msgctl - message control operations
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/msg.h>
int msgctl ( int msqid, int cmd, struct msqid_ds *buf )
The function performs the control operation specified by cmd on the
message queue with identifier msqid. Legal values for cmd are:
IPC_STAT Copy info from the message queue data structure into the
structure pointed to by buf. The user must have read
access privileges on the message queue.
IPC_SET 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. Considered members
from the user supplied struct msqid_ds pointed to by buf
are
msg_perm.uid
msg_perm.gid
msg_perm.mode /* only lowest 9-bits */
msg_qbytes
The calling process effective user-ID must be one among
super-user, creator or owner of the message queue. Only
the super-user can raise the msg_qbytes value beyond the
system parameter MSGMNB.
IPC_RMID Remove immediately the message queue and its data structures
awakening all waiting reader and writer processes
(with an error return and errno set to EIDRM). The calling
process effective user-ID must be one among super-user,
creator or owner of the message queue.
If successful, the return value will be 0, otherwise -1 with errno
indicating the error.
For a failing return, errno will be set to one among the following values:
EACCES The argument cmd is equal to IPC_STAT but the calling
process has no read access permissions on the message queue
msqid.
EFAULT The argument cmd has 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 value IPC_SET or IPC_RMID but the calling
process effective user-ID has insufficient privileges to
execute the command. Note this is also the case of a non
super-user process trying to increase the msg_qbytes value
beyond the value specified by the system parameter MSGMNB.
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 can 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)
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