1MSGCTL(2) Linux Programmer's Manual MSGCTL(2)
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6 msgctl - System V message control operations
7
9 #include <sys/msg.h>
10 int msgctl(int msqid, int cmd, struct msqid_ds *buf);
12
14 msgctl() performs the control operation specified by cmd on the Sys‐
15 tem V message queue with identifier msqid.
16 The msqid_ds data structure is defined in <sys/msg.h> as follows:
18 struct msqid_ds {
20 struct ipc_perm msg_perm; /* Ownership and permissions */
21 time_t msg_stime; /* Time of last msgsnd(2) */
22 time_t msg_rtime; /* Time of last msgrcv(2) */
23 time_t msg_ctime; /* Time of creation or last
24 modification by msgctl() */
25 unsigned long msg_cbytes; /* # of bytes in queue */
26 msgqnum_t msg_qnum; /* # number of messages in queue */
27 msglen_t msg_qbytes; /* Maximum # of bytes in queue */
28 pid_t msg_lspid; /* PID of last msgsnd(2) */
29 pid_t msg_lrpid; /* PID of last msgrcv(2) */
30 };
31 The fields of the msgid_ds structure are as follows:
33 msg_perm This is an ipc_perm structure (see below) that specifies the
35 access permissions on the message queue.
36 msg_stime Time of the last msgsnd(2) system call.
38 msg_rtime Time of the last msgrcv(2) system call.
40 msg_ctime Time of creation of queue or time of last msgctl() IPC_SET
42 operation.
43 msg_cbytes Number of bytes in all messages currently on the message
45 queue. This is a nonstandard Linux extension that is not
46 specified in POSIX.
47 msg_qnum Number of messages currently on the message queue.
49 msg_qbytes Maximum number of bytes of message text allowed on the mes‐
51 sage queue.
52 msg_lspid ID of the process that performed the last msgsnd(2) system
54 call.
55 msg_lrpid ID of the process that performed the last msgrcv(2) system
57 call.
58 The ipc_perm structure is defined as follows (the highlighted fields
60 are settable using IPC_SET):
61 struct ipc_perm {
63 key_t __key; /* Key supplied to msgget(2) */
64 uid_t uid; /* Effective UID of owner */
65 gid_t gid; /* Effective GID of owner */
66 uid_t cuid; /* Effective UID of creator */
67 gid_t cgid; /* Effective GID of creator */
68 unsigned short mode; /* Permissions */
69 unsigned short __seq; /* Sequence number */
70 };
71 The least significant 9 bits of the mode field of the ipc_perm struc‐
73 ture define the access permissions for the message queue. The permis‐
74 sion bits are as follows:
75 0400 Read by user
77 0200 Write by user
78 0040 Read by group
79 0020 Write by group
80 0004 Read by others
81 0002 Write by others
82 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
84 Valid values for cmd are:
86 IPC_STAT
88 Copy information from the kernel data structure associated with
89 msqid into the msqid_ds structure pointed to by buf. The caller
90 must have read permission on the message queue.
91 IPC_SET
93 Write the values of some members of the msqid_ds structure
94 pointed to by buf to the kernel data structure associated with
95 this message queue, updating also its msg_ctime member.
96 The 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.
100 The effective UID of the calling process must match the owner
102 (msg_perm.uid) or creator (msg_perm.cuid) of the message queue,
103 or the caller must be privileged. Appropriate privilege (Linux:
104 the CAP_SYS_RESOURCE capability) is required to raise the
105 msg_qbytes value beyond the system parameter MSGMNB.
106 IPC_RMID
108 Immediately remove the message queue, awakening all waiting
109 reader and writer processes (with an error return and errno set
110 to EIDRM). The calling process must have appropriate privileges
111 or its effective user ID must be either that of the creator or
112 owner of the message queue. The third argument to msgctl() is
113 ignored in this case.
