MSGGETSection: Linux Programmer's Manual (2)
Index Return to Main Contents
NAMEmsgget - get a message queue identifier
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
DESCRIPTIONThe 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:
- 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.
RETURN VALUEIf successful, the return value will be the message queue identifier (a non-negative integer), otherwise -1 with errno indicating the error.
ERRORSOn failure, errno is set to one of the following values:
- 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.
- A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL.
- No message queue exists for key and msgflg did not specify IPC_CREAT.
- A message queue has to be created but the system does not have enough memory for the new data structure.
- A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded.
CONFORMING TOSVr4, POSIX.1-2001.
NOTESIPC_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:
- System wide maximum number of message queues: policy dependent (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmni).
Linux NotesUntil version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled for deletion.
BUGSThe name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.
SEE ALSOmsgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7)