1MSGCTL(2) Linux Programmer's Manual MSGCTL(2)
2
6 msgctl - 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 message
17 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() */
24 time_t msg_rtime; /* Time of last msgrcv() */
25 time_t msg_ctime; /* Time of last change */
26 unsigned long __msg_cbytes; /* Current number of bytes in
27 queue (non-standard) */
28 msgqnum_t msg_qnum; /* Current number of messages
29 in queue */
30 msglen_t msg_qbytes; /* Maximum number of bytes
31 allowed in queue */
32 pid_t msg_lspid; /* PID of last msgsnd() */
33 pid_t msg_lrpid; /* PID of last msgrcv() */
34 };
35 The ipc_perm structure is defined in <sys/ipc.h> as follows (the high‐
37 lighted fields are settable using IPC_SET):
38 struct ipc_perm {
40 key_t key; /* Key supplied to msgget() */
41 uid_t uid; /* Effective UID of owner */
42 gid_t gid; /* Effective GID of owner */
43 uid_t cuid; /* Effective UID of creator */
44 gid_t cgid; /* Effective GID of creator */
45 unsigned short mode; /* Permissions */
46 unsigned short seq; /* Sequence number */
47 };
48 Valid values for cmd are:
50 IPC_STAT
52 Copy information from the kernel data structure associated with
53 msqid into the msqid_ds structure pointed to by buf. The caller
54 must have read permission on the message queue.
55 IPC_SET
57 Write the values of some members of the msqid_ds structure
58 pointed to by buf to the kernel data structure associated with
59 this message queue, updating also its msg_ctime member. The
60 following members of the structure are updated: msg_qbytes,
61 msg_perm.uid, msg_perm.gid, and (the least significant 9 bits
62 of) msg_perm.mode. The effective UID of the calling process
63 must match the owner (msg_perm.uid) or creator (msg_perm.cuid)
64 of the message queue, or the caller must be privileged. Appro‐
65 priate privilege (Linux: the CAP_IPC_RESOURCE capability) is
66 required to raise the msg_qbytes value beyond the system parame‐
67 ter MSGMNB.
68 IPC_RMID
70 Immediately remove the message queue, awakening all waiting
71 reader and writer processes (with an error return and errno set
72 to EIDRM). The calling process must have appropriate privileges
73 or its effective user ID must be either that of the creator or
74 owner of the message queue.
75 IPC_INFO (Linux specific)
77 Returns information about system-wide message queue limits and
78 parameters in the structure pointed to by buf. This structure
79 is of type msginfo (thus, a cast is required), defined in
80 <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
81 struct msginfo {
83 int msgpool; /* Size in bytes of buffer pool used
84 to hold message data; unused */
85 int msgmap; /* Max. # of entries in message
86 map; unused */
87 int msgmax; /* Max. # of bytes that can be
88 written in a single message */
89 int msgmnb; /* Max. # of bytes that can be written to
90 queue; used to initialize msg_qbytes
91 during queue creation (msgget()) */
92 int msgmni; /* Max. # of message queues */
93 int msgssz; /* Message segment size; unused */
94 int msgtql; /* Max. # of messages on all queues
95 in system; unused */
96 unsigned short int msgseg;
97 /* Max. # of segments; unused */
98 };
99 The msgmni, msgmax, and msgmnb settings can be changed via /proc
101 files of the same name; see proc(5) for details.
102 MSG_INFO (Linux specific)
104 Returns a msginfo structure containing the same information as
105 for IPC_INFO, except that the following fields are returned with
106 information about system resources consumed by message queues:
107 the msgpool field returns the number of message queues that cur‐
108 rently exist on the system; the msgmap field returns the total
109 number of messages in all queues on the system; and the msgtql
110 field returns the total number of bytes in all messages in all
111 queues on the system.
112 MSG_STAT (Linux specific)
114 Returns a msqid_ds structure as for IPC_STAT. However, the
115 msqid argument is not a queue identifier, but instead an index
116 into the kernel's internal array that maintains information
117 about all message queues on the system.
118
120 On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful
121 IPC_INFO or MSG_INFO operation returns the index of the highest used
122 entry in the kernel's internal array recording information about all
123 message queues. (This information can be used with repeated MSG_STAT
124 operations to obtain information about all queues on the system.) A
125 successful MSG_STAT operation returns the identifier of the queue whose
126 index was given in msqid.
127 On error, -1 is returned with errno indicating the error.
129
131 On failure, errno is set to one of the following:
132 EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the
134 calling process does not have read permission on the message
135 queue msqid, and does not have the CAP_IPC_OWNER capability.
136 EFAULT The argument cmd has the value IPC_SET or IPC_STAT, but the
138 address pointed to by buf isn't accessible.
139 EIDRM The message queue was removed.
141 EINVAL Invalid value for cmd or msqid. Or: for a MSG_STAT opera‐
143 tion, the index value specified in msqid referred to an
144 array slot that is currently unused.
145 EPERM The argument cmd has the value IPC_SET or IPC_RMID, but the
147 effective user ID of the calling process is not the creator
148 (as found in msg_perm.cuid) or the owner (as found in
149 msg_perm.uid) of the message queue, and the process is not
150 privileged (Linux: it does not have the CAP_SYS_ADMIN capa‐
151 bility).
152
154 The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the ipcs(8)
155 program to provide information on allocated resources. In the future
156 these may modified or moved to a /proc file system interface.
157 Various fields in the struct msqid_ds were shorts under Linux 2.2 and
159 have become longs under Linux 2.4. To take advantage of this, a recom‐
160 pilation under glibc-2.1.91 or later should suffice. (The kernel dis‐
161 tinguishes old and new calls by an IPC_64 flag in cmd.)
162
164 SVr4, POSIX.1-2001.
165
167 msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),
168 svipc(7)
169Linux 2.6.9 2004-11-10 MSGCTL(2)