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msgctl - Perform message control operations
struct msqid_ds *buf );
Application developers might want to specify #include statements for
<sys/types.h> and <sys/ipc.h> before the one for <sys/msg.h> if programs
are being developed for multiple platforms. The additional #include
statements are not required on Tru64 UNIX systems or by ISO or X/Open
standards, but may be required on other vendors' systems that conform to
Interfaces documented on this reference page conform to industry standards
msgctl(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
Specifies the message queue ID.
cmd Specifies the type of command. The possible commands and the operations
they perform are as follows:
Queries the message queue ID by copying the contents of its
associated data structure into the buf structure. The process must
have mandatory read access to the message queue with respect to all
access control policies.
Sets the message queue ID by copying values found in the buf
structure into corresponding fields in the msqid_ds structure
associated with the message queue ID. This operation is restricted.
The effective user ID of the calling process must be equal to that
of superuser or equal to the value of msg_perm.uid or msg_perm.cuid
in the associated msqid_ds structure. Only superuser can raise the
value of msg_qbytes.
Removes the message queue ID and deallocates its associated
msqid_ds structure. This operation is restricted. The effective
user ID of the calling process must be equal to that of superuser
or equal to the value of msg_perm.uid or msg_perm.cuid in the
associated msqid_ds structure.
buf Points to an msqid_ds structure. This structure is used only with the
IPC_STAT and IPC_SET commands. With IPC_STAT, the results of the query
are copied to this structure. With IPC_SET, the values in this
structure are used to set the corresponding fields in the msqid_ds
structure associated with the message queue ID. In either case, the
calling process must have allocated the structure before making the
The msgctl() function allows a process to query or set the contents of the
msqid_ds structure associated with the specified message queue ID. It also
allows a process to remove the message queue ID and its associated msqid_ds
structure. The cmd value determines which operation is performed.
The IPC_SET command uses the user-supplied contents of the buf structure to
set the following members of the msqid_ds structure associated with the
message queue ID:
The owner's user ID.
The owner's group ID.
The access modes for the queue. Only the low-order nine bits are set.
The maximum number of bytes on the queue.
The time of the last successful IPC_SET operation.
Upon successful completion, msgctl() returns a value of 0 (zero).
Otherwise, a value of -1 is returned and errno is set to indicate the
The msgctl() function sets errno to the specified values for the following
The cmd parameter is IPC_STAT, but the calling process does not have
[Tru64 UNIX] The cmd parameter is IPC_STAT or IPC_SET, and an error
occurred in accessing the buf structure.
The msqid parameter is not a valid message queue ID, or the cmd
parameter is not a valid command.
One of the following occurred:
· The cmd parameter is equal to either IPC_RMID or IPC_SET, and the
calling process does not have appropriate privilege.
· The cmd parameter is equal to IPC_SET, and an attempt is being
made to increase the value of the msg_qbytes parameter when the
calling process does not have the superuser privilege.
Functions: msgget(2), msgrcv(2), msgsnd(2)
Data Structures: msqid_ds(4)
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