msgget - get the message queue identifier
#include <sys/msg.h> int msgget(key_t key, int msgflg);
The msgget() function returns the message queue identifier associated with the argument key.
A message queue identifier, associated message queue and data structure, see <sys/msg.h>, are created for the argument key if one of the following is true:
- The argument key is equal to IPC_PRIVATE.
- The argument key does not already have a message queue identifier associated with it, and (msgflg & IPC_CREAT) is non-zero.
Upon creation, the data structure associated with the new message queue identifier is initialised 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 limit.
Upon successful completion, msgget() returns a non-negative integer, namely a message queue identifier. Otherwise, it returns -1 and errno is set to indicate the error.
The msgget() function will fail if:
- A message queue identifier exists for the argument key, but operation permission as specified by the low-order 9 bits of msgflg would not be granted, see IPC.
- A message queue identifier exists for the argument key but ((msgflg & IPC_CREAT) && (msgflg & IPC_EXCL)) is non-zero.
- A message queue identifier does not exist for the argument key and (msgflg & IPC_CREAT) is 0.
- 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.
The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in IPC can be easily modified to use the alternative interfaces.
mq_close(), mq_getattr(), mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgctl(), msgrcv(), msgsnd(), <sys/msg.h>, IPC.
Derived from Issue 2 of the SVID.