IPC Functions, Key Arguments, and Creation Flags

Processes requesting access to an IPC facility must be able to identify it. To do this, functions that initialize or provide access to an IPC facility use a key_t key argument. The key is an arbitrary value or one that can be derived from a common seed at run time. One way is with ftok(3C), which converts a filename to a key value that is unique within the system.

Functions that initialize or get access to messages, semaphores, or shared memory return an ID number of type int. IPC functions that perform read, write, and control operations use this ID.

If the key argument is specified as IPC_PRIVATE, the call initializes a new instance of an IPC facility that is private to the creating process.

When the IPC_CREAT flag is supplied in the flags argument appropriate to the call, the function tries to create the facility if it does not exist already.

When called with both the IPC_CREAT and IPC_EXCL flags, the function fails if the facility already exists. This can be useful when more than one process might attempt to initialize the facility. One such case might involve several server processes having access to the same facility. If they all attempt to create the facility with IPC_EXCL in effect, only the first attempt succeeds.

If neither of these flags is given and the facility already exists, the functions to get access simply return the ID of the facility. If IPC_CREAT is omitted and the facility is not already initialized, the calls fail.

These control flags are combined, using logical (bitwise) OR, with the octal permission modes to form the flags argument. For example, the statement below initializes a new message queue if the queue does not exist.

msqid = msgget(ftok("/tmp", 'A'), (IPC_CREAT | IPC_EXCL | 0400)); 

The first argument evaluates to a key ('A') based on the string ("/tmp"). The second argument evaluates to the combined permissions and control flags.