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msgget() - Unix, Linux System Call

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NAME

msgget - get a message queue identifier

SYNOPSIS

 
#include <sys/types.h>
 
#include <sys/ipc.h>
 
#include <sys/msg.h>

int msgget(key_t key, int msgflg);

DESCRIPTION

The msgget() system call returns the message queue identifier associated with the value of the key argument. A new message queue is created if key has the value IPC_PRIVATE or key isn’t IPC_PRIVATE, no message queue with the given key key exists, and IPC_CREAT is specified in msgflg.

If msgflg specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for key, then msgget() fails with errno set to EEXIST. (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

Upon creation, the least significant bits of the argument msgflg define the permissions of the message queue. These permission bits have the same format and semantics as the permissions specified for the mode argument of open(2). (The execute permissions are not used.)

If a new message queue is created, then its associated data structure msqid_ds (see msgctl(2)) is initialised as follows:

TagDescription
msg_perm.cuid and msg_perm.uid are set to the effective user ID of the calling process.
msg_perm.cgid and msg_perm.gid are set to the effective group ID of the calling process.
The least significant 9 bits of msg_perm.mode are set to the least significant 9 bits of msgflg.
msg_qnum, msg_lspid, msg_lrpid, msg_stime and msg_rtime are set to 0.
msg_ctime is set to the current time.
msg_qbytes is set to the system limit MSGMNB.
If the message queue already exists the permissions are verified, and a check is made to see if it is marked for destruction.

RETURN VALUE

If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error.

ERRORS

On failure, errno is set to one of the following values:
TagDescription
EACCES A message queue exists for key, but the calling process does not have permission to access the queue, and does not have the CAP_IPC_OWNER capability.
EEXIST A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL.
ENOENT No message queue exists for key and msgflg did not specify IPC_CREAT.
ENOMEM A message queue has to be created but the system does not have enough memory for the new data structure.
ENOSPC A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded.

NOTES

IPC_PRIVATE isn’t a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the least significant 9 bits of msgflg and creates a new message queue (on success).

The following is a system limit on message queue resources affecting a msgget() call:

TagDescription
MSGMNI System wide maximum number of message queues: policy dependent (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmni).

BUGS

The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

CONFORMING TO

SVr4, POSIX.1-2001.

LINUX NOTES

Until version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled for deletion.

SEE ALSO


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