sigqueue(2) System Calls Manual sigqueue(2)
sigqueue() - queue a signal to a process
The system call causes 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 system call.
The system call returns immediately. If is set for signo at the receiving process (see sigqueue(2)) and if resources are available to
queue the signal, the signal will be queued and sent to the receiving process. When the signal is delivered or accepted, the field
si_value of the siginfo parameter (see signal(5)) will be set to value. If is not set for signo, then signo, but not necessarily value,
will be sent at least once to the receiving process.
If the value of pid causes signo to be generated for the sending process, and if signo is not blocked, either signo or at least one pending
unblocked signal will be delivered to the sending process before the system call returns. Should any of multiple pending signals in the
range to be selected for delivery or acceptance, it will be the lowest numbered one. The selection order between realtime and non-realtime
signals, or between multiple pending non-realtime signals, is unspecified.
can be used to post signals to another process but can not be used to post signals to a specific thread in another process.
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 function for signo, either signo or at least one pending unblocked signal will be
delivered to the calling thread before the function returns.
Signals can not be posted to specific Lightweight Processes (LWPs) in another process.
Some or all of the actions associated with this system call are subject to compartmental restrictions. See compartments(5) for more infor-
mation about compartmentalization on systems that support that feature. Compartmental restrictions can be overridden if the process pos-
sesses the privilege (COMMALLOWED). Processes owned by the superuser may not have this privilege. Processes owned by any user may have
this privilege, depending on system configuration.
Some or all of the actions associated with this system call require one or more privileges. Processes owned by the superuser have many,
though not all, privileges. Processes owned by other users may have privilege(s), depending on system configuration. See privileges(5)
for more information about privileged access on systems that support fine-grained privileges.
Upon successful completion, the specified signal will be queued, and the function returns a value of (zero). Otherwise, a value of is
returned, and is set to indicate the error.
fails and no signal is sent if any of the following conditions occur:
No resources are available to queue the signal.
The process has already queued signals that are still pending at the receiver(s), or a systemwide resource limit has
The value of the
signo argument is an invalid or unsupported signal number.
The process does not have the appropriate privilege
to send the signal to the receiving process.
The process pid does not exist.
kill(2), sysconf(2), privileges(5), signal(5).