1MSGCTL(2) Linux Programmer's Manual MSGCTL(2)
2
6 msgctl - System V message control operations
7
9 #include <sys/types.h>
10 #include <sys/ipc.h>
11 #include <sys/msg.h>
12 int msgctl(int msqid, int cmd, struct msqid_ds *buf);
14
16 msgctl() performs the control operation specified by cmd on the Sys‐
17 tem V message queue with identifier msqid.
18 The msqid_ds data structure is defined in <sys/msg.h> as follows:
20 struct msqid_ds {
22 struct ipc_perm msg_perm; /* Ownership and permissions */
23 time_t msg_stime; /* Time of last msgsnd(2) */
24 time_t msg_rtime; /* Time of last msgrcv(2) */
25 time_t msg_ctime; /* Creation time/time of last
26 modification via msgctl() */
27 unsigned long __msg_cbytes; /* Current number of bytes in
28 queue (nonstandard) */
29 msgqnum_t msg_qnum; /* Current number of messages
30 in queue */
31 msglen_t msg_qbytes; /* Maximum number of bytes
32 allowed in queue */
33 pid_t msg_lspid; /* PID of last msgsnd(2) */
34 pid_t msg_lrpid; /* PID of last msgrcv(2) */
35 };
36 The fields of the msgid_ds structure are as follows:
38 msg_perm This is an ipc_perm structure (see below) that specifies the
40 access permissions on the message queue.
41 msg_qnum Number of messages currently on the message queue.
43 msg_qbytes Maximum number of bytes of message text allowed on the mes‐
45 sage queue.
46 msg_lspid ID of the process that performed the last msgsnd(2) system
48 call.
49 msg_lrpid ID of the process that performed the last msgrcv(2) system
51 call.
52 msg_stime Time of the last msgsnd(2) system call.
54 msg_rtime Time of the last msgrcv(2) system call.
56 msg_ctime Time of creation of queue or time of last msgctl() IPC_SET
58 operation.
59 The ipc_perm structure is defined as follows (the highlighted fields
61 are settable using IPC_SET):
62 struct ipc_perm {
64 key_t __key; /* Key supplied to msgget(2) */
65 uid_t uid; /* Effective UID of owner */
66 gid_t gid; /* Effective GID of owner */
67 uid_t cuid; /* Effective UID of creator */
68 gid_t cgid; /* Effective GID of creator */
69 unsigned short mode; /* Permissions */
70 unsigned short __seq; /* Sequence number */
71 };
72 The least significant 9 bits of the mode field of the ipc_perm struc‐
74 ture define the access permissions for the message queue. The permis‐
75 sion bits are as follows:
76 0400 Read by user
78 0200 Write by user
79 0040 Read by group
80 0020 Write by group
81 0004 Read by others
82 0002 Write by others
83 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
85 Valid values for cmd are:
87 IPC_STAT
89 Copy information from the kernel data structure associated with
90 msqid into the msqid_ds structure pointed to by buf. The caller
91 must have read permission on the message queue.
92 IPC_SET
94 Write the values of some members of the msqid_ds structure
95 pointed to by buf to the kernel data structure associated with
96 this message queue, updating also its msg_ctime member. The
97 following members of the structure are updated: msg_qbytes,
98 msg_perm.uid, msg_perm.gid, and (the least significant 9 bits
99 of) msg_perm.mode. The effective UID of the calling process
100 must match the owner (msg_perm.uid) or creator (msg_perm.cuid)
101 of the message queue, or the caller must be privileged. Appro‐
102 priate privilege (Linux: the CAP_SYS_RESOURCE capability) is
103 required to raise the msg_qbytes value beyond the system parame‐
104 ter MSGMNB.
105 IPC_RMID
107 Immediately remove the message queue, awakening all waiting
108 reader and writer processes (with an error return and errno set
109 to EIDRM). The calling process must have appropriate privileges
110 or its effective user ID must be either that of the creator or
111 owner of the message queue. The third argument to msgctl() is
112 ignored in this case.
