makecontext(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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 *restrict oucp,
                       const ucontext_t *restrict ucp);

DESCRIPTION         top

       In a System V-like environment, one has the type ucontext_t
       (defined in <ucontext.h> and described in getcontext(3)) 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.

       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 to indicate the error.

ERRORS         top

       ENOMEM Insufficient stack space left.

VERSIONS         top

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

ATTRIBUTES         top

       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         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
       instead.

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).

EXAMPLES         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
       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         top

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

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
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       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

GNU                            2021-03-22                 MAKECONTEXT(3)

Pages that refer to this page: sigaltstack(2)getcontext(3)signal(7)