MSGGET(2) — SYSTEM CALLS

NAME

msgget − get message queue

SYNOPSIS

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

int msgget(key, msgflg)
key_t key;
int msgflg;

DESCRIPTION

msgget() returns the message queue identifier associated with key.

A message queue identifier and associated message queue and data structure (see intro(2)) are created for key() if one of the following is true:

• key is equal to IPC_PRIVATE.

• key does not already have a message queue identifier associated with it, and (msgflg & IPC_CREAT) is “true”.

Upon creation, the data structure associated with the new message queue identifier is initialized as follows:

• msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process.

• The low-order 9 bits of msg_perm.mode are set equal to the low-order 9 bits of msgflg.

• msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are set equal to 0.

• msg_ctime is set equal to the current time.

• msg_qbytes is set equal to the system-wide standard value of the maximum number of bytes allowed on a message queue.

A message queue identifier (msqid) is a unique positive integer created by a msgget(2) system call. Each msqid has a message queue and a data structure associated with it. The data structure is referred to as msqid_ds() and contains the following members:

structipc_perm msg_perm; /∗ operation permission struct ∗/
ushortmsg_qnum;/∗ number of msgs on q ∗/
ushortmsg_qbytes;/∗ max number of bytes on q ∗/
ushortmsg_lspid;/∗ pid of last msgsnd operation ∗/
ushortmsg_lrpid;/∗ pid of last msgrcv operation ∗/
time_tmsg_stime;/∗ last msgsnd time ∗/
time_tmsg_rtime;/∗ last msgrcv time ∗/
time_tmsg_ctime;/∗ last change time ∗/
/∗ Times measured in secs since ∗/
/∗ 00:00:00 GMT, Jan. 1, 1970 ∗/

msg_perm() is an ipc_perm structure that specifies the message operation permission (see below). This structure includes the following members:

ushortcuid;/∗ creator user id ∗/
ushortcgid;/∗ creator group id ∗/
ushortuid;/∗ user id ∗/
ushortgid;/∗ group id ∗/
ushortmode;/∗ r/w permission ∗/

msg_qnum is the number of messages currently on the queue. msg_qbytes is the maximum number of bytes allowed on the queue. msg_lspid is the process ID of the last process that performed a msgsnd operation. msg_lrpid is the process ID of the last process that performed a msgrcv operation. msg_stime is the time of the last msgsnd operation, msg_rtime is the time of the last msgrcv operation, and msg_ctime is the time of the last msgctl(2) operation that changed a member of the above structure.

RETURN VALUES

msgget() returns A non-negative message queue identifier on success. On failure, it returns −1 and sets errno to indicate the error.

ERRORS

EACCES A message queue identifier exists for key, but operation permission (see intro(2)) as specified by the low-order 9 bits of msgflg would not be granted.

EEXIST A message queue identifier exists for key() but ( (msgflg & IPC_CREAT) & (msgflg & IPC_EXCL)) is “true”.

ENOENT A message queue identifier does not exist for key() and (msgflg & IPC_CREAT) is “false”.

ENOSPC A message queue identifier is to be created but the system-imposed limit on the maximum number of allowed message queue identifiers system wide would be exceeded.

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