MQ_OPEN
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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NAME
mq_open - open a message queue
SYNOPSIS
#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);
Link with -lrt.
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
- 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).
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
mq_open()
| Thread safety | MT-Safe
|
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
NOTES
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.
BUGS
In kernels before 2.6.14,
the process umask was not applied to the permissions specified in
mode.
SEE ALSO
mq_close(3),
mq_getattr(3),
mq_notify(3),
mq_receive(3),
mq_send(3),
mq_unlink(3),
mq_overview(7)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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- Running on 
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