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makecontext (3)
  • makecontext (3) ( Solaris man: Библиотечные вызовы )
  • makecontext (3) ( FreeBSD man: Библиотечные вызовы )
  • makecontext (3) ( Русские man: Библиотечные вызовы )
  • >> makecontext (3) ( Linux man: Библиотечные вызовы )
  • makecontext (3) ( POSIX man: Библиотечные вызовы )
  •  

    NAME

    makecontext, swapcontext - manipulate user context
     
    

    SYNOPSIS

    #include <ucontext.h>

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

    int swapcontext(ucontext_t *oucp, 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(2), setcontext(2), 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(2).

    The makecontext() function modifies the context pointed to by ucp (which was obtained from a call to getcontext(2)). 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(2) 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.  

    CONFORMING TO

    SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specifications of makecontext() and swapcontext().  

    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(3), to permit this on some 64-bit architectures (e.g., x86-64).  

    EXAMPLE

    The example program below demonstrates the use of getcontext(2), 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

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

    COLOPHON

    This page is part of release 3.14 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


     

    Index

    NAME
    SYNOPSIS
    DESCRIPTION
    RETURN VALUE
    ERRORS
    VERSIONS
    CONFORMING TO
    NOTES
    EXAMPLE
    Program source
    SEE ALSO
    COLOPHON


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