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),
       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 make-
       context(),  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 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 makecontext() 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.	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).

EXAMPLE

       The  example  program below demonstrates the use of getcontext(3), makecontext(), and swapcontext().  Running the program produces the fol-
       lowing 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
");
	   printf("func1: swapcontext(&uctx_func1, &uctx_func2)
");
	   if (swapcontext(&uctx_func1, &uctx_func2) == -1)
	       handle_error("swapcontext");
	   printf("func1: returning
");
       }

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

       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)
");
	   if (swapcontext(&uctx_main, &uctx_func2) == -1)
	       handle_error("swapcontext");

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

SEE ALSO

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

COLOPHON

       This page is part of release 4.15 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
								    2017-09-15							    MAKECONTEXT(3)