makecontext(3)             Library Functions Manual             makecontext(3)

NAME
       makecontext, swapcontext - manipulate user context

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <ucontext.h>

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

DESCRIPTION
       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
       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
       ENOMEM Insufficient stack space left.

ATTRIBUTES
       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           |
       +---------------+---------------+--------------------------------------+
STANDARDS
       None.

HISTORY
       glibc 2.1.  SUSv2, POSIX.1-2001.  Removed in POSIX.1-2008, 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 glibc 2.8,
       glibc makes some changes to makecontext(), to permit this on some
       64-bit architectures (e.g., x86-64).

EXAMPLES
       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 <stdio.h>
       #include <stdlib.h>
       #include <ucontext.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("%s: started\n", __func__);
           printf("%s: swapcontext(&uctx_func1, &uctx_func2)\n", __func__);
           if (swapcontext(&uctx_func1, &uctx_func2) == -1)
               handle_error("swapcontext");
           printf("%s: returning\n", __func__);
       }

       static void
       func2(void)
       {
           printf("%s: started\n", __func__);
           printf("%s: swapcontext(&uctx_func2, &uctx_func1)\n", __func__);
           if (swapcontext(&uctx_func2, &uctx_func1) == -1)
               handle_error("swapcontext");
           printf("%s: returning\n", __func__);
       }

       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("%s: swapcontext(&uctx_main, &uctx_func2)\n", __func__);
           if (swapcontext(&uctx_main, &uctx_func2) == -1)
               handle_error("swapcontext");

           printf("%s: exiting\n", __func__);
           exit(EXIT_SUCCESS);
       }

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

Linux man-pages 6.7               2023-10-31                    makecontext(3)