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mq_open

NAME

mq_open - open a message queue

SYNOPSIS

gcc [ flag... ] file ... -lmqueue [ library... ]

#include <mqueue.h>

mqd_t mq_open(const char *name, int oflag, /* unsigned long mode, struct mq_attr *attr */ ...);

DESCRIPTION

The mq_open() function establishes the connection between a process and a message queue with a message queue descriptor. It creates an open message queue description that refers to the message queue, and a message queue descriptor that refers to that open message queue description. The message queue descriptor is used by other functions to refer to that message queue.

The name argument points to a string naming a message queue. The name argument must conform to the construction rules for a pathname. It has to start with a / character. If name is not the name of an existing message queue and its creation is not requested, mq_open() fails and returns an error.
Processes calling mq_open() with the same value of name will refer to the same message queue object, as long as that name has not been removed.

The oflag argument requests the desired receive and/or send access to the message queue. The requested access permission to receive messages or send messages is granted if the calling process would be granted read or write access, respectively, to a file with the equivalent permissions.

The value of oflag is the bitwise inclusive OR of values from the following list. If application doesn't specify one of the first three values (access modes) below in the value of oflag, the O_RDONLY will be set as default.

O_RDONLY

Open the message queue for receiving messages. The process can use the returned message queue descriptor with mq_receive(), but not mq_send(). A message queue may be open multiple times in the same or different processes for receiving messages.

O_WRONLY

Open the queue for sending messages. The process can use the returned message queue descriptor with mq_send() but not mq_receive(). A message queue may be open multiple times in the same or different processes for sending messages.

O_RDWR

Open the queue for both receiving and sending messages. The process can use any of the functions allowed for O_RDONLY and O_WRONLY. A message queue may be open multiple times in the same or different processes for sending messages.

Any combination of the remaining flags may additionally be specified in the value of oflag:

O_CREAT

This option is used to create a message queue, and it requires two additional arguments: mode, which is of type mode_t, and attr, which is a pointer to an mq_attr structure. If the pathname, name, has already been used to create a message queue that still exists, then this flag has no effect, except as noted under O_EXCL (see below). Otherwise, a message queue is created without any messages in it.

The user ID of the message queue is set to the effective user ID of process, and the group ID of the message queue is set to the effective group ID of the process. The file permission bits are set to the value of mode (bits are NOT modified by the file mode creation mask of the process).

If attr is non-NULL and the process creates new message queue, mq_maxmsg and mq_msgsize attributes are set to the values of the corresponding members in the mq_attr structure referred to by attr. If attr is NULL and the process creates new queue, it is created with system limits.

O_EXCL

If both O_EXCL and O_CREAT are set, mq_open() will fail if the message queue name exists. The check for the existence of the message queue and the creation of the message queue if it does not exist are atomic with respect to other processes executing mq_open() naming the same name with both O_EXCL and O_CREAT set. If O_EXCL and O_CREAT are not set, the result is undefined.

O_NONBLOCK

The setting of this flag is associated with the open message queue description and determines whether an mq_send() or mq_receive() waits for resources or messages that are not currently available, or fails with errno set to EAGAIN. See mq_send() and mq_receive() for details.

The mq_open() function does not add or remove messages from the queue.

RETURN VALUES

Upon successful completion, mq_open() returns a message queue descriptor; otherwise, the function returns (mqd_t)-1 and sets errno to indicate the error.

DIAGNOSTICS

The mq_open() function will fail if:

EACCESS

The message queue exists and the permissions specified by oflag are denied.

EEXIST

O_CREAT and O_EXCL are set and the named message queue already exists.

EINTR

The mq_open() operation was interrupted by a signal.

EINVAL

The mq_open() operation is not supported for the given name, or O_CREAT was specified in oflag, the value of attr is not NULL, and either mq_maxmsg or mq_msgsize was less than or equal to zero or greater than system limits. Also when the given name contains more then one '/'.

ENAMETOOLONG

The length of the name argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.

ENOENT

O_CREAT is not set and the named message queue does not exist.

ENOMEM

Insufficient kernel memory was available.

ENOSPC

There is insufficient space for the creation of the new message queue.

EFAULT

name points outside your accessible address space.

EMFILE

The process already has the maximum number of files open.

ENFILE

The limit on the total number of files open on the system has been reached.

NOTES

You can't use slash characters other than the leading slash character in name (mqueuefs doesn't support directories).

Opening of message queue is possilble with normal open() when mqueue filesystem is mounted. When using open() actual path to the queue file (in mounted filesystem) must be provided. If open() will be used to create message queue it will have default (i.e. maximum) system limits set. Note that using open() to manipulate POSIX message queues is NOT portable.

AUTHORS

Krzysztof Benedyczak <golbi@mat.uni.torun.pl>
Michal Wronski <wrona@mat.uni.torun.pl>

CONFORMING TO

IEEE Std 1003.1-2001

SEE ALSO

mq_close(2), mq_send(2), mq_receive(2)