1 .\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
3 .\" %%%LICENSE_START(VERBATIM)
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13 .\" Since the Linux kernel and libraries are constantly changing, this
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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 2016-10-08 "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 A new message queue is created if
61 no message queue with the given key
74 and a message queue already exists for
82 (This is analogous to the effect of the combination
87 Upon creation, the least significant bits of the argument
89 define the permissions of the message queue.
90 These permission bits have the same format and semantics
91 as the permissions specified for the
95 (The execute permissions are not used.)
97 If a new message queue is created,
98 then its associated data structure
102 is initialized as follows:
107 are set to the effective user ID of the calling process.
112 are set to the effective group ID of the calling process.
114 The least significant 9 bits of
116 are set to the least significant 9 bits of
128 is set to the current time.
131 is set to the system limit
134 If the message queue already exists the permissions are
135 verified, and a check is made to see if it is marked for
138 If successful, the return value will be the message queue identifier (a
139 nonnegative integer), otherwise \-1
142 indicating the error.
146 is set to one of the following values:
149 A message queue exists for
151 but the calling process does not have permission to access the queue,
152 and does not have the
154 capability in the user namespace that governs its IPC namespace.
162 but a message queue already exists for
166 No message queue exists for
174 A message queue has to be created but the system does not have enough
175 memory for the new data structure.
178 A message queue has to be created but the system limit for the maximum
179 number of message queues
183 POSIX.1-2001, POSIX.1-2008, SVr4.
189 isn't required on Linux or by any version of POSIX.
191 some old implementations required the inclusion of these header files,
192 and the SVID also documented their inclusion.
193 Applications intended to be portable to such old systems may need
194 to include these header files.
195 .\" Like Linux, the FreeBSD man pages still document
196 .\" the inclusion of these header files.
199 isn't a flag field but a
202 If this special value is used for
204 the system call ignores everything but the least significant 9 bits of
206 and creates a new message queue (on success).
208 The following is a system limit on message queue resources affecting a
213 System-wide limit on the number of message queues.
215 .\" commit 0050ee059f7fc86b1df2527aaa14ed5dc72f9973
216 the default value for this limit was calculated using a formula
217 based on available system memory.
218 Since Linux 3.19, the default value is 32,000.
219 On Linux, this limit can be read and modified via
220 .IR /proc/sys/kernel/msgmni .
222 Until version 2.3.20, Linux would return
226 on a message queue scheduled for deletion.
230 was perhaps unfortunate,
232 would more clearly show its function.
238 .BR capabilities (7),