1SIGQUEUE(3P)               POSIX Programmer's Manual              SIGQUEUE(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

13       sigqueue — queue a signal to a process
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SYNOPSIS

16       #include <signal.h>
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18       int sigqueue(pid_t pid, int signo, const union sigval value);
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DESCRIPTION

21       The sigqueue() function shall cause the signal specified by signo to be
22       sent with the value specified by value to the process specified by pid.
23       If signo is zero (the null signal), error checking is performed but  no
24       signal  is  actually  sent.  The  null  signal can be used to check the
25       validity of pid.
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27       The conditions required for a process to have  permission  to  queue  a
28       signal to another process are the same as for the kill() function.
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30       The  sigqueue() function shall return immediately. If SA_SIGINFO is set
31       for signo and if the resources were available to queue the signal,  the
32       signal shall be queued and sent to the receiving process. If SA_SIGINFO
33       is not set for signo, then signo shall be sent at  least  once  to  the
34       receiving process; it is unspecified whether value shall be sent to the
35       receiving process as a result of this call.
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37       If the value of pid causes  signo  to  be  generated  for  the  sending
38       process,  and  if signo is not blocked for the calling thread and if no
39       other thread has signo unblocked or is waiting in a sigwait()  function
40       for signo, either signo or at least the pending, unblocked signal shall
41       be delivered to the  calling  thread  before  the  sigqueue()  function
42       returns.  Should  any multiple pending signals in the range SIGRTMIN to
43       SIGRTMAX be selected for delivery, it shall be the lowest numbered one.
44       The  selection  order  between  realtime  and  non-realtime signals, or
45       between multiple pending non-realtime signals, is unspecified.
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RETURN VALUE

48       Upon successful  completion,  the  specified  signal  shall  have  been
49       queued,  and the sigqueue() function shall return a value of zero. Oth‐
50       erwise, the function shall return a value of −1 and set errno to  indi‐
51       cate the error.
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ERRORS

54       The sigqueue() function shall fail if:
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56       EAGAIN No  resources are available to queue the signal. The process has
57              already queued {SIGQUEUE_MAX} signals that are still pending  at
58              the  receiver(s),  or  a  system-wide  resource  limit  has been
59              exceeded.
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61       EINVAL The value of the signo argument is  an  invalid  or  unsupported
62              signal number.
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64       EPERM  The  process  does  not  have appropriate privileges to send the
65              signal to the receiving process.
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67       ESRCH  The process pid does not exist.
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69       The following sections are informative.
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EXAMPLES

72       None.
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APPLICATION USAGE

75       None.
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RATIONALE

78       The sigqueue() function allows an application to queue a realtime  sig‐
79       nal to itself or to another process, specifying the application-defined
80       value. This is common practice in  realtime  applications  on  existing
81       realtime  systems.  It was felt that specifying another function in the
82       sig...  name space already carved out for  signals  was  preferable  to
83       extending the interface to kill().
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85       Such a function became necessary when the put/get event function of the
86       message queues was removed. It should  be  noted  that  the  sigqueue()
87       function  implies reduced performance in a security-conscious implemen‐
88       tation as the access permissions between the sender and  receiver  have
89       to  be  checked  on  each  send  when the pid is resolved into a target
90       process. Such access checks were necessary only at message  queue  open
91       in the previous interface.
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93       The  standard  developers required that sigqueue() have the same seman‐
94       tics with respect to the null signal as kill(), and that the same  per‐
95       mission checking be used. But because of the difficulty of implementing
96       the ``broadcast'' semantic of kill() (for example, to  process  groups)
97       and  the  interaction  with  resource allocation, this semantic was not
98       adopted. The sigqueue() function queues a signal to  a  single  process
99       specified by the pid argument.
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101       The  sigqueue()  function  can  fail  if  the  system  has insufficient
102       resources to queue the signal. An  explicit  limit  on  the  number  of
103       queued  signals  that  a  process  could send was introduced. While the
104       limit is ``per-sender'', this volume of POSIX.1‐2008 does  not  specify
105       that  the  resources  be  part  of  the state of the sender. This would
106       require either that the sender be maintained after exit until all  sig‐
107       nals  that it had sent to other processes were handled or that all such
108       signals that had not yet been acted upon be removed from  the  queue(s)
109       of  the  receivers.  This volume of POSIX.1‐2008 does not preclude this
110       behavior, but an implementation that allocated queuing resources from a
111       system-wide  pool  (with per-sender limits) and that leaves queued sig‐
112       nals pending after the sender exits is also permitted.
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FUTURE DIRECTIONS

115       None.
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SEE ALSO

118       Section 2.8.1, Realtime Signals
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120       The Base Definitions volume of POSIX.1‐2008, <signal.h>
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123       Portions of this text are reprinted and reproduced in  electronic  form
124       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
125       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
126       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
127       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
128       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) In the
129       event of any discrepancy between this version and the original IEEE and
130       The  Open Group Standard, the original IEEE and The Open Group Standard
131       is the referee document. The original Standard can be  obtained  online
132       at http://www.unix.org/online.html .
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134       Any  typographical  or  formatting  errors that appear in this page are
135       most likely to have been introduced during the conversion of the source
136       files  to  man page format. To report such errors, see https://www.ker
137       nel.org/doc/man-pages/reporting_bugs.html .
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141IEEE/The Open Group                  2013                         SIGQUEUE(3P)
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