114 IPC_INFO (Linux-specific)
116 Return information about system-wide message queue limits and
117 parameters in the structure pointed to by buf. This structure
118 is of type msginfo (thus, a cast is required), defined in
119 <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
120 struct msginfo {
122 int msgpool; /* Size in kibibytes of buffer pool
123 used to hold message data;
124 unused within kernel */
125 int msgmap; /* Maximum number of entries in message
126 map; unused within kernel */
127 int msgmax; /* Maximum number of bytes that can be
128 written in a single message */
129 int msgmnb; /* Maximum number of bytes that can be
130 written to queue; used to initialize
131 msg_qbytes during queue creation
132 (msgget(2)) */
133 int msgmni; /* Maximum number of message queues */
134 int msgssz; /* Message segment size;
135 unused within kernel */
136 int msgtql; /* Maximum number of messages on all queues
137 in system; unused within kernel */
138 unsigned short msgseg;
139 /* Maximum number of segments;
140 unused within kernel */
141 };
142 The msgmni, msgmax, and msgmnb settings can be changed via /proc
144 files of the same name; see proc(5) for details.
145 MSG_INFO (Linux-specific)
147 Return a msginfo structure containing the same information as
148 for IPC_INFO, except that the following fields are returned with
149 information about system resources consumed by message queues:
150 the msgpool field returns the number of message queues that cur‐
151 rently exist on the system; the msgmap field returns the total
152 number of messages in all queues on the system; and the msgtql
153 field returns the total number of bytes in all messages in all
154 queues on the system.
155 MSG_STAT (Linux-specific)
157 Return a msqid_ds structure as for IPC_STAT. However, the msqid
158 argument is not a queue identifier, but instead an index into
159 the kernel's internal array that maintains information about all
160 message queues on the system.
161 MSG_STAT_ANY (Linux-specific, since Linux 4.17)
163 Return a msqid_ds structure as for MSG_STAT. However,
164 msg_perm.mode is not checked for read access for msqid meaning
165 that any user can employ this operation (just as any user may
166 read /proc/sysvipc/msg to obtain the same information).
167
169 On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful
170 IPC_INFO or MSG_INFO operation returns the index of the highest used
171 entry in the kernel's internal array recording information about all
172 message queues. (This information can be used with repeated MSG_STAT
173 or MSG_STAT_ANY operations to obtain information about all queues on
174 the system.) A successful MSG_STAT or MSG_STAT_ANY operation returns
175 the identifier of the queue whose index was given in msqid.
176 On failure, -1 is returned and errno is set to indicate the error.
178
180 EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the call‐
181 ing process does not have read permission on the message queue
182 msqid, and does not have the CAP_IPC_OWNER capability in the
183 user namespace that governs its IPC namespace.
184 EFAULT The argument cmd has the value IPC_SET or IPC_STAT, but the ad‐
186 dress pointed to by buf isn't accessible.
187 EIDRM The message queue was removed.
189 EINVAL Invalid value for cmd or msqid. Or: for a MSG_STAT operation,
191 the index value specified in msqid referred to an array slot
192 that is currently unused.
193 EPERM The argument cmd has the value IPC_SET or IPC_RMID, but the ef‐
195 fective user ID of the calling process is not the creator (as
196 found in msg_perm.cuid) or the owner (as found in msg_perm.uid)
197 of the message queue, and the caller is not privileged (Linux:
198 does not have the CAP_SYS_ADMIN capability).
199 EPERM An attempt (IPC_SET) was made to increase msg_qbytes beyond the
201 system parameter MSGMNB, but the caller is not privileged
202 (Linux: does not have the CAP_SYS_RESOURCE capability).
203
205 POSIX.1-2001, POSIX.1-2008, SVr4.
206
208 The IPC_INFO, MSG_STAT, and MSG_INFO operations are used by the ipcs(1)
209 program to provide information on allocated resources. In the future
210 these may modified or moved to a /proc filesystem interface.
211 Various fields in the struct msqid_ds were typed as short under Linux
213 2.2 and have become long under Linux 2.4. To take advantage of this, a
214 recompilation under glibc-2.1.91 or later should suffice. (The kernel
215 distinguishes old and new calls by an IPC_64 flag in cmd.)
216
218 msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),
219 sysvipc(7)
220
222 This page is part of release 5.13 of the Linux man-pages project. A
223 description of the project, information about reporting bugs, and the
224 latest version of this page, can be found at
225 https://www.kernel.org/doc/man-pages/.
226Linux 2021-03-22 MSGCTL(2)