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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