## NAME

mq_open - open a message queue

## SYNOPSIS

#include <mqueue.h>

mqd_t mq_open(const char *name , int oflag ,
  /* unsigned long mode , mq_attr attr */ ... );
struct mq_attr {
  long mq_flags; /* message queue flags */
  long mq_maxmsg; /* maximum number of messages */
  long mq_msgsize; /* maximum message size */
  long mq_curmsgs; /* number of messages currently queued */
  ...
};

## DESCRIPTION

mq_open() establishes a connection to a named message queue, name, returning the address of the message queue descriptor to the caller for subsequent calls to mq_send() or mq_receive(). The message queue once opened remains usable until the message queue is closed by a successful call to mq_close().

name points to a string naming a message queue. The name argument must conform to the construction rules for a path-name. If name is not the name of an existing message queue and its creation is not requested, mq_open() fails and returns an error.

oflag 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 thread 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. Applications must specify exactly one of the first three values (access modes) below in the value of oflag :

O_RDONLY
  Open the message queue for receiving messages. The thread 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 threads for receiving messages.
O_WRONLY
  Open the queue for sending messages. The thread 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 thread for sending messages.
O_RDWR
  Open the queue for both receiving and sending messages. The thread 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 threads 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 pointer to a 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, unless combined with O_EXCL (see below). Otherwise, a message queue is created without any messages in it. The mode field is not used in the current implementation. If attr is NULL, the message queue is created with the default message queue attributes, these can be set by the user in the global variable mqAttrDefault. If attr is non-NULL, the 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. The mq_msgsize must be specified in 8-bit bytes. Therefore C30/40 users may use the BSIZEOF() function instead of sizeof to do the conversion.

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.
O_NONBLOCK
  The setting of this flag is associated with the open message queue descriptor and determines whether a calling mq_send() waits for message buffer space or a calling mq_receive() waits for messages that are not currently available; or whether the calling function fails, thereby setting errno to EAGAIN.

## 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 condition.

## ERRORS

EACCES
  The message queue exists and the permissions specified by oflag are denied, or the message queue does not exist and permission to create the message queue is denied.
EEXIST
  O_CREAT and O_EXCL are set and the named message queue already exists.
EINVAL
  name is not a valid name.
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.
ENOENT
  O_CREAT is not set and the named message queue, name , does not exist.
ENOSPC
  There is insufficient space for the creation of the new message queue.

## SEE ALSO

mq_close(), mq_receive(), mq_send(), mq_setattr(), mq_unlink()

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