The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


mq_open - open a message queue (REALTIME)


#include <mqueue.h>

mqd_t mq_open(const char *name, int oflag, ...);


The mq_open() function establishes the connection between a process and a message queue with a message queue descriptor. It creates a 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. It is unspecified whether the name appears in the file system and is visible to other functions that take pathnames as arguments. The name argument conforms to the construction rules for a pathname. If name begins with the slash character, then processes calling mq_open() with the same value of name refer to the same message queue object, as long as that name has not been removed. If name does not begin with the slash character, the effect is implementation-dependent. The interpretation of slash characters other than the leading slash character in name is implementation-dependent. If the name argument is not the name of an existing message queue and creation is not requested, mq_open() fails and returns an error.

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 an equivalently protected file.

The value of oflag is the bitwise inclusive OR of values from the following list. Applications specify exactly one of the first three values (access modes) below in the value of oflag:

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.
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.
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 be specified in the value of oflag:

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 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, except as noted under O_EXCL. 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 the 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. When bits in mode other than file permission bits are set, the effect is implementation-dependent. If attr is NULL, the message queue is created with implementation-dependent default message queue attributes. If attr is non-NULL and the calling process has the appropriate privilege on name, 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. If attr is non-NULL, but the calling process does not have the appropriate privilege on name, the mq_open() function fails and returns an error without creating the message queue.
If O_EXCL and O_CREAT are set, mq_open() fails 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 O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the result is undefined.
The setting of this flag is associated with the open message queue description and determines whether a 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.


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


The mq_open() function will fail if:
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.
O_CREAT and O_EXCL are set and the named message queue already exists.
The mq_open() operation was interrupted by a signal.
The mq_open() operation is not supported for the given 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.
Too many message queue descriptors or file descriptors are currently in use by this process.
The length of the name string exceeds {PATH_MAX}, or a pathname component is longer than {NAME_MAX} while _POSIX_NO_TRUNC is in effect.
Too many message queues are currently open in the system.
O_CREAT is not set and the named message queue does not exist.
There is insufficient space for the creation of the new message queue.
The function mq_open() is not supported by this implementation.






mq_close(), mq_receive(), mq_send(), mq_setattr(), mq_getattr(), mq_unlink(), <mqueue.h>, msgctl(), msgget(), msgrcv(), msgsnd().


Derived from the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995)

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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