mq_open() returns a message queue descriptor that establishes a connection between a process and a message queue. That descriptor refers to the open message queue description referring to the message queue created by mq_open(). The message queue descriptor returned by mq_open() can be used by other functions to refer to the message queue created. name is the message queue name (string). Note: It is not specified if this name actually appears as part of the filesystem and/or if functions on files apply to the message queue. name must follow the correct construction rules for a pathname - If the first letter of name is "/" then any other process calling the message queue with that name will refer to that message queue object (unless it's been removed). - If the first letter of name is not "/" the behavior is implementation defined. - How other "/"s in name are interpreted is implementation-defined. mq_open() returns an error if creation is not requested and name does not refer to an already existing message queue If the message queue descriptor is implemented with a file descriptor, at least {OPEN_MAX} file and message queues can be opened. oflag refers to the send/receive access to the message queue. They behave just as read/write, respectively, access on a file with equivalent protections. oflag is a bitwise-inclusive OR of: exactly ONE of O_RDONLY, O_WRONLY, O_RDWR (access modes) any combination of O_CREAT, O_EXCL, O_NONBLOCK If the access mode is O_RDONLY, the message queue can receive messages, but not send. (i.e., the message queue descriptor returned can be used with mq_receive, but not mq_send.) A message queue can be opened more than once in either the same process or a different process for receiving messages. If the access mode is O_WRONLY, the message queue can send messages, but not receive. (i.e., the message queue descriptor can be used with mq_send, but not ms_receive) A message queue can be opened more than once in either the same process or a different process for sending messages. If the access mode is O_RDWR, the message queue can send or receive messages (i.e., both mq_receive and mq_send can be used). A message queue can be opened more than once in either the same process or a different process for sending messages. If the O_CREAT flag is set, a message queue is created. In addition, the arguments mode_t mode and mq_attr *attr should be sent. If name has not been used to create a currently-existing message queue, then an empty message queue will be created with a user ID == the effective user ID of the process and group ID == effective group ID of the process. The file permissions == mode. (It's undefined what happens if non-file permissions bits are set in mode). If the O_CREAT flag is set and name has already been used to create a currently existing message queue, then the current call has no effect (except for exceptions noted with O_EXCL assertions). If the O_CREAT flag is set and attr == NULL, then the implementation-defined default message queue attributes will be used. If the O_CREAT flag is set and attr != NULL, then, if the calling process has the allowed privileges on name, mq_maxmsg and ms_msgsize will be set as defined in attr. If the O_CREAT flag is set and attr != NULL, but the calling process does not have the allowed privileges on name, mq_open() will fail and return an error. No message queue will have been created. If O_EXCL and O_CREAT are set, and message queue name already exists, mq_open() fails. When O_EXCL and O_CREAT are set, this check to see if the message queue exists and creation of the message queue (if it doesn't already exist) are atomic w.r.t. other threads calling mq_open with O_EXCL and O_CREAT and using the same name. If O_EXCL is set without O_CREAT set, the results are undefined. O_NONBLOCK determines the blocking/non-blocking behavior of the mq_send() and mq_receive() calls. Tests will be done in the mq_send() and mq_receive() function tests. mq_open() does not add messages to the queue or remove messages from the queue If mq_open() is successful, it will return a message queue descriptor. If mq_open() is not successful, it will return (mqd_t)-1 and set errno to indicate the error. mq_open() fails with EACCES if either: - the message queue exists, but the oflag permissions are denied or - the message queue doesn't exist, but permission to create it is denied mq_open() fails with EEXIST if O_CREAT and O_EXCL are set in the flags, but the message queue exists already mq_open() fails with EINTR if it is interrupted by a signal mq_open() fails with EINVAL if either: - mq_open() is not supported for the name parameter - O_CREAT was set in oflag, attr != NULL, and either mq_maxmsq less than or equal to 0 or mq_msgsize less than or equal to 0 mq_open() fails with EMFILE if there are too many message queue descriptors or file descriptors in use by the process calling mq_open mq_open() fails with ENAMETOOLONG if the name parameter's length greater than PATH_MAX or a component of the pathname in name greater than NAME_MAX mq_open() fails with ENFILE if the system has too many message queues open at the time mq_open() is called mq_open() fails with ENOENT if the named message queue does not yet exist, but O_CREAT is not set mq_open() fails with ENOSPC if there is not enough space to create the message queue