Section (2) msgget
msgget — get a System V message queue identifier
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
msgget() system call
returns the System V message queue identifier associated with
the value of the
argument. It may be used either to obtain the identifier of a
previously created message queue (when
msgflg is zero and
key does not have the value
IPC_PRIVATE), or to create a
A new message queue is created if
key has the value
IPC_PRIVATE, no message queue with the
IPC_CREAT is specified in
IPC_EXCL and a message queue
already exists for
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
argument of open(2). (The execute
permissions are not used.)
If a new message queue is created, then its associated
msqid_ds (see msgctl(2)) is initialized
msg_perm.uidare set to the effective user ID of the calling process.
msg_perm.gidare set to the effective group ID of the calling process.
The least significant 9 bits of
msg_perm.modeare set to the least significant 9 bits of
msg_rtimeare set to 0.
msg_ctimeis set to the current time.
msg_qbytesis set to the system limit
If the message queue already exists the permissions are verified, and a check is made to see if it is marked for destruction.
If successful, the return value will be the message queue
identifier (a nonnegative integer), otherwise −1 with
errno indicating the error.
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_OWNERcapability in the user namespace that governs its IPC namespace.
IPC_EXCLwere specified in
msgflg, but a message queue already exists for
No message queue exists for
msgflgdid not specify
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.
The inclusion of
isn_zsingle_quotesz_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.
IPC_PRIVATE isn_zsingle_quotesz_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
System-wide limit on the number of message queues. Before Linux 3.19, the default value for this limit was calculated using a formula based on available system memory. Since Linux 3.19, the default value is 32,000. On Linux, this limit can be read and modified via
The name choice
was perhaps unfortunate,
IPC_NEW would more clearly show its
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and the latest version of this page, can be found at
Copyright 1993 Giorgio Ciucci <giorgiocrcc.it>
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Added correction due to Nick Duffek <nsdbbc.com>, aeb, 960426
Modified Wed Nov 6 04:00:31 1996 by Eric S. Raymond <esrthyrsus.com>
Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpagesgmail.com>
Removed EIDRM from errors - that can_zsingle_quotesz_t happen...
Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com>
Added notes on capability requirements
Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com>
Language and formatting clean-ups
Added notes on /proc files