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

NAME         top

       makecontext, swapcontext - manipulate user context

SYNOPSIS         top

       #include <ucontext.h>

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

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

DESCRIPTION         top

       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 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         top

       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.

ERRORS         top

       ENOMEM Insufficient stack space left.

VERSIONS         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface     Attribute     Value                      │
       │makecontext() │ Thread safety │ MT-Safe race:ucp           │
       │swapcontext() │ Thread safety │ MT-Safe race:oucp race:ucp │

CONFORMING TO         top

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

NOTES         top

       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         top

       The example program below demonstrates the use of getcontext(3),
       makecontext(), and swapcontext().  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");

SEE ALSO         top

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

COLOPHON         top

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GNU                              2015-03-02                   MAKECONTEXT(3)