SIGQUEUE(3P)                                        POSIX Programmer's Manual                                       SIGQUEUE(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       sigqueue - 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  receiv-
       ing 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. 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 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 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().

       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(),  and
       that  the  same  permission  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  sys-
       tem-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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                  SIGQUEUE(3P)