The sigqueue() function shall cause the signal specified by signo
to be sent with the value specified by value to the process specified
by pid. If signo is zero (the null signal), error checking is
performed but no signal is actually sent. The null signal can be used to check
the validity of pid.
The conditions required for a process to have permission to queue a signal to
another process are the same as for the kill() function.
The sigqueue() function shall return immediately. If SA_SIGINFO is set
for signo and if the resources were available to queue the signal, the
signal shall be queued and sent to the receiving process. If SA_SIGINFO is not
set for signo, then signo shall be sent at least once to the
receiving process; it is unspecified whether value shall be sent to the
receiving process as a result of this call.
If the value of pid causes signo to be generated for the sending
process, and if signo is not blocked for the calling thread and if no
other thread has signo unblocked or is waiting in a sigwait()
function for signo, either signo or at least the pending,
unblocked signal shall be delivered to the calling thread before the
sigqueue() function returns. Should any multiple pending signals in the
range SIGRTMIN to SIGRTMAX be selected for delivery, it shall be the lowest
numbered one. The selection order between realtime and non-realtime signals,
or between multiple pending non-realtime signals, is unspecified.
Upon successful completion, the specified signal shall have been queued, and the
sigqueue() function shall return a value of zero. Otherwise, the
function shall return a value of -1 and set errno to indicate the
The sigqueue() function allows an application to queue a realtime signal
to itself or to another process, specifying the application-defined value.
This is common practice in realtime applications on existing realtime systems.
It was felt that specifying another function in the sig... name space
already carved out for signals was preferable to extending the interface to
Such a function became necessary when the put/get event function of the message
queues was removed. It should be noted that the sigqueue() function
implies reduced performance in a security-conscious implementation as the
access permissions between the sender and receiver have to be checked on each
send when the pid is resolved into a target process. Such access checks
were necessary only at message queue open in the previous interface.
The standard developers required that sigqueue() have the same semantics
with respect to the null signal as kill(), and that the same permission
checking be used. But because of the difficulty of implementing the
"broadcast" semantic of kill() (for example, to process
groups) and the interaction with resource allocation, this semantic was not
adopted. The sigqueue() function queues a signal to a single process
specified by the pid argument.
The sigqueue() function can fail if the system has insufficient resources
to queue the signal. An explicit limit on the number of queued signals that a
process could send was introduced. While the limit is "per-sender",
this volume of IEEE Std 1003.1-2001 does not specify that the
resources be part of the state of the sender. This would require either that
the sender be maintained after exit until all signals that it had sent to
other processes were handled or that all such signals that had not yet been
acted upon be removed from the queue(s) of the receivers. This volume of
IEEE Std 1003.1-2001 does not preclude this behavior, but an
implementation that allocated queuing resources from a system-wide pool (with
per-sender limits) and that leaves queued signals pending after the sender
exits is also permitted.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html