MAKECONTEXT(3)                                      Linux Programmer's Manual                                     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, 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  con-
       trol 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(), 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 guaran-
       teed 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 3.25 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/.



GNU                                                        2009-03-31                                             MAKECONTEXT(3)