getcontext(2)							System Calls Manual						     getcontext(2)

NAME

       getcontext, setcontext - Initiates and restores user level context switching

SYNOPSIS

       #include <ucontext.h>

       int  getcontext(
	       ucontext_t *ucp ); int setcontext(
	       const ucontext_t *ucp );

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       getcontext(), setcontext():  XSH5.0

       Refer to the standards(5) reference page for more information about industry standards and associated tags.

PARAMETERS

       Provides a pointer to a ucontext structure, defined in the <ucontext.h> header file.  The ucontext structure contains the signal mask, exe-
       cution stack, and machine registers. (See ucontext(5) for more information about the format of the ucontext structure.)

DESCRIPTION

       Using both the getcontext() and setcontext() functions enables you to initiate user  level  context  control,  switching  between  multiple
       threads of control within a single process.

       When you call getcontext(), it initializes the ucp argument to the current user context of the calling process.

       Use  the setcontext() function to restore the state of the user context pointed to by the ucp argument.	The setcontext() function, if suc-
       cessful, does not return; application execution continues from the point specified by the ucontext structure you pass to  the  setcontext()
       function.

       The  ucontext  structure  that  you  pass to the setcontext() function must have been created by a call to the getcontext() function or the
       makecontext() function, or have been passed as the third argument to a signal handler.  (The third argument in a call  to  the  sigaction()
       function determines the action to be performed when a signal is delivered. For more information, see sigaction(2).)

       When  a	context  structure is created by the getcontext() function, execution of the program continues as if the corresponding call of the
       getcontext() function had just returned.

       When a context structure is created by the makecontext() function, program execution continues with the function passed	to  makecontext().
       When  that  function returns, the thread continues as if after a call to setcontext() with the context structure argument that was input to
       makecontext().

       If the uc_link member of the ucontext_t structure pointed to by the ucp argument is 0 (zero), then this context is the  main  context,  and
       the thread will exit when this context returns.	The effects of passing a ucp argument from any other source are unspecified.

NOTES

       When  a	signal	handler executes, the current user context is saved and a new context is created by the kernel.  If the process leaves the
       signal handler using the longjmp() function, the original context cannot be restored, and the result of future calls  to  the  getcontext()
       function are unpredictable.  Use the siglongjmp() or setcontext() functions in signal handlers, instead of the longjmp() function.

RETURN VALUES

       The  setcontext()  function does not return upon success.  The getcontext() function returns 0 (zero) upon success.  Upon failure, both the
       setcontext() and getcontext() functions return a value of -1.

SEE ALSO

       Functions:  bsd_signal(2), makecontext(2), sigaction(2), sigaltstack(2), sigprocmask(2), setjmp(3), sigsetjmp(3)

       Files:  ucontext(5)

       Standards:  standards(5)

																     getcontext(2)