Provided by: glibc-doc_2.9-4ubuntu6_all
pthread_sigmask, pthread_kill, sigwait - handling of signals in threads
int pthread_sigmask(int how, const sigset_t *newmask, sigset_t
int pthread_kill(pthread_t thread, int signo);
int sigwait(const sigset_t *set, int *sig);
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
sigprocmask(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
signal 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.
sigwait is a cancellation point.
On success, 0 is returned. On failure, a non-zero error code is
The pthread_sigmask function returns the following error codes on
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
The sigwait function never returns an error.
Xavier Leroy <Xavier.Leroy@inria.fr>
sigprocmask(2), kill(2), sigaction(2), sigsuspend(2).
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.
Signal handling in LinuxThreads departs significantly from the POSIX
standard. According to the standard, ‘‘asynchronous’’ (external)
signals 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
handlers 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).