sigqueue.3p (6161B)
- '\" et
- .TH SIGQUEUE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- sigqueue
- \(em queue a signal to a process
- .SH SYNOPSIS
- .LP
- .nf
- #include <signal.h>
- .P
- int sigqueue(pid_t \fIpid\fP, int \fIsigno\fP, union sigval \fIvalue\fP);
- .fi
- .SH DESCRIPTION
- The
- \fIsigqueue\fR()
- function shall cause the signal specified by
- .IR signo
- to be sent with the value specified by
- .IR value
- to the process specified by
- .IR pid .
- If
- .IR 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
- .IR pid .
- .P
- The conditions required for a process to have permission to queue a
- signal to another process are the same as for the
- \fIkill\fR()
- function.
- .P
- The
- \fIsigqueue\fR()
- function shall return immediately. If SA_SIGINFO is set for
- .IR 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
- .IR signo ,
- then
- .IR signo
- shall be sent at least once to the receiving process; it is unspecified
- whether
- .IR value
- shall be sent to the receiving process as a result of this call.
- .P
- If the value of
- .IR pid
- causes
- .IR signo
- to be generated for the sending process, and if
- .IR signo
- is not blocked for the calling thread and if no other thread has
- .IR signo
- unblocked or is waiting in a
- \fIsigwait\fR()
- function for
- .IR signo ,
- either
- .IR signo
- or at least the pending, unblocked signal shall be delivered to the
- calling thread before the
- \fIsigqueue\fR()
- 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"
- Upon successful completion, the specified signal shall have been
- queued, and the
- \fIsigqueue\fR()
- function shall return a value of zero. Otherwise, the function shall
- return a value of \-1 and set
- .IR errno
- to indicate the error.
- .SH ERRORS
- The
- \fIsigqueue\fR()
- function shall fail if:
- .TP
- .BR 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
- .BR EINVAL
- The value of the
- .IR signo
- argument is an invalid or unsupported signal number.
- .TP
- .BR EPERM
- The process does not have appropriate privileges to send the signal
- to the receiving process.
- .TP
- .BR ESRCH
- The process
- .IR pid
- does not exist.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- None.
- .SH "APPLICATION USAGE"
- None.
- .SH RATIONALE
- The
- \fIsigqueue\fR()
- 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
- .IR sig .\|.\|.
- name space already carved out for signals was preferable to extending
- the interface to
- \fIkill\fR().
- .P
- Such a function became necessary when the put/get event function of
- the message queues was removed. It should be noted that the
- \fIsigqueue\fR()
- 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
- .IR pid
- is resolved into a target process. Such access checks were necessary
- only at message queue open in the previous interface.
- .P
- The standard developers required that
- \fIsigqueue\fR()
- have the same semantics with respect to the null signal as
- \fIkill\fR(),
- and that the same permission checking be used. But because of the
- difficulty of implementing the ``broadcast'' semantic of
- \fIkill\fR()
- (for example, to process groups) and the interaction with resource
- allocation, this semantic was not adopted. The
- \fIsigqueue\fR()
- function queues a signal to a single process specified by the
- .IR pid
- argument.
- .P
- The
- \fIsigqueue\fR()
- 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\(hy2017 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\(hy2017 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"
- None.
- .SH "SEE ALSO"
- .IR "Section 2.8.1" ", " "Realtime Signals"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "\fB<signal.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 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 .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .