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mq_open - Man Page

open a message queue (REALTIME)


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <mqueue.h>

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


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 conforms to the construction rules for a pathname, except that the interpretation of <slash> characters other than the leading <slash> character in name is implementation-defined, and that the length limits for the name argument are implementation-defined and need not be the same as the pathname limits {PATH_MAX} and {NAME_MAX}. 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. 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. The group ID of the message queue shall be set to the effective group ID of the process; however, if the name argument is visible in the file system, the group ID may be set to the group ID of the containing directory. 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 appropriate privileges 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. The values of the mq_flags and mq_curmsgs members of the mq_attr structure shall be ignored. If attr is non-NULL, but the calling process does not have appropriate privileges 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.

Return Value

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.


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.

If any of the following conditions occur, the mq_open() function may return (mqd_t)-1 and set errno to the corresponding value.


The length of the name argument exceeds {_POSIX_PATH_MAX} on systems that do not support the XSI option or exceeds {_XOPEN_PATH_MAX} on XSI systems, or has a pathname component that is longer than {_POSIX_NAME_MAX} on systems that do not support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI systems.

The following sections are informative.



Application Usage




Future Directions

A future version might require the mq_open() and mq_unlink() functions to have semantics similar to normal file system operations.

See Also

mq_close(), mq_getattr(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgctl(), msgget(), msgrcv(), msgsnd()

The Base Definitions volume of POSIX.1-2017, <mqueue.h>

Referenced By

mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_receive(3p), mq_send(3p), mq_setattr(3p), mqueue.h(0p), mq_unlink(3p), msgctl(3p), msgget(3p), msgrcv(3p), msgsnd(3p), umask(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual