rtsigqueueinfo

rt_sigqueueinfo is a Linux system call used to queue a real-time signal to a specific process (or thread) with an accompanying value. Real-time signals are delivered in a defined order and can carry additional data through a sigval payload. When delivered, the receiver receives a siginfo_t structure containing the sender’s PID and UID, along with the si_value from the queued signal.

The typical usage is through the C library wrapper int rt_sigqueueinfo(pid_t pid, int sig, const union sigval *val); The sig parameter must be a real-time signal (within the SIGRTMIN to SIGRTMAX range). The val argument can provide a payload via union sigval, which can hold either an integer or a pointer. The target process must exist and the caller must have permission to signal it. If the target cannot be signaled or if the real-time signal queue is full, the call may fail with an appropriate error.

Delivery semantics are that the signal is queued for the target’s real-time signal queue and delivered in a first-in, first-out order relative to other real-time signals for that target. The accompanying data becomes available to the receiver in the siginfo_t si_value field when the signal is handled. RT signals and their queueing differ from standard signals in their guaranteed delivery order and the payload mechanism.

Related facilities include rt_tgsigqueueinfo for targeting a specific thread within a thread group, sigqueue for non-real-time signals, and the broader signal handling interfaces such as sigaction and SIGRTMIN/SIGRTMAX.