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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

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