1SIGQUEUE(3P) POSIX Programmer's Manual SIGQUEUE(3P)
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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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13 sigqueue — queue a signal to a process
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16 #include <signal.h>
17 int sigqueue(pid_t pid, int signo, const union sigval value);
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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.
26 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.
29 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.
36 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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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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54 The sigqueue() function shall fail if:
55 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.
60 EINVAL The value of the signo argument is an invalid or unsupported
62 signal number.
63 EPERM The process does not have appropriate privileges to send the
65 signal to the receiving process.
66 ESRCH The process pid does not exist.
68 The following sections are informative.
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72 None.
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75 None.
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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().
84 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.
92 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.
100 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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115 None.
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118 Section 2.8.1, Realtime Signals
119 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 .
133 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 .
138IEEE/The Open Group 2013 SIGQUEUE(3P)