Inter process communication using Message Queues (mq) -1

Message queues are one of the many ways of communicating among processes in linux.

Message queues communicate by sending and reciveing messages, over a common queue. The sender sends the message by writing to the queue and recieves by reading from the queue.

The following are some of the commonly used functions with respect to message queues.

To create or open a message queue we use the function mq_open:

mqd_t mq_open(const char *name, int oflag, mode_t mode,struct mq_attr *attr) Arguments: name : Name of the message queue oflag : Flags which control the operation of the queue. The various possible flags are : One of the flags O_RDONLY , O_WRONLY, O_RDWR Optionally ored with: O_NONBLOCK, O_CREAT, O_EXCL. Note: The description of the flags can be found in the man pages. When a new message queue has to be created along with the above two argumetns these two are also mandatory mode: Which specifies the permissions of the queue. attr: A structure which sets the attributes, like message size, number of messages etc. If set to NULL it is initalized to default values. It has been explained further below.

Retrun:

The function returns a data type mqd_t which is a descrpitor for the message queues. In case of a error it returns -1.

Setting and getting attributes :

The attributes of the message queue are stored in a structure mq_attr.

struct mq_attr { long mq_flags; /* Flags: 0 or O_NONBLOCK */ long mq_maxmsg; /* Max. # of messages on queue */ long mq_msgsize; /* Max. message size (bytes) */ long mq_curmsgs; /* # of messages currently in queue */ };

The fields of these structures can be read by using the function int mq_getattr(mqd_t mqdes, struct mq_attr *attr); Arguments : mqdes : The queue descriptor returned by the open call. mq_attr: A pointer to a buffer to which the attribute structure will be returned.

To change the attributes of an already existing queue we can use the function

int mq_setattr(mqd_t mqdes, struct mq_attr *newattr,struct mq_attr *oldattr); Arguments: mqdes : The queue descriptor returned by the open call. newattr: Pointer to the structure with the new attribute values oldattr: Pointer to the structure of the current attr gets returned to this buffer like the mq_getattr or it can be left as NULL.

Sending and recieving messages :

Once the message queue is opened successfully, the process can communicate with the queue using the queue descriptor along with the functions mq_send to send a message and mq_recieve to recieve a message.

Sending:

int mq_send(mqd_t mqdes, const char *msg_ptr,size_t msg_len, unsigned msg_prio);Arguments : Arguments mqdes : The queue descriptor returned by the open call msg_ptr: Pointer to buffer that holds the message to be transfered msg_len: Length of the message that has to be transfered msg_prio: Priority of the message, Messages are placed on the queue in decreasing order of priority

Receieving:

ssize_t mq_receive(mqd_t mqdes, char *msg_ptr,size_t msg_len, unsigned *msg_prio); Arguments mqdes : The queue descriptor returned by the open call msg_ptr: Pointer to buffer where the received message has to be placed msg_len: mq_msgszie for the queue which is being read from. msg_prio: This recives the priority of the message that was recieved.

Using the above set of functions we can set up a message queue and communicate between processes using them. In the next few posts we will see examples of how to use the above mentioned functions.