1MSGCTL(2) Linux Programmer's Manual MSGCTL(2)
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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; /* Time of creation or last
26 modification by msgctl() */
27 unsigned long msg_cbytes; /* # of bytes in queue */
28 msgqnum_t msg_qnum; /* # number of messages in queue */
29 msglen_t msg_qbytes; /* Maximum # of bytes in queue */
30 pid_t msg_lspid; /* PID of last msgsnd(2) */
31 pid_t msg_lrpid; /* PID of last msgrcv(2) */
32 };
33 The fields of the msgid_ds structure are as follows:
35 msg_perm This is an ipc_perm structure (see below) that specifies the
37 access permissions on the message queue.
38 msg_stime Time of the last msgsnd(2) system call.
40 msg_rtime Time of the last msgrcv(2) system call.
42 msg_ctime Time of creation of queue or time of last msgctl() IPC_SET
44 operation.
45 msg_cbytes Number of bytes in all messages currently on the message
47 queue. This is a nonstandard Linux extension that is not
48 specified in POSIX.
49 msg_qnum Number of messages currently on the message queue.
51 msg_qbytes Maximum number of bytes of message text allowed on the mes‐
53 sage queue.
54 msg_lspid ID of the process that performed the last msgsnd(2) system
56 call.
57 msg_lrpid ID of the process that performed the last msgrcv(2) system
59 call.
60 The ipc_perm structure is defined as follows (the highlighted fields
62 are settable using IPC_SET):
63 struct ipc_perm {
65 key_t __key; /* Key supplied to msgget(2) */
66 uid_t uid; /* Effective UID of owner */
67 gid_t gid; /* Effective GID of owner */
68 uid_t cuid; /* Effective UID of creator */
69 gid_t cgid; /* Effective GID of creator */
70 unsigned short mode; /* Permissions */
71 unsigned short __seq; /* Sequence number */
72 };
73 The least significant 9 bits of the mode field of the ipc_perm struc‐
75 ture define the access permissions for the message queue. The permis‐
76 sion bits are as follows:
77 0400 Read by user
79 0200 Write by user
80 0040 Read by group
81 0020 Write by group
82 0004 Read by others
83 0002 Write by others
84 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
86 Valid values for cmd are:
88 IPC_STAT
90 Copy information from the kernel data structure associated with
91 msqid into the msqid_ds structure pointed to by buf. The caller
92 must have read permission on the message queue.
93 IPC_SET
95 Write the values of some members of the msqid_ds structure
96 pointed to by buf to the kernel data structure associated with
97 this message queue, updating also its msg_ctime member.
98 The following members of the structure are updated: msg_qbytes,
100 msg_perm.uid, msg_perm.gid, and (the least significant 9 bits
101 of) msg_perm.mode.
102 The effective UID of the calling process must match the owner
104 (msg_perm.uid) or creator (msg_perm.cuid) of the message queue,
105 or the caller must be privileged. Appropriate privilege (Linux:
106 the CAP_SYS_RESOURCE capability) is required to raise the
107 msg_qbytes value beyond the system parameter MSGMNB.
108 IPC_RMID
110 Immediately remove the message queue, awakening all waiting
111 reader and writer processes (with an error return and errno set
112 to EIDRM). The calling process must have appropriate privileges
113 or its effective user ID must be either that of the creator or
114 owner of the message queue. The third argument to msgctl() is
115 ignored in this case.
116 IPC_INFO (Linux-specific)
118 Return information about system-wide message queue limits and
119 parameters in the structure pointed to by buf. This structure
120 is of type msginfo (thus, a cast is required), defined in
121 <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
122 struct msginfo {
124 int msgpool; /* Size in kibibytes of buffer pool
125 used to hold message data;
126 unused within kernel */
127 int msgmap; /* Maximum number of entries in message
128 map; unused within kernel */
129 int msgmax; /* Maximum number of bytes that can be
130 written in a single message */
131 int msgmnb; /* Maximum number of bytes that can be
132 written to queue; used to initialize
133 msg_qbytes during queue creation
134 (msgget(2)) */
135 int msgmni; /* Maximum number of message queues */
136 int msgssz; /* Message segment size;
137 unused within kernel */
138 int msgtql; /* Maximum number of messages on all queues
139 in system; unused within kernel */
140 unsigned short msgseg;
141 /* Maximum number of segments;
142 unused within kernel */
143 };
144 The msgmni, msgmax, and msgmnb settings can be changed via /proc
146 files of the same name; see proc(5) for details.
147 MSG_INFO (Linux-specific)
149 Return a msginfo structure containing the same information as
150 for IPC_INFO, except that the following fields are returned with
151 information about system resources consumed by message queues:
152 the msgpool field returns the number of message queues that cur‐
153 rently exist on the system; the msgmap field returns the total
154 number of messages in all queues on the system; and the msgtql
155 field returns the total number of bytes in all messages in all
156 queues on the system.
157 MSG_STAT (Linux-specific)
159 Return a msqid_ds structure as for IPC_STAT. However, the msqid
160 argument is not a queue identifier, but instead an index into
161 the kernel's internal array that maintains information about all
162 message queues on the system.
163 MSG_STAT_ANY (Linux-specific, since Linux 4.17)
165 Return a msqid_ds structure as for MSG_STAT. However,
166 msg_perm.mode is not checked for read access for msqid meaning
167 that any user can employ this operation (just as any user may
168 read /proc/sysvipc/msg to obtain the same information).
169
171 On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful
172 IPC_INFO or MSG_INFO operation returns the index of the highest used
173 entry in the kernel's internal array recording information about all
174 message queues. (This information can be used with repeated MSG_STAT
175 or MSG_STAT_ANY operations to obtain information about all queues on
176 the system.) A successful MSG_STAT or MSG_STAT_ANY operation returns
177 the identifier of the queue whose index was given in msqid.
178 On error, -1 is returned with errno indicating the error.
180
182 On failure, errno is set to one of the following:
183 EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the call‐
185 ing process does not have read permission on the message queue
186 msqid, and does not have the CAP_IPC_OWNER capability in the
187 user namespace that governs its IPC namespace.
188 EFAULT The argument cmd has the value IPC_SET or IPC_STAT, but the ad‐
190 dress pointed to by buf isn't accessible.
191 EIDRM The message queue was removed.
193 EINVAL Invalid value for cmd or msqid. Or: for a MSG_STAT operation,
195 the index value specified in msqid referred to an array slot
196 that is currently unused.
197 EPERM The argument cmd has the value IPC_SET or IPC_RMID, but the ef‐
199 fective user ID of the calling process is not the creator (as
200 found in msg_perm.cuid) or the owner (as found in msg_perm.uid)
201 of the message queue, and the caller is not privileged (Linux:
202 does not have the CAP_SYS_ADMIN capability).
203 EPERM An attempt (IPC_SET) was made to increase msg_qbytes beyond the
205 system parameter MSGMNB, but the caller is not privileged
206 (Linux: does not have the CAP_SYS_RESOURCE capability).
207
209 POSIX.1-2001, POSIX.1-2008, SVr4.
210
212 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
213 or by any version of POSIX. However, some old implementations required
214 the inclusion of these header files, and the SVID also documented their
215 inclusion. Applications intended to be portable to such old systems
216 may need to include these header files.
217 The IPC_INFO, MSG_STAT, and MSG_INFO operations are used by the ipcs(1)
219 program to provide information on allocated resources. In the future
220 these may modified or moved to a /proc filesystem interface.
221 Various fields in the struct msqid_ds were typed as short under Linux
223 2.2 and have become long under Linux 2.4. To take advantage of this, a
224 recompilation under glibc-2.1.91 or later should suffice. (The kernel
225 distinguishes old and new calls by an IPC_64 flag in cmd.)
226
228 msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),
229 sysvipc(7)
230
232 This page is part of release 5.10 of the Linux man-pages project. A
233 description of the project, information about reporting bugs, and the
234 latest version of this page, can be found at
235 https://www.kernel.org/doc/man-pages/.
236Linux 2020-11-01 MSGCTL(2)