swapcontext


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 4.15 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)
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