The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
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mq_open - open a message queue (REALTIME)


[MSG] [Option Start] #include <mqueue.h>

mqd_t mq_open(const char *
name, int oflag, ...); [Option End]


The mq_open() function shall establish the connection between a process and a message queue with a message queue descriptor. It shall create 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. 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 shall conform 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 shall 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-defined. The interpretation of slash characters other than the leading slash character in name is implementation-defined. If the name argument is not the name of an existing message queue and creation is not requested, mq_open() shall fail and return an error.

A message queue descriptor may be implemented using a file descriptor, in which case applications can open up to at least {OPEN_MAX} file and message queues.

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 shall be 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 shall 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:

Create a message queue. It requires two additional arguments: mode, which shall be of type mode_t, and attr, which shall be 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 shall have no effect, except as noted under O_EXCL. Otherwise, a message queue shall be created without any messages in it. The user ID of the message queue shall be set to the effective user ID of the process, and the group ID of the message queue shall be set to the effective group ID of the process. The permission bits of the message queue shall be set to the value of the mode argument, except those set in the file mode creation mask of the process. When bits in mode other than the file permission bits are specified, the effect is unspecified. If attr is NULL, the message queue shall be created with implementation-defined 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 shall be 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 shall fail and return an error without creating the message queue.
If O_EXCL and O_CREAT are set, mq_open() shall 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 shall be atomic with respect to other threads 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.
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.


Upon successful completion, the function shall return a message queue descriptor; otherwise, the function shall return (mqd_t)-1 and set errno to indicate the error.


The mq_open() function shall 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() function was interrupted by a signal.
The mq_open() function 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 argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.
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 following sections are informative.










mq_close(), mq_getattr(), mq_receive(), mq_send(), mq_setattr(), mq_timedreceive(), mq_timedsend(), mq_unlink(), msgctl(), msgget(), msgrcv(), msgsnd(), the Base Definitions volume of IEEE Std 1003.1-2001, <mqueue.h>


First released in Issue 5. Included for alignment with the POSIX Realtime Extension.

Issue 6

The mq_open() function is marked as part of the Message Passing option.

The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Message Passing option.

The mq_timedreceive() and mq_timedsend() functions are added to the SEE ALSO section for alignment with IEEE Std 1003.1d-1999.

The DESCRIPTION of O_EXCL is updated in response to IEEE PASC Interpretation 1003.1c #48.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/62 is applied, updating the description of the permission bits in the DESCRIPTION section. The change is made for consistency with the shm_open() and sem_open() functions.

End of informative text.

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