makecontext man page on Manjaro

Man page or keyword search:  
man Server   11224 pages
apropos Keyword Search (all sections)
Output format
Manjaro logo
[printable version]


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(3), setcontext(3), make‐
       context()  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(3).

       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  swapcon‐
       text())	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 swapcon‐
       text() 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.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The makecontext() and swapcontext() functions are thread-safe.

CONFORMING TO
       SUSv2, POSIX.1-2001.  POSIX.1-2008 removes the specifications of	 make‐
       context()  and swapcontext(), citing portability issues, and recommend‐
       ing 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.	  How‐
       ever, doing this is not guaranteed to be portable, is undefined accord‐
       ing 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(), to	permit	this  on  some	64-bit
       architectures (e.g., x86-64).

EXAMPLE
       The  example program below demonstrates the use of getcontext(3), make‐
       context(), and swapcontext().  Running the program produces the follow‐
       ing 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
       sigaction(2),	 sigaltstack(2),     sigprocmask(2),	getcontext(3),
       sigsetjmp(3)

COLOPHON
       This page is part of release 3.65 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				  2014-04-14			MAKECONTEXT(3)
[top]

List of man pages available for Manjaro

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Tweet
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
....................................................................
Vote for polarhome
Free Shell Accounts :: the biggest list on the net