MSGGET(2) | System Calls Manual | MSGGET(2)

# NAME

`msgget` —
get message queue identifier

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <sys/msg.h>`

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.

May 13, 2004 | NetBSD 9.2