1 .\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
3 .\" %%%LICENSE_START(VERBATIM)
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9 .\" manual under the conditions for verbatim copying, provided that the
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13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
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18 .\" which is licensed free of charge, as they might when working
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25 .\" Added correction due to Nick Duffek <nsd@bbc.com>, aeb, 960426
26 .\" Modified Wed Nov 6 04:00:31 1996 by Eric S. Raymond <esr@thyrsus.com>
27 .\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com>
28 .\" Removed EIDRM from errors - that can't happen...
29 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
30 .\" Added notes on capability requirements
31 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
32 .\" Language and formatting clean-ups
33 .\" Added notes on /proc files
35 .TH MSGGET 2 2018-04-30 "Linux" "Linux Programmer's Manual"
37 msgget \- get a System V message queue identifier
40 .B #include <sys/types.h>
41 .B #include <sys/ipc.h>
42 .B #include <sys/msg.h>
44 .BI "int msgget(key_t " key ", int " msgflg );
49 system call returns the System\ V message queue identifier associated
53 It may be used either to obtain the identifier of a previously created
58 does not have the value
60 or to create a new set.
62 A new message queue is created if
70 no message queue with the given key
83 and a message queue already exists for
91 (This is analogous to the effect of the combination
96 Upon creation, the least significant bits of the argument
98 define the permissions of the message queue.
99 These permission bits have the same format and semantics
100 as the permissions specified for the
104 (The execute permissions are not used.)
106 If a new message queue is created,
107 then its associated data structure
111 is initialized as follows:
116 are set to the effective user ID of the calling process.
121 are set to the effective group ID of the calling process.
123 The least significant 9 bits of
125 are set to the least significant 9 bits of
137 is set to the current time.
140 is set to the system limit
143 If the message queue already exists the permissions are
144 verified, and a check is made to see if it is marked for
147 If successful, the return value will be the message queue identifier (a
148 nonnegative integer), otherwise \-1
151 indicating the error.
155 is set to one of the following values:
158 A message queue exists for
160 but the calling process does not have permission to access the queue,
161 and does not have the
163 capability in the user namespace that governs its IPC namespace.
171 but a message queue already exists for
175 No message queue exists for
183 A message queue has to be created but the system does not have enough
184 memory for the new data structure.
187 A message queue has to be created but the system limit for the maximum
188 number of message queues
192 POSIX.1-2001, POSIX.1-2008, SVr4.
198 isn't required on Linux or by any version of POSIX.
200 some old implementations required the inclusion of these header files,
201 and the SVID also documented their inclusion.
202 Applications intended to be portable to such old systems may need
203 to include these header files.
204 .\" Like Linux, the FreeBSD man pages still document
205 .\" the inclusion of these header files.
208 isn't a flag field but a
211 If this special value is used for
213 the system call ignores everything but the least significant 9 bits of
215 and creates a new message queue (on success).
217 The following is a system limit on message queue resources affecting a
222 System-wide limit on the number of message queues.
224 .\" commit 0050ee059f7fc86b1df2527aaa14ed5dc72f9973
225 the default value for this limit was calculated using a formula
226 based on available system memory.
227 Since Linux 3.19, the default value is 32,000.
228 On Linux, this limit can be read and modified via
229 .IR /proc/sys/kernel/msgmni .
231 Until version 2.3.20, Linux would return
235 on a message queue scheduled for deletion.
239 was perhaps unfortunate,
241 would more clearly show its function.
247 .BR capabilities (7),