GETCONTEXT(2)                                       Linux Programmer's Manual                                      GETCONTEXT(2)



NAME
       getcontext, setcontext - get or set the user context

SYNOPSIS
       #include <ucontext.h>

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

DESCRIPTION
       In  a  System  V-like  environment,  one has the two types mcontext_t and ucontext_t defined in <ucontext.h> and the four
       functions getcontext(), setcontext(), makecontext(3) and swapcontext(3) that allow user-level context  switching  between
       multiple threads of control within a process.

       The  mcontext_t type is machine-dependent and opaque.  The ucontext_t type is a structure that has at least the following
       fields:

           typedef struct ucontext {
               struct ucontext *uc_link;
               sigset_t         uc_sigmask;
               stack_t          uc_stack;
               mcontext_t       uc_mcontext;
               ...
           } ucontext_t;

       with sigset_t and stack_t defined in <signal.h>.  Here uc_link points to the context that will be resumed when  the  cur-
       rent  context terminates (in case the current context was created using makecontext(3)), uc_sigmask is the set of signals
       blocked in this context (see sigprocmask(2)), uc_stack is the stack  used  by  this  context  (see  sigaltstack(2)),  and
       uc_mcontext  is the machine-specific representation of the saved context, that includes the calling thread's machine reg-
       isters.

       The function getcontext() initializes the structure pointed at by ucp to the currently active context.

       The function setcontext() restores the user context pointed at by ucp.  A successful call does not return.   The  context
       should have been obtained by a call of getcontext(), or makecontext(3), or passed as third argument to a signal handler.

       If the context was obtained by a call of getcontext(), program execution continues as if this call just returned.

       If the context was obtained by a call of makecontext(3), program execution continues by a call to the function func spec-
       ified as the second argument of that call to makecontext(3).  When the  function  func  returns,  we  continue  with  the
       uc_link  member of the structure ucp specified as the first argument of that call to makecontext(3).  When this member is
       NULL, the thread exits.

       If the context was obtained by a call to a signal handler, then old standard text says that "program execution  continues
       with the program instruction following the instruction interrupted by the signal".  However, this sentence was removed in
       SUSv2, and the present verdict is "the result is unspecified".

RETURN VALUE
       When successful, getcontext() returns 0 and setcontext() does not return.  On error, both return -1 and set errno  appro-
       priately.

ERRORS
       None defined.

CONFORMING TO
       SUSv2, POSIX.1-2001.  POSIX.1-2008 removes the specification of getcontext(), citing portability issues, and recommending
       that applications be rewritten to use POSIX threads instead.

NOTES
       The earliest incarnation of this mechanism was the setjmp(3)/longjmp(3) mechanism.  Since that does not define  the  han-
       dling  of  the  signal context, the next stage was the sigsetjmp(3)/siglongjmp(3) pair.  The present mechanism gives much
       more control.  On the other hand, there is no easy way to detect whether a return from getcontext()  is  from  the  first
       call,  or  via  a setcontext() call.  The user has to invent her own bookkeeping device, and a register variable won't do
       since registers are restored.

       When a signal occurs, the current user context is saved and a new context is created by the kernel for  the  signal  han-
       dler.   Do not leave the handler using longjmp(3): it is undefined what would happen with contexts.  Use siglongjmp(3) or
       setcontext() instead.

SEE ALSO
       sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecontext(3), 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/.



Linux                                                      2009-03-15                                              GETCONTEXT(2)