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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



sigqueue - queue a signal to a process


#include <sys/types.h>
#include <signal.h>

int sigqueue(pid_t pid, int signo, const union sigval value);
int sigqueue_wait(pid_t pid, int signo, const union sigval value,
     const struct timespec *timeout);


The sigqueue() and sigqueue_wait() functions cause the signal specified by signo to be sent with the value specified by value to the process specified by pid. If signo is 0 (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(2) function.

The sigqueue() function returns immediately. In contrast, sigqueue_wait() will wait until enough resources become available to queue the signal if the caller has the maximum number of queued signals allowed per process pending. If the timeout argument is not NULL, sigqueue_wait() will wait up to the specified time for the necessary resources to become available.

If SA_SIGINFO is set for signo and if the resources were available to queue the signal, the signal is queued and sent to the receiving process. If SA_SIGINFO is not set for signo, then signo is sent at least once to the receiving process; it is unspecified whether value will 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(2) function for signo, either signo or at least the pending, unblocked signal will be delivered to the calling thread before the sigqueue() function returns. Should any of multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected for delivery, 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.

Return Values

Upon successful completion, the specified signal will have been queued, and the sigqueue() and sigqueue_wait() functions return 0. Otherwise, the functions return −1 and set errno to indicate the error.


The sigqueue() and sigqueue_wait() functions will fail if:


No resources are available to queue the signal within the specified time, if any. The process has already queued its maximum number of signals that are still pending at the receiver(s), or a system wide resource limit has been exceeded. The maximum number of outstanding queued signals that a process can have is defined by its process.max-siqueue-size resource control.


The value of signo is an invalid or unsupported signal number or the timeout argument specifies an invalid time.


The process does not have the appropriate privilege to send the signal to the receiving process.


The process pid does not exist.

The sigqueue_wait() function will fail if:


The timeout argument to sigqueue_wait() is an invalid address.


The sigqueue_wait() function was interrupted while waiting.


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

kill(2), pthread_kill(3C), pthread_sigqueue(3C), sigwaitinfo(3C), siginfo.h(3HEAD), signal.h(3HEAD), attributes(7), resource-controls(7), standards(7)