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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
.TH "SIGQUEUE" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" sigqueue
.SH NAME
sigqueue \- queue a signal to a process (\fBREALTIME\fP)
.SH SYNOPSIS
.LP
\fB#include <signal.h>
.br
.sp
int sigqueue(pid_t\fP \fIpid\fP\fB, int\fP \fIsigno\fP\fB, const union
sigval\fP \fIvalue\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIsigqueue\fP() function shall cause the signal specified by
\fIsigno\fP to be sent with the value specified by
\fIvalue\fP to the process specified by \fIpid\fP. If \fIsigno\fP
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 \fIpid\fP.
.LP
The conditions required for a process to have permission to queue
a signal to another process are the same as for the \fIkill\fP() function.
.LP
The \fIsigqueue\fP() function shall return immediately. If SA_SIGINFO
is set for \fIsigno\fP 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
\fIsigno\fP, then \fIsigno\fP shall be sent at least once to the receiving
process; it is unspecified whether \fIvalue\fP shall
be sent to the receiving process as a result of this call.
.LP
If the value of \fIpid\fP causes \fIsigno\fP to be generated for the
sending process, and if \fIsigno\fP is not blocked for
the calling thread and if no other thread has \fIsigno\fP unblocked
or is waiting in a \fIsigwait\fP() function for \fIsigno\fP, either
\fIsigno\fP or at least the pending, unblocked
signal shall be delivered to the calling thread before the \fIsigqueue\fP()
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.
.SH RETURN VALUE
.LP
Upon successful completion, the specified signal shall have been queued,
and the \fIsigqueue\fP() function shall return a value
of zero. Otherwise, the function shall return a value of -1 and set
\fIerrno\fP to indicate the error.
.SH ERRORS
.LP
The \fIsigqueue\fP() function shall fail if:
.TP 7
.B 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.
.TP 7
.B EINVAL
The value of the \fIsigno\fP argument is an invalid or unsupported
signal number.
.TP 7
.B EPERM
The process does not have the appropriate privilege to send the signal
to the receiving process.
.TP 7
.B ESRCH
The process \fIpid\fP does not exist.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
The \fIsigqueue\fP() 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 \fIsig\fP... name space already
carved out for signals was preferable to extending the
interface to \fIkill\fP().
.LP
Such a function became necessary when the put/get event function of
the message queues was removed. It should be noted that the
\fIsigqueue\fP() 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 \fIpid\fP
is resolved into a target process. Such access checks were
necessary only at message queue open in the previous interface.
.LP
The standard developers required that \fIsigqueue\fP() have the same
semantics with respect to the null signal as \fIkill\fP(), and that
the same permission checking be used. But because of the difficulty
of
implementing the "broadcast" semantic of \fIkill\fP() (for example,
to process groups) and
the interaction with resource allocation, this semantic was not adopted.
The \fIsigqueue\fP() function queues a signal to a single
process specified by the \fIpid\fP argument.
.LP
The \fIsigqueue\fP() 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.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIRealtime Signals\fP , the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<signal.h>\fP
.SH 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 .
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