1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" %%%LICENSE_START(VERBATIM)
5 .\" Permission is granted to make and distribute verbatim copies of this
6 .\" manual provided the copyright notice and this permission notice are
7 .\" preserved on all copies.
9 .\" Permission is granted to copy and distribute modified versions of this
10 .\" manual under the conditions for verbatim copying, provided that the
11 .\" entire resulting derived work is distributed under the terms of a
12 .\" permission notice identical to this one.
14 .\" Since the Linux kernel and libraries are constantly changing, this
15 .\" manual page may be incorrect or out-of-date. The author(s) assume no
16 .\" responsibility for errors or omissions, or for damages resulting from
17 .\" the use of the information contained herein. The author(s) may not
18 .\" have taken the same level of care in the production of this manual,
19 .\" which is licensed free of charge, as they might when working
22 .\" Formatted or processed versions of this manual, if unaccompanied by
23 .\" the source, must acknowledge the copyright and authors of this work.
26 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
27 .\" Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aeb@cwi.nl>
28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Added notes on CAP_IPC_OWNER requirement
30 .\" Modified, 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
31 .\" Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
32 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
33 .\" Language and formatting clean-ups
34 .\" Added msqid_ds and ipc_perm structure definitions
35 .\" 2005-08-02, mtk: Added IPC_INFO, MSG_INFO, MSG_STAT descriptions
36 .\" 2018-03-20, dbueso: Added MSG_STAT_ANY description.
38 .TH MSGCTL 2 2021-03-22 "Linux" "Linux Programmer's Manual"
40 msgctl \- System V message control operations
43 .B #include <sys/ipc.h>
44 .B #include <sys/msg.h>
46 .BI "int msgctl(int " msqid ", int " cmd ", struct msqid_ds *" buf );
50 performs the control operation specified by
52 on the System\ V message queue with identifier
57 data structure is defined in \fI<sys/msg.h>\fP as follows:
62 struct ipc_perm msg_perm; /* Ownership and permissions */
63 time_t msg_stime; /* Time of last msgsnd(2) */
64 time_t msg_rtime; /* Time of last msgrcv(2) */
65 time_t msg_ctime; /* Time of creation or last
66 modification by msgctl() */
67 unsigned long msg_cbytes; /* # of bytes in queue */
68 msgqnum_t msg_qnum; /* # number of messages in queue */
69 msglen_t msg_qbytes; /* Maximum # of bytes in queue */
70 pid_t msg_lspid; /* PID of last msgsnd(2) */
71 pid_t msg_lrpid; /* PID of last msgrcv(2) */
78 structure are as follows:
83 structure (see below) that specifies the access permissions on the message
97 Time of creation of queue or time of last
103 Number of bytes in all messages currently on the message queue.
104 This is a nonstandard Linux extension that is not specified in POSIX.
107 Number of messages currently on the message queue.
110 Maximum number of bytes of message text allowed on the message
114 ID of the process that performed the last
119 ID of the process that performed the last
125 structure is defined as follows
126 (the highlighted fields are settable using
132 key_t __key; /* Key supplied to msgget(2) */
133 uid_t \fBuid\fP; /* Effective UID of owner */
134 gid_t \fBgid\fP; /* Effective GID of owner */
135 uid_t cuid; /* Effective UID of creator */
136 gid_t cgid; /* Effective GID of creator */
137 unsigned short \fBmode\fP; /* Permissions */
138 unsigned short __seq; /* Sequence number */
143 The least significant 9 bits of the
147 structure define the access permissions for the message queue.
148 The permission bits are as follows:
159 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
166 Copy information from the kernel data structure associated with
170 structure pointed to by
172 The caller must have read permission on the message queue.
175 Write the values of some members of the
177 structure pointed to by
179 to the kernel data structure associated with this message queue,
184 The following members of the structure are updated:
188 and (the least significant 9 bits of)
191 The effective UID of the calling process must match the owner
194 .RI ( msg_perm.cuid )
195 of the message queue, or the caller must be privileged.
196 Appropriate privilege (Linux: the
198 capability) is required to raise the
200 value beyond the system parameter
204 Immediately remove the message queue,
205 awakening all waiting reader and writer processes (with an error
210 The calling process must have appropriate privileges
211 or its effective user ID must be either that of the creator or owner
212 of the message queue.
213 The third argument to
215 is ignored in this case.
217 .BR IPC_INFO " (Linux-specific)"
218 Return information about system-wide message queue limits and
219 parameters in the structure pointed to by
221 This structure is of type
223 (thus, a cast is required),
228 feature test macro is defined:
233 int msgpool; /* Size in kibibytes of buffer pool
234 used to hold message data;
235 unused within kernel */
236 int msgmap; /* Maximum number of entries in message
237 map; unused within kernel */
238 int msgmax; /* Maximum number of bytes that can be
239 written in a single message */
240 int msgmnb; /* Maximum number of bytes that can be
241 written to queue; used to initialize
242 msg_qbytes during queue creation
244 int msgmni; /* Maximum number of message queues */
245 int msgssz; /* Message segment size;
246 unused within kernel */
247 int msgtql; /* Maximum number of messages on all queues
248 in system; unused within kernel */
249 unsigned short msgseg;
250 /* Maximum number of segments;
251 unused within kernel */
261 settings can be changed via
263 files of the same name; see
267 .BR MSG_INFO " (Linux-specific)"
270 structure containing the same information as for
272 except that the following fields are returned with information
273 about system resources consumed by message queues: the
275 field returns the number of message queues that currently exist
278 field returns the total number of messages in all queues
279 on the system; and the
281 field returns the total number of bytes in all messages
282 in all queues on the system.
284 .BR MSG_STAT " (Linux-specific)"
291 argument is not a queue identifier, but instead an index into
292 the kernel's internal array that maintains information about
293 all message queues on the system.
295 .BR MSG_STAT_ANY " (Linux-specific, since Linux 4.17)"
302 is not checked for read access for
304 meaning that any user can employ this operation (just as any user may read
305 .IR /proc/sysvipc/msg
306 to obtain the same information).
318 operation returns the index of the highest used entry in the
319 kernel's internal array recording information about all
321 (This information can be used with repeated
325 operations to obtain information about all queues on the system.)
330 operation returns the identifier of the queue whose index was given in
333 On failure, \-1 is returned and
335 is set to indicate the error.
345 but the calling process does not have read permission on the message queue
347 and does not have the
349 capability in the user namespace that governs its IPC namespace.
358 but the address pointed to by
363 The message queue was removed.
372 operation, the index value specified in
374 referred to an array slot that is currently unused.
383 but the effective user ID of the calling process is not the creator
389 of the message queue,
390 and the caller is not privileged (Linux: does not have the
399 beyond the system parameter
401 but the caller is not privileged (Linux: does not have the
405 POSIX.1-2001, POSIX.1-2008, SVr4.
406 .\" SVID does not document the EIDRM error condition.
410 isn't required on Linux or by any version of POSIX.
412 some old implementations required the inclusion of this header file,
413 and the SVID also documented its inclusion.
414 Applications intended to be portable to such old systems may need
415 to include this header file.
416 .\" Like Linux, the FreeBSD man pages still document
417 .\" the inclusion of this header file.
424 operations are used by the
426 program to provide information on allocated resources.
427 In the future these may modified or moved to a
429 filesystem interface.
431 Various fields in the \fIstruct msqid_ds\fP were
438 To take advantage of this,
439 a recompilation under glibc-2.1.91 or later should suffice.
440 (The kernel distinguishes old and new calls by an
448 .BR capabilities (7),