NAME
msgget - get a System V message queue identifier SYNOPSIS
#include
#include
#include
int msgget(keyt key, int msgflg); DESCRIPTION The msgget() system call returns the System V message queue identifier associated with the value of the key argument. A new message queue is created if key has the value IPCPRIVATE or key isn't IPCPRIVATE, no message queue with the given key key exists, and IPCCREAT is specified in msgflg. If msgflg specifies both IPCCREAT and IPCEXCL 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 OCREAT | OEXCL 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 msqidds (see msgctl(2)) is initialized as follows: msgperm.cuid and msgperm.uid are set to the effective user ID of the calling process. msgperm.cgid and msgperm.gid are set to the effective group ID of the calling process. The least significant 9 bits of msgperm.mode are set to the least significant 9 bits of msgflg. msgqnum, msglspid, msglrpid, msgstime and msgrtime are set to 0. msgctime is set to the current time. msgqbytes is set to the system limit MSGMNB. If the message queue already exists the permissions are verified, and a check is made to see if it is marked for destruction. RETURN VALUE If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error. ERRORS On failure, errno is set to one of the following values: EACCES A message queue exists for key, but the calling process does not have permission to access the queue, and does not have the CAPIPCOWNER capability. EEXIST A message queue exists for key and msgflg specified both IPCCREAT and IPCEXCL. ENOENT No message queue exists for key and msgflg did not specify IPCCREAT. ENOMEM A message queue has to be created but the system does not have enough memory for the new data structure. ENOSPC A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded. CONFORMING TO
SVr4, POSIX.1-2001. NOTES The inclusion of
and isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files. IPCPRIVATE isn't a flag field but a keyt type. If this special value is used for key, the system call ignores everything but the least sig‐ nificant 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: MSGMNI System wide maximum number of message queues: policy dependent (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmni). Linux notes Until version 2.3.20 Linux would return EIDRM for a msgget() on a mes‐ sage queue scheduled for deletion. BUGS The name choice IPCPRIVATE was perhaps unfortunate, IPCNEW would more clearly show its function. SEE ALSO msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mqover‐ view(7), svipc(7) COLOPHON This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-05-31 MSGGET(2)