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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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mq_open(3) Library Functions Manual mq_open(3)
mq_open - open a message queue
Real-time library (librt, -lrt)
#include <fcntl.h> /* For O_* constants */
#include <sys/stat.h> /* For mode constants */
#include <mqueue.h>
mqd_t mq_open(const char *name, int oflag);
mqd_t mq_open(const char *name, int oflag, mode_t mode,
struct mq_attr *attr);
mq_open() creates a new POSIX message queue or opens an existing
queue. The queue is identified by name. For details of the
construction of name, see mq_overview(7).
The oflag argument specifies flags that control the operation of
the call. (Definitions of the flags values can be obtained by
including <fcntl.h>.) Exactly one of the following must be
specified in oflag:
O_RDONLY
Open the queue to receive messages only.
O_WRONLY
Open the queue to send messages only.
O_RDWR Open the queue to both send and receive messages.
Zero or more of the following flags can additionally be ORed in
oflag:
O_CLOEXEC (since Linux 2.6.26)
Set the close-on-exec flag for the message queue
descriptor. See open(2) for a discussion of why this flag
is useful.
O_CREAT
Create the message queue if it does not exist. The owner
(user ID) of the message queue is set to the effective user
ID of the calling process. The group ownership (group ID)
is set to the effective group ID of the calling process.
O_EXCL If O_CREAT was specified in oflag, and a queue with the
given name already exists, then fail with the error EEXIST.
O_NONBLOCK
Open the queue in nonblocking mode. In circumstances where
mq_receive(3) and mq_send(3) would normally block, these
functions instead fail with the error EAGAIN.
If O_CREAT is specified in oflag, then two additional arguments
must be supplied. The mode argument specifies the permissions to
be placed on the new queue, as for open(2). (Symbolic definitions
for the permissions bits can be obtained by including
<sys/stat.h>.) The permissions settings are masked against the
process umask.
The fields of the struct mq_attr pointed to attr specify the
maximum number of messages and the maximum size of messages that
the queue will allow. This structure is defined as follows:
struct mq_attr {
long mq_flags; /* Flags (ignored for mq_open()) */
long mq_maxmsg; /* Max. # of messages on queue */
long mq_msgsize; /* Max. message size (bytes) */
long mq_curmsgs; /* # of messages currently in queue
(ignored for mq_open()) */
};
Only the mq_maxmsg and mq_msgsize fields are employed when calling
mq_open(); the values in the remaining fields are ignored.
If attr is NULL, then the queue is created with implementation-
defined default attributes. Since Linux 3.5, two /proc files can
be used to control these defaults; see mq_overview(7) for details.
On success, mq_open() returns a message queue descriptor for use
by other message queue functions. On error, mq_open() returns
(mqd_t) -1, with errno set to indicate the error.
EACCES The queue exists, but the caller does not have permission
to open it in the specified mode.
EACCES name contained more than one slash.
EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a
queue with this name already exists.
EINVAL name doesn't follow the format in mq_overview(7).
EINVAL O_CREAT was specified in oflag, and attr was not NULL, but
attr->mq_maxmsg or attr->mq_msqsize was invalid. Both of
these fields must be greater than zero. In a process that
is unprivileged (does not have the CAP_SYS_RESOURCE
capability), attr->mq_maxmsg must be less than or equal to
the msg_max limit, and attr->mq_msgsize must be less than
or equal to the msgsize_max limit. In addition, even in a
privileged process, attr->mq_maxmsg cannot exceed the
HARD_MAX limit. (See mq_overview(7) for details of these
limits.)
EMFILE The per-process limit on the number of open file and
message queue descriptors has been reached (see the
description of RLIMIT_NOFILE in getrlimit(2)).
ENAMETOOLONG
name was too long.
ENFILE The system-wide limit on the total number of open files and
message queues has been reached.
ENOENT The O_CREAT flag was not specified in oflag, and no queue
with this name exists.
ENOENT name was just "/" followed by no other characters.
ENOMEM Insufficient memory.
ENOSPC Insufficient space for the creation of a new message queue.
This probably occurred because the queues_max limit was
encountered; see mq_overview(7).
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ mq_open() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C library/kernel differences
The mq_open() library function is implemented on top of a system
call of the same name. The library function performs the check
that the name starts with a slash (/), giving the EINVAL error if
it does not. The kernel system call expects name to contain no
preceding slash, so the C library function passes name without the
preceding slash (i.e., name+1) to the system call.
POSIX.1-2008.
POSIX.1-2001.
Before Linux 2.6.14, the process umask was not applied to the
permissions specified in mode.
mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3),
mq_send(3), mq_unlink(3), mq_overview(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 mq_open(3)
Pages that refer to this page: getrlimit(2), syscalls(2), umask(2), mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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