May 13, 2004 MSGGET 2

### NAME

msgget - get message queue identifier

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```int` msgget(```key_t key``, int msgflg`)

### DESCRIPTION

The msgget(``) system call returns the message queue identifier associated with ```key`. A message queue identifier is a unique integer greater than zero.

A message queue is created if either ```key` is equal to ```IPC_PRIVATE`, or ```key` does not have a message queue identifier associated with it and the ```IPC_CREAT` bit is set in ```msgflg`. If both the ```IPC_CREAT` bit and the ```IPC_EXCL` bit are set in ```msgflg`, and ```key` has a message queue identifier associated with it already, the operation will fail.

If a new message queue is created, the data structure associated with it (the msqid_ds structure, see msgctl(2)) is initialized as follows:

- msg_perm.cuid and msg_perm.uid are set to the effective uid of the calling process.

- msg_perm.gid and msg_perm.cgid are set to the effective gid of the calling process.

- msg_perm.mode is set to the lower 9 bits of ```msgflg`.

- msg_qnum, msg_lspid, msg_lrpid, msg_rtime, and msg_stime are set to 0.

- msg_qbytes is set to the system wide maximum value for the number of bytes in a queue (```MSGMNB`).

- msg_ctime is set to the current time.

### RETURN VALUES

Upon successful completion a positive message queue identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

### ERRORS

[```EACCES`]
  A message queue is already associated with ```key` and the caller has no permission to access it.

[```EEXIST`]
  Both ```IPC_CREAT` and ```IPC_EXCL` are set in ```msgflg`, and a message queue is already associated with ```key`.

[```ENOSPC`]
  A new message queue could not be created because the system limit for the number of message queues has been reached.

[```ENOENT`]
  ```IPC_CREAT` is not set in ```msgflg` and no message queue associated with ```key` was found.

### SEE ALSO

msgctl(2), msgrcv(2), msgsnd(2), ftok(3)

### STANDARDS

The msgget system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'') .

### HISTORY

Message queues appeared in the first release of `AT&T` System V `UNIX`.