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)