sigqueue(2)

NAME

sigqueue() — queue a signal to a process

SYNOPSIS

#include <signal.h>

int sigqueue(pid_t pid, int signo, const union sigval value);

DESCRIPTION

The sigqueue() 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 kill() system call.

The sigqueue() system call returns immediately. If SA_SIGINFO 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 SA_SIGINFO 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 sigqueue() system call returns. Should any of multiple pending signals in the range SIGRTMIN to SIGRTMAX 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.

Application Usage

Threads Considerations

sigqueue() 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 sigwait() function for signo, either signo or at least one pending unblocked signal will be delivered to the calling thread before the sigqueue() function returns.

LWP Considerations

Signals can not be posted to specific Lightweight Processes (LWPs) in another process.

RETURN VALUE

Upon successful completion, the specified signal will be queued, and the sigqueue() function returns a value of 0 (zero). Otherwise, a value of -1 is returned, and errno is set to indicate the error.

ERRORS

sigqueue() fails and no signal is sent if any of the following conditions occur:

[EAGAIN]: No resources are available to queue the signal. The process has already queued {SIGQUEUE_MAX} signals that are still pending at the receiver(s), or a systemwide resource limit has been exceeded.

[EINVAL]: The value of the signo argument is an invalid or unsupported signal number.

[EPERM]: The process does not have the appropriate privilege to send the signal to the receiving process.

[ESRCH]: The process pid does not exist.

sigqueue(2)

Technical documentation

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