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 |
POSIX.1-2001, POSIX.1-2008.
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.
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)