sigwait

PTHREAD_SIGNAL(3)                                            PTHREAD_SIGNAL(3)



NAME
       pthread_sigmask, pthread_kill, sigwait - handling of signals in threads


SYNOPSIS
       #include <pthread.h>
       #include <signal.h>

       int pthread_sigmask(int how, const sigset_t  *newmask,  sigset_t  *old-
       mask);

       int pthread_kill(pthread_t thread, int signo);

       int sigwait(const sigset_t *set, int *sig);


DESCRIPTION
       pthread_sigmask  changes  the  signal  mask  for  the calling thread as
       described by the how and newmask arguments. If oldmask is not NULL, the
       previous signal mask is stored in the location pointed to by oldmask.

       The  meaning  of  the how and newmask arguments is the same as for sig-
       procmask(2).  If how is SIG_SETMASK, the signal mask is set to newmask.
       If  how is SIG_BLOCK, the signals specified to newmask are added to the
       current signal mask.  If how is SIG_UNBLOCK, the signals  specified  to
       newmask are removed from the current signal mask.

       Recall  that  signal  masks  are  set on a per-thread basis, but signal
       actions and signal handlers,  as  set  with  sigaction(2),  are  shared
       between all threads.

       pthread_kill send signal number signo to the thread thread.  The signal
       is delivered and handled as described in kill(2).

       sigwait suspends the calling thread until one of the signals in set  is
       delivered  to the calling thread. It then stores the number of the sig-
       nal received in the location pointed to by sig and returns. The signals
       in  set must be blocked and not ignored on entrance to sigwait.  If the
       delivered signal has a signal handler function attached, that  function
       is not called.


CANCELLATION
       sigwait is a cancellation point.


RETURN VALUE
       On  success,  0  is  returned.  On  failure,  a  non-zero error code is
       returned.


ERRORS
       The pthread_sigmask function  returns  the  following  error  codes  on
       error:

              EINVAL how is not one of SIG_SETMASK, SIG_BLOCK, or SIG_UNBLOCK


              EFAULT newmask or oldmask point to invalid addresses

       The pthread_kill function returns the following error codes on error:

              EINVAL signo is not a valid signal number


              ESRCH  the  thread  thread  does  not exist (e.g. it has already
                     terminated)

       The sigwait function never returns an error.


AUTHOR
       Xavier Leroy <Xavier.Leroy@inria.fr>


SEE ALSO
       sigprocmask(2), kill(2), sigaction(2), sigsuspend(2).


NOTES
       For sigwait to work reliably, the signals  being  waited  for  must  be
       blocked in all threads, not only in the calling thread, since otherwise
       the POSIX semantics for signal delivery do not guarantee that it's  the
       thread doing the sigwait that will receive the signal.  The best way to
       achieve this is block those signals before any threads are created, and
       never unblock them in the program other than by calling sigwait.


BUGS
       Signal  handling  in  LinuxThreads departs significantly from the POSIX
       standard. According to the standard, ``asynchronous''  (external)  sig-
       nals  are  addressed  to  the  whole  process  (the  collection  of all
       threads), which then delivers them to one particular thread. The thread
       that actually receives the signal is any thread that does not currently
       block the signal.

       In LinuxThreads, each thread is actually a kernel process with its  own
       PID,  so external signals are always directed to one particular thread.
       If, for instance, another thread is blocked in sigwait on that  signal,
       it will not be restarted.

       The  LinuxThreads  implementation of sigwait installs dummy signal han-
       dlers for the signals in set for the duration of the wait. Since signal
       handlers  are shared between all threads, other threads must not attach
       their own signal handlers  to  these  signals,  or  alternatively  they
       should  all block these signals (which is recommended anyway -- see the
       Notes section).



                                 LinuxThreads                PTHREAD_SIGNAL(3)