#include <signal.h> int sigqueue(pid_t pid, int signo, union sigval value);
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.
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 POSIX.1-2017 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 POSIX.1-2017 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.
The Base Definitions volume of POSIX.1-2017, <signal.h>
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