NAME

       getcontext, setcontext - get or set the user context

LIBRARY

       Standard C library (libc, -lc)

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_t {
               struct ucontext_t *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 to by  ucp  to  the  currently
       active context.

       The  function setcontext() restores the user context pointed to by ucp.  A successful call
       does not return.  The context should have been obtained by  a  call  of  getcontext(),  or
       makecontext(3),  or received as the third argument to a signal handler (see the discussion
       of the SA_SIGINFO flag in sigaction(2)).

       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 to indicate the error.

ERRORS

       None defined.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────┬───────────────┬──────────────────┐
       │Interface                                             │ Attribute     │ Value            │
       ├──────────────────────────────────────────────────────┼───────────────┼──────────────────┤
       │getcontext(), setcontext()                            │ Thread safety │ MT-Safe race:ucp │
       └──────────────────────────────────────────────────────┴───────────────┴──────────────────┘

STANDARDS

       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 their 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),
       signal(7)