mq_open(3c)mq_open(3c)NAMEmq_open - open/create a message queue
C SYNOPSIS
#include <mqueue.h>
mqd_t mq_open (const char *mq_name, int oflag, ... /* mode_t mode, struct
mq_attr *mq_attr */);
DESCRIPTIONmq_open is used to open or to create and open a message queue. It returns
a message queue descriptor that is used as the handle for other
operations on the queue. mq_name points to a path name that is
constructed using the standard filename conventions.
The oflag argument is used to specify send, receive or send/receive
access to the queue. It is also used to specify creation of the queue.
The following oflags may be selected (the first three flags are mutually
exclusive):
O_RDONLY Open the queue for receiving messages, but not for sending
messages.
O_WRONLY Open the queue for sending messages, but not for receiving
messages.
O_RDWR Open the queue for both sending and receiving messages.
O_NONBLOCK Do not block on mq_send when sending messages to a full
queue (or if required resources are unavailable) or on
mq_receive when receiving messages from a empty queue (or if
required resources are unavailable). [see mq_send(3c) and
mq_receive(3c)].
O_CREAT Create the message queue. If the message queue exists, this
flag has no effect, except as noted under O_EXCL below.
Otherwise, the third argument, mode and the fourth argument
mq_attr are used to create the message queue. The uid and
gid of the queue are set to the effective uid and gid,
respectively, of the process. The mode for the queue is set
to the value of the mode argument, similar to that for file
creation. The mode of queue specifies the read and write
(corresponding to receive and send) permission for user,
group and the world. The queue is created according to the
specifications in the fourth argument, mq_attr. If this
argument is non-NULL, the mq_msgsize field of the mq_attr
structure specifies the maximum size of a single message in
the queue. Messages larger than this size sent to the queue
will generate an error. The mq_maxmsg field specifies the
maximum number of messages on the queue at any one time. If
the mq_attr argument is NULL, the queue is created using the
default values of 32 and 64 for mq_maxmsg and mq_msgsize,
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respectively.
O_EXCL If O_EXCL and O_CREAT are set, mq_open will fail if the
queue exists. The check for the existence of the file and
the creation of the queue if it does not exist is atomic
with respect to other processes executing mq_open with the
same name in the same directory with O_EXCL and O_CREAT set.
When a new message queue is created, the queue is void of messages.
Opening an existing message queue has no effect on its contents.
Message queue state is preserved across application termination, but the
contents of a queue after system reboot are undefined.
POSIX message queues are compatible across all MIPS ABIs: o32, n32 and
64.
mq_open will fail if one or more of the following conditions are true:
[EACCES] The message queue does not exist and permission to create
the queue is denied.
[EACCES] The message queue exists, but the permissions in oflag are
denied.
[EACCES] A component of mq_name has no search permission or does
not exist.
[EEXIST] O_CREAT and O_EXCL are set, and the message queue exists.
[EFAULT] mq_name points outside the allocated address space of the
process.
[EINTR] A signal interrupted the mq_open function call.
[EINVAL] The values of mq_msgsize and mq_maxmsg fields in the
mq_attr structure are outside the valid range. Invalid
flags specified in the oflag argument.
[EMFILE] The process has too many open files or message queues[see
getrlimit(2)].
[ENAMETOOLONG] The length of the mq_name argument exceeds {PATH_MAX}, or
the length of a mq_name component exceeds {NAME_MAX} while
{_POSIX_NO_TRUNC} is in effect.
[ENFILE] There are too many files and/or message queues in the
system.
[ENOENT] O_CREAT is not set and the named queue does not exist.
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[ENOENT] O_CREAT is set and a component of the mq_name does not
exist or is not directory.
[ENOMEM] The system is unable to allocate a message queue
descriptor.
[ENOSPC] O_CREAT is set, the message queue does not exist and the
system cannot create the queue due to lack of resources.
SEE ALSOmq_close(3c), open(3c), getdtablesize(2), getrlimit(2), intro(3c),
mq_receive(3c), mq_send(3c), mq_getattr(3c), mq_setattr(3c),
mq_notify(3c), umask(2)DIAGNOSTICS
Upon successful completion, the message queue descriptor is returned.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
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