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), 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
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

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
 | Thread safety | MT-Safe race:ucp
 | Thread safety | MT-Safe race:oucp race:ucp

CONFORMING TO
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
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

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

COLOPHON
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