SIGQUEUE(P) POSIX Programmer's Manual SIGQUEUE(P)NAMEsigqueue - queue a signal to a process (REALTIME)
SYNOPSIS
#include <signal.h>
int sigqueue(pid_t pid, int signo, const 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() 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.
RETURN VALUE
Upon successful completion, the specified signal shall have been
queued, and the sigqueue() function shall return a value of zero. Oth‐
erwise, the function shall return a value of -1 and set errno to indi‐
cate 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 the appropriate privilege to send the
signal to the receiving process.
ESRCH The process pid does not exist.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
The sigqueue() function allows an application to queue a realtime sig‐
nal 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().
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 implemen‐
tation 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 seman‐
tics with respect to the null signal as kill(), and that the same per‐
mission 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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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
Realtime Signals , the Base Definitions volume of IEEE Std 1003.1-2001,
<signal.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SIGQUEUE(P)
