46.1
Creating or Opening a Message Queue
The msgget() system call creates a new message queue or obtains the identifier of an
existing queue.
The key argument is a key generated using one of the methods described in Sec-
tion 45.2 (i.e., usually the value
IPC_PRIVATE
or a key returned by ftok()). The msgflg
argument is a bit mask that specifies the permissions (Table 15-4, on page 295) to
be placed on a new message queue or checked against an existing queue. In addition,
zero or more of the following flags can be ORed (
|
) in msgflg to control the operation
of msgget():
IPC_CREAT
If no message queue with the specified key exists, create a new queue.
IPC_EXCL
If
IPC_CREAT
was also specified, and a queue with the specified key already
exists, fail with the error
EEXIST
.
These flags are described in more detail in Section 45.1.
The msgget() system call begins by searching the set of all existing message
queues for one with the specified key. If a matching queue is found, the identifier
of that queue is returned (unless both
IPC_CREAT
and
IPC_EXCL
were specified in msgflg,
in which case an error is returned). If no matching queue was found and
IPC_CREAT
was specified in msgflg, a new queue is created and its identifier is returned.
The program in Listing 46-1 provides a command-line interface to the msgget()
system call. The program permits the use of command-line options and arguments
to specify all possibilities for the key and msgflg arguments to msgget(). Details of the
command format accepted by this program are shown in the usageError() function.
Upon successful queue creation, the program prints the queue identifier. We dem-
onstrate the use of this program in Section 46.2.2.
Listing 46-1: Using
msgget()
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