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.

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.