), sigsetjmp(3)

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

GNU					    2013-02-12				   MAKECONTEXT(3)
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