LIBRARY

Lb librt

SYNOPSIS

In mqueue.h Ft mqd_t Fn mq_open const char *name int oflag ...

DESCRIPTION

The Fn mq_open system call establishes the connection between a process and a message queue with a message queue descriptor. It creates 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 Fa name argument points to a string naming a message queue. The Fa name argument should conform to the construction rules for a pathname. The Fa name should begin with a slash character. Processes calling Fn mq_open with the same value of Fa name refers to the same message queue object, as long as that name has not been removed. If the Fa name argument is not the name of an existing message queue and creation is not requested, Fn mq_open will fail and return an error.

The Fa oflag argument requests the desired receive and/or send access to the message queue. The requested access permission to receive messages or send messages would be granted if the calling process would be granted read or write access, respectively, to an equivalently protected file.

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

O_RDONLY

Open the message queue for receiving messages. The process can use the returned message queue descriptor with Fn mq_receive , but not Fn mq_send . A message queue may be open multiple times in the same or different processes for receiving messages.

O_WRONLY

Open the queue for sending messages. The process can use the returned message queue descriptor with Fn mq_send but not Fn mq_receive . A message queue may be open multiple times in the same or different processes for sending messages.

O_RDWR

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 Fa oflag :

O_CREAT

Create a message queue. It requires two additional arguments: Fa mode , which is of type Vt mode_t , and Fa attr , which is a pointer to an Vt mq_attr structure. If the pathname Fa 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 will be created without any messages in it. The user ID of the message queue will be set to the effective user ID of the process, and the group ID of the message queue will be set to the effective group ID of the process. The permission bits of the message queue will be set to the value of the Fa mode argument, except those set in the file mode creation mask of the process. When bits in Fa mode other than the file permission bits are specified, the effect is unspecified. If Fa attr is NULL the message queue is 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 will be set to the values of the corresponding members in the Vt mq_attr structure referred to by Fa attr . If Fa attr is non- NULL but the calling process does not have the appropriate privilege on name, the Fn mq_open function will fail and return an error without creating the message queue.

O_EXCL

If O_EXCL and O_CREAT are set, Fn mq_open will fail if the message queue name exists.

O_NONBLOCK

Determines whether an Fn mq_send or Fn mq_receive waits for resources or messages that are not currently available, or fails with errno set to Er EAGAIN ; see mq_send2 and mq_receive2 for details.

The Fn mq_open system call does not add or remove messages from the queue.

NOTES

Fx implements message queue based on file descriptor. The descriptor is inherited by child after fork(2). The descriptor is closed in a new image after exec(3). The select(2) and kevent(2) system calls are supported for message queue descriptor.

RETURN VALUES

Upon successful completion, the function returns a message queue descriptor; otherwise, the function returns Po Vt mqd_t Pc Ns -1 and sets the global variable errno to indicate the error.

STANDARDS

The Fn mq_open system call conforms to St -p1003.1-2004 .

HISTORY

Support for POSIX message queues first appeared in Fx 7.0 .

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html.