1msgctl(2) System Calls Manual msgctl(2)
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6 msgctl - System V message control operations
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9 Standard C library (libc, -lc)
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12 #include <sys/msg.h>
13 int msgctl(int msqid, int cmd, struct msqid_ds *buf);
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17 msgctl() performs the control operation specified by cmd on the Sys‐
18 tem V message queue with identifier msqid.
19 The msqid_ds data structure is defined in <sys/msg.h> as follows:
21 struct msqid_ds {
23 struct ipc_perm msg_perm; /* Ownership and permissions */
24 time_t msg_stime; /* Time of last msgsnd(2) */
25 time_t msg_rtime; /* Time of last msgrcv(2) */
26 time_t msg_ctime; /* Time of creation or last
27 modification by msgctl() */
28 unsigned long msg_cbytes; /* # of bytes in queue */
29 msgqnum_t msg_qnum; /* # number of messages in queue */
30 msglen_t msg_qbytes; /* Maximum # of bytes in queue */
31 pid_t msg_lspid; /* PID of last msgsnd(2) */
32 pid_t msg_lrpid; /* PID of last msgrcv(2) */
33 };
34 The fields of the msqid_ds structure are as follows:
36 msg_perm This is an ipc_perm structure (see below) that specifies the
38 access permissions on the message queue.
39 msg_stime Time of the last msgsnd(2) system call.
41 msg_rtime Time of the last msgrcv(2) system call.
43 msg_ctime Time of creation of queue or time of last msgctl() IPC_SET
45 operation.
46 msg_cbytes Number of bytes in all messages currently on the message
48 queue. This is a nonstandard Linux extension that is not
49 specified in POSIX.
50 msg_qnum Number of messages currently on the message queue.
52 msg_qbytes Maximum number of bytes of message text allowed on the mes‐
54 sage queue.
55 msg_lspid ID of the process that performed the last msgsnd(2) system
57 call.
58 msg_lrpid ID of the process that performed the last msgrcv(2) system
60 call.
61 The ipc_perm structure is defined as follows (the highlighted fields
63 are settable using IPC_SET):
64 struct ipc_perm {
66 key_t __key; /* Key supplied to msgget(2) */
67 uid_t uid; /* Effective UID of owner */
68 gid_t gid; /* Effective GID of owner */
69 uid_t cuid; /* Effective UID of creator */
70 gid_t cgid; /* Effective GID of creator */
71 unsigned short mode; /* Permissions */
72 unsigned short __seq; /* Sequence number */
73 };
74 The least significant 9 bits of the mode field of the ipc_perm struc‐
76 ture define the access permissions for the message queue. The permis‐
77 sion bits are as follows:
78 0400 Read by user
80 0200 Write by user
81 0040 Read by group
82 0020 Write by group
83 0004 Read by others
84 0002 Write by others
85 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
87 Valid values for cmd are:
89 IPC_STAT
91 Copy information from the kernel data structure associated with
92 msqid into the msqid_ds structure pointed to by buf. The caller
93 must have read permission on the message queue.
94 IPC_SET
96 Write the values of some members of the msqid_ds structure
97 pointed to by buf to the kernel data structure associated with
98 this message queue, updating also its msg_ctime member.
99 The following members of the structure are updated: msg_qbytes,
101 msg_perm.uid, msg_perm.gid, and (the least significant 9 bits
102 of) msg_perm.mode.
103 The effective UID of the calling process must match the owner
105 (msg_perm.uid) or creator (msg_perm.cuid) of the message queue,
106 or the caller must be privileged. Appropriate privilege (Linux:
107 the CAP_SYS_RESOURCE capability) is required to raise the
108 msg_qbytes value beyond the system parameter MSGMNB.
109 IPC_RMID
111 Immediately remove the message queue, awakening all waiting
112 reader and writer processes (with an error return and errno set
113 to EIDRM). The calling process must have appropriate privileges
114 or its effective user ID must be either that of the creator or
115 owner of the message queue. The third argument to msgctl() is
116 ignored in this case.
