MQ_OPEN(3) Linux Programmer's Manual MQ_OPEN(3)
mq_open - open a message queue
#include <<fcntl.h>> /* For O_* constants */
#include <<sys/stat.h>> /* For mode constants */
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);
Link with -lrt.
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:
Open the queue to receive messages only.
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:
Open the queue in non-blocking mode. In circumstances where
mq_receive(3) and mq_send(3) would normally block, these func-
tions instead fail with the error EAGAIN.
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.
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 permis-
sions bits can be obtained by including <sys/stat.h>.) The permissions
settings are masked against the process umask. The attr argument spec-
ifies attributes for the queue. See mq_getattr(3) for details. If
attr is NULL, then the queue is created with implementation-defined
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.
EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a queue
with this name already exists.
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 unprivi-
leged (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 msg-
size_max limit. In addition, even in a privileged process,
attr->mq_maxmsg cannot exceed the HARD_MAX limit. (See mq_over-
view(7) for details of these limits.)
EMFILE The process already has the maximum number of files and message
name was too long.
ENFILE The system 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.
ENOMEM Insufficient memory.
ENOSPC Insufficient space for the creation of a new message queue.
This probably occurred because the queues_max limit was encoun-
tered; see mq_overview(7).
In kernels before 2.6.14, the process umask was not applied to the per-
missions specified in mode.
mq_close(3), mq_getattr(3), mq_notify(3), mq_receive(3), mq_send(3),
This page is part of release 3.05 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 2008-05-08 MQ_OPEN(3)