1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" %%%LICENSE_START(verbatim)
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14 .\" Since the Linux kernel and libraries are constantly changing, this
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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
37 .TH MSGCTL 2 2012-05-31 "Linux" "Linux Programmer's Manual"
39 msgctl \- message control operations
42 .B #include <sys/types.h>
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 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 last change */
66 unsigned long __msg_cbytes; /* Current number of bytes in
67 queue (nonstandard) */
68 msgqnum_t msg_qnum; /* Current number of messages
70 msglen_t msg_qbytes; /* Maximum number of bytes
72 pid_t msg_lspid; /* PID of last msgsnd(2) */
73 pid_t msg_lrpid; /* PID of last msgrcv(2) */
80 structure is defined as follows
81 (the highlighted fields are settable using
87 key_t __key; /* Key supplied to msgget(2) */
88 uid_t \fBuid\fP; /* Effective UID of owner */
89 gid_t \fBgid\fP; /* Effective GID of owner */
90 uid_t cuid; /* Effective UID of creator */
91 gid_t cgid; /* Effective GID of creator */
92 unsigned short \fBmode\fP; /* Permissions */
93 unsigned short __seq; /* Sequence number */
103 Copy information from the kernel data structure associated with
107 structure pointed to by
109 The caller must have read permission on the message queue.
112 Write the values of some members of the
114 structure pointed to by
116 to the kernel data structure associated with this message queue,
120 The following members of the structure are updated:
124 and (the least significant 9 bits of)
126 The effective UID of the calling process must match the owner
129 .RI ( msg_perm.cuid )
130 of the message queue, or the caller must be privileged.
131 Appropriate privilege (Linux: the
133 capability) is required to raise the
135 value beyond the system parameter
139 Immediately remove the message queue,
140 awakening all waiting reader and writer processes (with an error
145 The calling process must have appropriate privileges
146 or its effective user ID must be either that of the creator or owner
147 of the message queue.
149 .BR IPC_INFO " (Linux-specific)"
150 Returns information about system-wide message queue limits and
151 parameters in the structure pointed to by
153 This structure is of type
155 (thus, a cast is required),
160 feature test macro is defined:
165 int msgpool; /* Size in kibibytes of buffer pool
166 used to hold message data;
167 unused within kernel */
168 int msgmap; /* Maximum number of entries in message
169 map; unused within kernel */
170 int msgmax; /* Maximum number of bytes that can be
171 written in a single message */
172 int msgmnb; /* Maximum number of bytes that can be
173 written to queue; used to initialize
174 msg_qbytes during queue creation
176 int msgmni; /* Maximum number of message queues */
177 int msgssz; /* Message segment size;
178 unused within kernel */
179 int msgtql; /* Maximum number of messages on all queues
180 in system; unused within kernel */
181 unsigned short int msgseg;
182 /* Maximum number of segments;
183 unused within kernel */
193 settings can be changed via
195 files of the same name; see
199 .BR MSG_INFO " (Linux-specific)"
202 structure containing the same information as for
204 except that the following fields are returned with information
205 about system resources consumed by message queues: the
207 field returns the number of message queues that currently exist
210 field returns the total number of messages in all queues
211 on the system; and the
213 field returns the total number of bytes in all messages
214 in all queues on the system.
216 .BR MSG_STAT " (Linux-specific)"
223 argument is not a queue identifier, but instead an index into
224 the kernel's internal array that maintains information about
225 all message queues on the system.
237 operation returns the index of the highest used entry in the
238 kernel's internal array recording information about all
240 (This information can be used with repeated
242 operations to obtain information about all queues on the system.)
245 operation returns the identifier of the queue whose index was given in
248 On error, \-1 is returned with
250 indicating the error.
254 is set to one of the following:
263 but the calling process does not have read permission on the message queue
265 and does not have the
276 but the address pointed to by
281 The message queue was removed.
290 operation, the index value specified in
292 referred to an array slot that is currently unused.
301 but the effective user ID of the calling process is not the creator
307 of the message queue,
308 and the process is not privileged (Linux: it does not have the
313 .\" SVID does not document the EIDRM error condition.
319 isn't required on Linux or by any version of POSIX.
321 some old implementations required the inclusion of these header files,
322 and the SVID also documented their inclusion.
323 Applications intended to be portable to such old systems may need
324 to include these header files.
325 .\" Like Linux, the FreeBSD man pages still document
326 .\" the inclusion of these header files.
333 operations are used by the
335 program to provide information on allocated resources.
336 In the future these may modified or moved to a /proc file system
339 Various fields in the \fIstruct msqid_ds\fP were
346 To take advantage of this,
347 a recompilation under glibc-2.1.91 or later should suffice.
348 (The kernel distinguishes old and new calls by an
356 .BR capabilities (7),