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



NAME
       makecontext, swapcontext - manipulate user context

SYNOPSIS
       #include <ucontext.h>

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

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

DESCRIPTION
       In  a System V-like environment, one has the type ucontext_t defined in
       <ucontext.h> and the four functions getcontext(3), setcontext(3), make-
       context()  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  swapcon-
       text())	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 exits.

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

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

ERRORS
       ENOMEM Insufficient stack space left.

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

ATTRIBUTES
       For  an	explanation  of	 the  terms  used   in	 this	section,   see
       attributes(7).

       +--------------+---------------+----------------------------+
       |Interface     | Attribute     | Value			   |
       +--------------+---------------+----------------------------+
       |makecontext() | Thread safety | MT-Safe race:ucp	   |
       +--------------+---------------+----------------------------+
       |swapcontext() | Thread safety | MT-Safe race:oucp race:ucp |
       +--------------+---------------+----------------------------+
CONFORMING TO
       SUSv2,  POSIX.1-2001.  POSIX.1-2008 removes the specifications of make-
       context() and swapcontext(), citing portability issues, and  recommend-
       ing that applications be rewritten to use POSIX threads instead.

NOTES
       The  interpretation  of	ucp->uc_stack  is  just	 as in sigaltstack(2),
       namely, this struct contains 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.  How-
       ever, doing this is not guaranteed to be portable, is undefined accord-
       ing  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).

EXAMPLE
       The example program below demonstrates the use of getcontext(3),	 make-
       context(), and swapcontext().  Running the program produces the follow-
       ing 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
       func1(void)
       {
	   printf("func1: started\n");
	   printf("func1: swapcontext(&uctx_func1, &uctx_func2)\n");
	   if (swapcontext(&uctx_func1, &uctx_func2) == -1)
	       handle_error("swapcontext");
	   printf("func1: returning\n");
       }

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

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

	   if (getcontext(&uctx_func1) == -1)
	       handle_error("getcontext");
	   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)
	       handle_error("getcontext");
	   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)
	       handle_error("swapcontext");

	   printf("main: exiting\n");
	   exit(EXIT_SUCCESS);
       }

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

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



GNU				  2015-03-02			MAKECONTEXT(3)