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.

VERSIONS

       makecontext() and swapcontext() are provided since glibc 2.1.

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

       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 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)