msgget - Man Page

get the XSI message queue identifier


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <sys/msg.h>

int msgget(key_t key, int msgflg);


The msgget() function operates on XSI message queues (see the Base Definitions volume of POSIX.1-2017, Section 3.226, 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.


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 shall be initialized as follows:


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.


msg_qbytes shall be set to the system limit.

Return Value

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:


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.


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 following sections are informative.



Application Usage

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.



Future Directions


See Also

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.1-2017, Section 3.226, Message Queue, <sys_msg.h>

Referenced By

ftok(3p), mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_open(3p), mq_receive(3p), mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgrcv(3p), msgsnd(3p), sys_msg.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual