NAME
msgget —
get message queue
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgget(key_t
key, int
msgflg);
DESCRIPTION
Themsgget()
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 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_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.
ENVIRONMENT
The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the sysvipcd(8) daemon has to be running.
If the USR_SYSVIPC variable is set in a
process' environment, the process and its children will use the userspace
implementation.
ERRORS
- [
EACCES] - A message queue is already associated with key and the caller has no permission to access it.
- [
EEXIST] - Both
IPC_CREATandIPC_EXCLare 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_CREATwas not set in msgflg and no message queue associated with key was found.
SEE ALSO
HISTORY
Message queues appeared in the first release of AT&T System V UNIX.
AUTHORS
The DragonFly specific userspace implementation (see ENVIRONMENT) was written by Larisa Grigore.