MSGCTL(2) Linux Programmer's Manual MSGCTL(2)
msgctl - message control operations
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
This function performs the control operation specified by cmd on the
message queue with identifier msqid. Legal values for cmd are:
Copy info from the message queue data structure associated with
msqid into the structure pointed to by buf. The caller must
have read permission on the message queue.
Write the values of some members of the msqid_ds structure
pointed to by buf to the message queue data structure, updating
also its msg_ctime member. The following members of the struc-
ture can be updated:
msg_perm.mode /* only lowest 9-bits */
The calling process must have appropriate (probably, root) priv-
ileges or its effective user-ID must be that of the creator
(msg_perm.cuid) or owner (msg_perm.uid) of the message queue.
Appropriate (probably, root) privileges are required to raise
the msg_qbytes value beyond the system parameter MSGMNB.
Immediately remove the message queue and its associated data
structure, awakening all waiting reader and writer processes
(with an error return and errno set to EIDRM). The calling pro-
cess must have appropriate (probably, root) privileges or its
effective user-ID must be either that of the creator or owner of
the message queue.
On success, the return value will be 0, otherwise -1 with errno indi-
cating the error.
On failure, errno is set to one of the following:
EACCES The argument cmd is equal to IPC_STAT but the calling pro-
cess does not have read permission on the message queue
EFAULT The argument cmd has the value IPC_SET or IPC_STAT, but the
address pointed to by buf isn't accessible.
EIDRM The message queue was removed.
EINVAL Invalid value for cmd or msqid.
EPERM The argument cmd has the value IPC_SET or IPC_RMID, but the
calling process has insufficient privileges to execute the
The IPC_INFO, MSG_STAT and MSG_INFO control calls are used by the
ipcs(8) program to provide information on allocated resources. In the
future these may be modified as needed or moved to a proc file system
Various fields in a struct msqid_ds were shorts under Linux 2.2 and
have become longs under Linux 2.4. To take advantage of this, a recom-
pilation under glibc-2.1.91 or later should suffice. (The kernel dis-
tinguishes old and new calls by a IPC_64 flag in cmd.)
SVr4, SVID. SVID does not document the EIDRM error condition.
ipc(5), msgget(2), msgsnd(2), msgrcv(2)
Linux 2.4.1 2001-02-18 MSGCTL(2)