Reference

msgget(2)

get message queue

NAME

msgget – get message queue

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/msg.h>

int

msgget(key_t key, int msgflg);

DESCRIPTION

The msgget() function 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 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 which are set by ORing these constants:

    0400

    Read access for user.

    0200

    Write access for user.

    0040

    Read access for group.

    0020

    Write access for group.

    0004

    Read access for other.

    0002

    Write access for other.

  • msg_cbytes, 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 was not set in msgflg and no message queue associated with key was found.

SEE ALSO

msgctl(2), msgrcv(2), msgsnd(2)

HISTORY

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

History