![[Go to CFHT Home Page]](/images/cfht-icon.gif){kind=icon}
Man Pages 
Back to Software Index 
Manpage Top Level
cc [ flag ... ] file ... -lposix4 [ library ... ]
#include <mqueue.h>
mqd_t mq_open(const char *name, int oflag, /* unsigned long mode, mq_attr attr */ ... );
struct mq_attr {
long mq_flags; /* message queue flags */
long mq_maxmsg; /* maximum number of messages */
long mq_msgsize; /* maximum message size */
long mq_curmsgs; /* number of messages currently queued */
...
};
name points to a string naming a message queue. The name argument must conform to the construction rules for a path-name. If name is not the name of an existing message queue and its creation is not requested, mq_open() fails and returns an error.
oflag 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 a file with the equivalent permissions.
The value of oflag is the bitwise inclusive OR of values from the following list. Applications must specify exactly one of the first three values (access modes) below in the value of oflag :
- O_RDONLY
- Open the message queue for receiving messages. The process can use the returned message queue descriptor with mq_receive(3R) , but not mq_send(3R) . 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 mq_send(3R) but not mq_receive(3R) . 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 additionally be specified in the value of oflag:
- O_CREAT
- 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 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, unless combined with O_EXCL (see below). Otherwise, a message queue is created without any messages in it.
- The message queue’s user ID is set to the process’s effective user ID,
- and the message queue’s group ID is set to the process’s effective group ID. The message queue’s permission bits will be set to the value of mode, and modified by clearing all bits set in the file mode creation mask of the process (see umask(2) ). ‘AND-NOT’) those already set in the file mode creation mask of the process.
- If
- attr is NULL, the message queue is created with the default message queue attributes, (mq_maxmsg = 128 and mq_maxsize = 1024). If attr is non-NULL , 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.
- O_EXCL
- If both O_EXCL and O_CREAT are set, mq_open() will 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 are atomic with respect to other processes executing mq_open() naming the same name with both O_EXCL and O_CREAT set.
- O_NONBLOCK
- The setting of this flag is associated with the open message queue descriptor and determines whether a calling mq_send(3R) waits for message buffer space or a calling mq_receive(3R) waits for messages that are not currently available; or whether the calling function fails, thereby setting errno to EAGAIN.
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.