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
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 structure can be updated:
msg_perm.mode /* only lowest 9-bits */
The calling process must have appropriate (probably, root) privileges or its effec-
tive 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 process must have appropriate (probably, root) privileges or
its effective user-ID must be either that of the creator or owner of the message
On success, the return value will be 0, otherwise -1 with errno indicating the error.
On failure, errno is set to one of the following:
EACCES The argument cmd is equal to IPC_STAT but the calling process does not have
read permission on the message queue msqid.
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 command.
The IPC_INFO, MSG_STAT and MSG_INFO control calls are used by the ipcs(8) program to pro-
vide information on allocated resources. In the future these may be modified as needed or
moved to a proc file system interface.
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 recompilation under glibc-2.1.91 or later
should suffice. (The kernel distinguishes 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)