Section: Linux Programmer's Manual (2)
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msgget - get a System V message queue identifier
int msgget(key_t key, int msgflg);
system call returns the System V message queue identifier associated
with the value of the
A new message queue is created if
has the value
no message queue with the given key
is specified in
and a message queue already exists for
(This is analogous to the effect of the combination
O_CREAT | O_EXCL
Upon creation, the least significant bits of the argument
define the permissions of the message queue.
These permission bits have the same format and semantics
as the permissions specified for the
(The execute permissions are not used.)
If a new message queue is created,
then its associated data structure
is initialized as follows:
are set to the effective user ID of the calling process.
are set to the effective group ID of the calling process.
The least significant 9 bits of
are set to the least significant 9 bits of
are set to 0.
is set to the current time.
is 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
If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise -1
indicating the error.
is set to one of the following values:
A message queue exists for
but the calling process does not have permission to access the queue,
and does not have the
capability in the user namespace that governs its IPC namespace.
were specified in
but a message queue already exists for
No message queue exists for
did 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
would be exceeded.
POSIX.1-2001, POSIX.1-2008, SVr4.
The inclusion of
isn't required on Linux or by any version of POSIX.
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.
isn't a flag field but a
If this special value is used for
the system call ignores everything but the least significant 9 bits of
and creates a new message queue (on success).
The following is a system limit on message queue resources affecting a
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
Until version 2.3.20, Linux would return
on a message queue scheduled for deletion.
The name choice
was perhaps unfortunate,
would more clearly show its function.
This page is part of release 4.15 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- RETURN VALUE
- CONFORMING TO
- Linux notes
- SEE ALSO
This document was created by
using the manual pages.
Time: 12:25:17 GMT, September 26, 2018