113 IPC_INFO (Linux-specific)
115 Return information about system-wide message queue limits and
116 parameters in the structure pointed to by buf. This structure
117 is of type msginfo (thus, a cast is required), defined in
118 <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
119 struct msginfo {
121 int msgpool; /* Size in kibibytes of buffer pool
122 used to hold message data;
123 unused within kernel */
124 int msgmap; /* Maximum number of entries in message
125 map; unused within kernel */
126 int msgmax; /* Maximum number of bytes that can be
127 written in a single message */
128 int msgmnb; /* Maximum number of bytes that can be
129 written to queue; used to initialize
130 msg_qbytes during queue creation
131 (msgget(2)) */
132 int msgmni; /* Maximum number of message queues */
133 int msgssz; /* Message segment size;
134 unused within kernel */
135 int msgtql; /* Maximum number of messages on all queues
136 in system; unused within kernel */
137 unsigned short int msgseg;
138 /* Maximum number of segments;
139 unused within kernel */
140 };
141 The msgmni, msgmax, and msgmnb settings can be changed via /proc
143 files of the same name; see proc(5) for details.
144 MSG_INFO (Linux-specific)
146 Return a msginfo structure containing the same information as
147 for IPC_INFO, except that the following fields are returned with
148 information about system resources consumed by message queues:
149 the msgpool field returns the number of message queues that cur‐
150 rently exist on the system; the msgmap field returns the total
151 number of messages in all queues on the system; and the msgtql
152 field returns the total number of bytes in all messages in all
153 queues on the system.
154 MSG_STAT (Linux-specific)
156 Return a msqid_ds structure as for IPC_STAT. However, the msqid
157 argument is not a queue identifier, but instead an index into
158 the kernel's internal array that maintains information about all
159 message queues on the system.
160 MSG_STAT_ANY (Linux-specific, since Linux 4.17)
162 Return a msqid_ds structure as for MSG_STAT. However,
163 msg_perm.mode is not checked for read access for msqid meaning
164 that any user can employ this operation (just as any user may
165 read /proc/sysvipc/msg to obtain the same information).
166
168 On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful
169 IPC_INFO or MSG_INFO operation returns the index of the highest used
170 entry in the kernel's internal array recording information about all
171 message queues. (This information can be used with repeated MSG_STAT
172 or MSG_STAT_ANY operations to obtain information about all queues on
173 the system.) A successful MSG_STAT or MSG_STAT_ANY operation returns
174 the identifier of the queue whose index was given in msqid.
175 On error, -1 is returned with errno indicating the error.
177
179 On failure, errno is set to one of the following:
180 EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the call‐
182 ing process does not have read permission on the message queue
183 msqid, and does not have the CAP_IPC_OWNER capability in the
184 user namespace that governs its IPC namespace.
185 EFAULT The argument cmd has the value IPC_SET or IPC_STAT, but the
187 address pointed to by buf isn't accessible.
188 EIDRM The message queue was removed.
190 EINVAL Invalid value for cmd or msqid. Or: for a MSG_STAT operation,
192 the index value specified in msqid referred to an array slot
193 that is currently unused.
194 EPERM The argument cmd has the value IPC_SET or IPC_RMID, but the
196 effective user ID of the calling process is not the creator (as
197 found in msg_perm.cuid) or the owner (as found in msg_perm.uid)
198 of the message queue, and the caller is not privileged (Linux:
199 does not have the CAP_SYS_ADMIN capability).
200 EPERM An attempt (IPC_SET) was made to increase msg_qbytes beyond the
202 system parameter MSGMNB, but the caller is not privileged
203 (Linux: does not have the CAP_SYS_RESOURCE capability).
204
206 POSIX.1-2001, POSIX.1-2008, SVr4.
207
209 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
210 or by any version of POSIX. However, some old implementations required
211 the inclusion of these header files, and the SVID also documented their
212 inclusion. Applications intended to be portable to such old systems
213 may need to include these header files.
214 The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the ipcs(1)
216 program to provide information on allocated resources. In the future
217 these may modified or moved to a /proc filesystem interface.
218 Various fields in the struct msqid_ds were typed as short under Linux
220 2.2 and have become long under Linux 2.4. To take advantage of this, a
221 recompilation under glibc-2.1.91 or later should suffice. (The kernel
222 distinguishes old and new calls by an IPC_64 flag in cmd.)
223
225 msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),
226 sysvipc(7)
227
229 This page is part of release 5.07 of the Linux man-pages project. A
230 description of the project, information about reporting bugs, and the
231 latest version of this page, can be found at
232 https://www.kernel.org/doc/man-pages/.
233Linux 2020-04-11 MSGCTL(2)