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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, const 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), 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.

       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

       ENOMEM Insufficient stack space left.

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

   Multithreading (see pthreads(7))
       The makecontext() and swapcontext() functions are thread-safe.

       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.

       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).

       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
           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),

       This page is part of release 3.74 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

GNU                               2014-05-28                    MAKECONTEXT(3)

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