117 IPC_INFO (Linux-specific)
119 Return information about system-wide message queue limits and
120 parameters in the structure pointed to by buf. This structure
121 is of type msginfo (thus, a cast is required), defined in
122 <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
123 struct msginfo {
125 int msgpool; /* Size in kibibytes of buffer pool
126 used to hold message data;
127 unused within kernel */
128 int msgmap; /* Maximum number of entries in message
129 map; unused within kernel */
130 int msgmax; /* Maximum number of bytes that can be
131 written in a single message */
132 int msgmnb; /* Maximum number of bytes that can be
133 written to queue; used to initialize
134 msg_qbytes during queue creation
135 (msgget(2)) */
136 int msgmni; /* Maximum number of message queues */
137 int msgssz; /* Message segment size;
138 unused within kernel */
139 int msgtql; /* Maximum number of messages on all queues
140 in system; unused within kernel */
141 unsigned short msgseg;
142 /* Maximum number of segments;
143 unused within kernel */
144 };
145 The msgmni, msgmax, and msgmnb settings can be changed via /proc
147 files of the same name; see proc(5) for details.
148 MSG_INFO (Linux-specific)
150 Return a msginfo structure containing the same information as
151 for IPC_INFO, except that the following fields are returned with
152 information about system resources consumed by message queues:
153 the msgpool field returns the number of message queues that cur‐
154 rently exist on the system; the msgmap field returns the total
155 number of messages in all queues on the system; and the msgtql
156 field returns the total number of bytes in all messages in all
157 queues on the system.
158 MSG_STAT (Linux-specific)
160 Return a msqid_ds structure as for IPC_STAT. However, the msqid
161 argument is not a queue identifier, but instead an index into
162 the kernel's internal array that maintains information about all
163 message queues on the system.
164 MSG_STAT_ANY (Linux-specific, since Linux 4.17)
166 Return a msqid_ds structure as for MSG_STAT. However,
167 msg_perm.mode is not checked for read access for msqid meaning
168 that any user can employ this operation (just as any user may
169 read /proc/sysvipc/msg to obtain the same information).
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172 On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful
173 IPC_INFO or MSG_INFO operation returns the index of the highest used
174 entry in the kernel's internal array recording information about all
175 message queues. (This information can be used with repeated MSG_STAT
176 or MSG_STAT_ANY operations to obtain information about all queues on
177 the system.) A successful MSG_STAT or MSG_STAT_ANY operation returns
178 the identifier of the queue whose index was given in msqid.
179 On failure, -1 is returned and errno is set to indicate the error.
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183 EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the call‐
184 ing process does not have read permission on the message queue
185 msqid, and does not have the CAP_IPC_OWNER capability in the
186 user namespace that governs its IPC namespace.
187 EFAULT The argument cmd has the value IPC_SET or IPC_STAT, but the ad‐
189 dress pointed to by buf isn't accessible.
190 EIDRM The message queue was removed.
192 EINVAL Invalid value for cmd or msqid. Or: for a MSG_STAT operation,
194 the index value specified in msqid referred to an array slot
195 that is currently unused.
196 EPERM The argument cmd has the value IPC_SET or IPC_RMID, but the ef‐
198 fective user ID of the calling process is not the creator (as
199 found in msg_perm.cuid) or the owner (as found in msg_perm.uid)
200 of the message queue, and the caller is not privileged (Linux:
201 does not have the CAP_SYS_ADMIN capability).
202 EPERM An attempt (IPC_SET) was made to increase msg_qbytes beyond the
204 system parameter MSGMNB, but the caller is not privileged
205 (Linux: does not have the CAP_SYS_RESOURCE capability).
206
208 POSIX.1-2008.
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211 POSIX.1-2001, SVr4.
212 Various fields in the struct msqid_ds were typed as short under Linux
214 2.2 and have become long under Linux 2.4. To take advantage of this, a
215 recompilation under glibc-2.1.91 or later should suffice. (The kernel
216 distinguishes old and new calls by an IPC_64 flag in cmd.)
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219 The IPC_INFO, MSG_STAT, and MSG_INFO operations are used by the ipcs(1)
220 program to provide information on allocated resources. In the future
221 these may modified or moved to a /proc filesystem interface.
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224 msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),
225 sysvipc(7)
226Linux man-pages 6.04 2023-03-30 msgctl(2)