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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 (consult 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
       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(3P)