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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 .