1SIGQUEUE(3P) POSIX Programmer's Manual SIGQUEUE(3P)
2
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.
10
12 sigqueue — queue a signal to a process
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15 #include <signal.h>
16 int sigqueue(pid_t pid, int signo, union sigval value);
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20 The sigqueue() function shall cause the signal specified by signo to be
21 sent with the value specified by value to the process specified by pid.
22 If signo is zero (the null signal), error checking is performed but no
23 signal is actually sent. The null signal can be used to check the
24 validity of pid.
25 The conditions required for a process to have permission to queue a
27 signal to another process are the same as for the kill() function.
28 The sigqueue() function shall return immediately. If SA_SIGINFO is set
30 for signo and if the resources were available to queue the signal, the
31 signal shall be queued and sent to the receiving process. If SA_SIGINFO
32 is not set for signo, then signo shall be sent at least once to the
33 receiving process; it is unspecified whether value shall be sent to the
34 receiving process as a result of this call.
35 If the value of pid causes signo to be generated for the sending
37 process, and if signo is not blocked for the calling thread and if no
38 other thread has signo unblocked or is waiting in a sigwait() function
39 for signo, either signo or at least the pending, unblocked signal shall
40 be delivered to the calling thread before the sigqueue() function
41 returns. Should any multiple pending signals in the range SIGRTMIN to
42 SIGRTMAX be selected for delivery, it shall be the lowest numbered one.
43 The selection order between realtime and non-realtime signals, or
44 between multiple pending non-realtime signals, is unspecified.
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47 Upon successful completion, the specified signal shall have been
48 queued, and the sigqueue() function shall return a value of zero. Oth‐
49 erwise, the function shall return a value of -1 and set errno to indi‐
50 cate the error.
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53 The sigqueue() function shall fail if:
54 EAGAIN No resources are available to queue the signal. The process has
56 already queued {SIGQUEUE_MAX} signals that are still pending at
57 the receiver(s), or a system-wide resource limit has been
58 exceeded.
59 EINVAL The value of the signo argument is an invalid or unsupported
61 signal number.
62 EPERM The process does not have appropriate privileges to send the
64 signal to the receiving process.
65 ESRCH The process pid does not exist.
67 The following sections are informative.
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71 None.
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74 None.
75
77 The sigqueue() function allows an application to queue a realtime sig‐
78 nal to itself or to another process, specifying the application-defined
79 value. This is common practice in realtime applications on existing
80 realtime systems. It was felt that specifying another function in the
81 sig... name space already carved out for signals was preferable to
82 extending the interface to kill().
83 Such a function became necessary when the put/get event function of the
85 message queues was removed. It should be noted that the sigqueue()
86 function implies reduced performance in a security-conscious implemen‐
87 tation as the access permissions between the sender and receiver have
88 to be checked on each send when the pid is resolved into a target
89 process. Such access checks were necessary only at message queue open
90 in the previous interface.
91 The standard developers required that sigqueue() have the same seman‐
93 tics with respect to the null signal as kill(), and that the same per‐
94 mission checking be used. But because of the difficulty of implementing
95 the ``broadcast'' semantic of kill() (for example, to process groups)
96 and the interaction with resource allocation, this semantic was not
97 adopted. The sigqueue() function queues a signal to a single process
98 specified by the pid argument.
99 The sigqueue() function can fail if the system has insufficient
101 resources to queue the signal. An explicit limit on the number of
102 queued signals that a process could send was introduced. While the
103 limit is ``per-sender'', this volume of POSIX.1‐2017 does not specify
104 that the resources be part of the state of the sender. This would
105 require either that the sender be maintained after exit until all sig‐
106 nals that it had sent to other processes were handled or that all such
107 signals that had not yet been acted upon be removed from the queue(s)
108 of the receivers. This volume of POSIX.1‐2017 does not preclude this
109 behavior, but an implementation that allocated queuing resources from a
110 system-wide pool (with per-sender limits) and that leaves queued sig‐
111 nals pending after the sender exits is also permitted.
112
114 None.
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117 Section 2.8.1, Realtime Signals
118 The Base Definitions volume of POSIX.1‐2017, <signal.h>
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122 Portions of this text are reprinted and reproduced in electronic form
123 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
124 table Operating System Interface (POSIX), The Open Group Base Specifi‐
125 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
126 Electrical and Electronics Engineers, Inc and The Open Group. In the
127 event of any discrepancy between this version and the original IEEE and
128 The Open Group Standard, the original IEEE and The Open Group Standard
129 is the referee document. The original Standard can be obtained online
130 at http://www.opengroup.org/unix/online.html .
131 Any typographical or formatting errors that appear in this page are
133 most likely to have been introduced during the conversion of the source
134 files to man page format. To report such errors, see https://www.ker‐
135 nel.org/doc/man-pages/reporting_bugs.html .
136IEEE/The Open Group 2017 SIGQUEUE(3P)