setcontext(3)

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.

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 │
   └───────────────────────────┴───────────────┴──────────────────┘

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 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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