NAME
sigqueue — queue a
signal to a process
SYNOPSIS
int sigqueue(pid_t pid, int signo, union sigval value);
DESCRIPTION
The sigqueue () function shall cause 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(3) 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(3) 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.
RETURN VALUE
Upon successful completion, the specified signal shall have been
queued, and the sigqueue () function shall return a
value of zero. Otherwise, the function shall return a value of -1 and set
errno to
indicate the error.
ERRORS
The sigqueue () function shall fail
if:
- [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 system-wide 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 appropriate privileges to send the signal to the receiving process.
- [ESRCH]
- The process pid does not exist.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
The sigqueue () function allows an
application to queue a realtime signal to itself or to another process,
specifying the application-defined value. This is common practice in
realtime applications on existing realtime systems. It was felt that
specifying another function in the
sig... name
space already carved out for signals was preferable to extending the
interface to kill(3).
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(3), and that the same permission
checking be used. But because of the difficulty of implementing the
"broadcast" semantic of kill(3) (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.1-2024 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.1-2024 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.
FUTURE DIRECTIONS
None.
SEE ALSO
XBD <signal.h>
CHANGE HISTORY
First released in Issue 5. Included for alignment with the POSIX Realtime Extension and the POSIX Threads Extension.
Issue 6
The sigqueue () function is marked as part
of the Realtime Signals Extension option.
The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Realtime Signals Extension option.
Issue 7
The sigqueue () function is moved from the
Realtime Signals Extension option to the Base.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0332 [844] is applied.