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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 current 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
       registers.

       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 specified 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 appropriately.

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 handling 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 handler.  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 3.23 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/.