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msgget — get the XSI message queue identifier
int msgget(key_t key, int msgflg);
The msgget() function operates on XSI message queues (see the Base Definitions volume of POSIX.1‐2008, Section 3.225, Message Queue). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime.
The msgget() function shall return the message queue identifier associated with the argument key.
A message queue identifier, associated message queue, and data structure (see <sys/msg.h>), shall be created for the argument key if one of the following is true:
- The argument key is equal to IPC_PRIVATE.
Upon creation, the data structure associated with the new message queue identifier shall be initialized as follows:
- The argument key does not already have a message queue identifier associated with it, and ( msgflg & IPC_CREAT) is non-zero.
- msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid shall be set to the effective user ID and effective group ID, respectively, of the calling process.
- The low-order 9 bits of msg_perm.mode shall be set to the low-order 9 bits of msgflg.
- msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime shall be set to 0.
- msg_ctime shall be set to the current time, as described in Section 2.7.1, IPC General Description.
Upon successful completion, msgget() shall return a non-negative integer, namely a message queue identifier. Otherwise, it shall return −1 and set errno to indicate the error.
The msgget() function shall fail if:
- msg_qbytes shall be set to the system limit.
- 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 Section 2.7, XSI Interprocess Communication.
- 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.
The following sections are informative.
The POSIX Realtime Extension defines alternative interfaces for interprocess communication (IPC). Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces.
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, ftok(), mq_close(), mq_getattr(), mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgctl(), msgrcv(), msgsnd()
The Base Definitions volume of POSIX.1‐2008, Section 3.225, Message Queue, <sys_msg.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
- 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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