MAKECONTEXT(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
makecontext, swapcontext - manipulate user contexts

SYNOPSIS

#include <ucontext.h>

void makecontext(ucontext_t *ucp, void (*func)(void),
int argc, ...);
int swapcontext(ucontext_t *restrict oucp,
const ucontext_t *restrict ucp);

DESCRIPTION

The makecontext() function shall modify the context specified by ucp, which has been initialized using getcontext(). When this context is resumed using swapcontext() or setcontext(), program execution shall continue by calling func, passing it the arguments that follow argc in the makecontext() call.

Before a call is made to makecontext(), the application shall ensure that the context being modified has a stack allocated for it. The application shall ensure that the value of argc matches the number of arguments of type int passed to func; otherwise, the behavior is undefined.

The uc_link member is used to determine the context that shall be resumed when the context being modified by makecontext() returns. The application shall ensure that the uc_link member is initialized prior to the call to makecontext().

The swapcontext() function shall save the current context in the context structure pointed to by oucp and shall set the context to the context structure pointed to by ucp.

RETURN VALUE

Upon successful completion, swapcontext() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error.

ERRORS

The swapcontext() function shall fail if:

ENOMEM
    The ucp argument does not have enough stack left to complete the operation.

The following sections are informative.

EXAMPLES

The following example illustrates the use of makecontext():

    #include <stdio.h>
    #include <ucontext.h>

    static ucontext_t ctx[3];

    static void
    f1 (void)
    {
        puts("start f1");
        swapcontext(&ctx[1], &ctx[2]);
        puts("finish f1");
    }

    static void
    f2 (void)
    {
        puts("start f2");
        swapcontext(&ctx[2], &ctx[1]);
        puts("finish f2");
    }

    int
    main (void)
    {
        char st1[8192];
        char st2[8192];

        getcontext(&ctx[1]);
        ctx[1].uc_stack.ss_sp = st1;
        ctx[1].uc_stack.ss_size = sizeof st1;
        ctx[1].uc_link = &ctx[0];
        makecontext(&ctx[1], f1, 0);

        getcontext(&ctx[2]);
        ctx[2].uc_stack.ss_sp = st2;
        ctx[2].uc_stack.ss_size = sizeof st2;
        ctx[2].uc_link = &ctx[1];
        makecontext(&ctx[2], f2, 0);

        swapcontext(&ctx[0], &ctx[2]);
        return 0;
    }

APPLICATION USAGE

None.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

exit(), getcontext(), sigaction(), sigprocmask(), the Base Definitions volume of IEEE Std 1003.1-2001, <ucontext.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .