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## Class message_queue

boost::interprocess::message_queue

## Synopsis

    // In header: <boost/interprocess/ipc/message_queue.hpp>

    class message_queue {
    public:
      // types
      typedef boost::posix_time::ptime ptime;  // @ private:

      // construct/copy/destruct
      message_queue();
      message_queue(create_only_t, const char *, std::size_t, std::size_t,
                    const permissions & = permissions());
      message_queue(open_or_create_t, const char *, std::size_t, std::size_t,
                    const permissions & = permissions());
      message_queue(open_only_t, const char *);
      ~message_queue();

      // public member functions
      *void send(const void *, std::size_t, unsigned int);
      bool try_send(const void *, std::size_t, unsigned int);
      bool timed_send(const void *, std::size_t, unsigned int,
                      const boost::posix_time::ptime &);
      void receive(void *, std::size_t, std::size_t &, unsigned int &);
      bool try_receive(void *, std::size_t, std::size_t &, unsigned int &);
      bool timed_receive(void *, std::size_t, std::size_t &, unsigned int &,
                         const boost::posix_time::ptime &);
      std::size_t get_max_msg() const;
      std::size_t get_max_msg_size() const;
      std::size_t get_num_msg();
      bool do_receive(block_t, void *, std::size_t, std::size_t &, unsigned int &,
                      const ptime &);
      bool do_send(block_t, const void *, std::size_t, unsigned int,
                   const ptime &);

      // public static functions
      static bool remove(const char *);
      static std::size_t get_mem_size(std::size_t, std::size_t);
      unspecified m_shmem;
    };

## Description

A class that allows sending messages between processes.

### `message_queue` public construct/copy/destruct

1.     message_queue();

2.     message_queue(create_only_t create_only, const char * name,
                     std::size_t max_num_msg, std::size_t max_msg_size,
                     const permissions & perm = permissions());

   Creates a process shared message queue with name "name". For this message queue, the maximum number of messages will be "max_num_msg" and the maximum message size will be "max_msg_size". Throws on error and if the queue was previously created.

3.     message_queue(open_or_create_t open_or_create, const char * name,
                     std::size_t max_num_msg, std::size_t max_msg_size,
                     const permissions & perm = permissions());

   Opens or creates a process shared message queue with name "name". If the queue is created, the maximum number of messages will be "max_num_msg" and the maximum message size will be "max_msg_size". If queue was previously created the queue will be opened and "max_num_msg" and "max_msg_size" parameters are ignored. Throws on error.

4.     message_queue(open_only_t open_only, const char * name);

   Opens a previously created process shared message queue with name "name". If the was not previously created or there are no free resources, throws an error.

5.     ~message_queue();

   Destroys *this and indicates that the calling process is finished using the resource. All opened message queues are still valid after destruction. The destructor function will deallocate any system resources allocated by the system for use by this process for this resource. The resource can still be opened again calling the open constructor overload. To erase the message queue from the system use remove().

### `message_queue` public member functions

1.     *void send(const void * buffer, std::size_t buffer_size,
                  unsigned int priority);

   Sends a message stored in buffer "buffer" with size "buffer_size" in the message queue with priority "priority". If the message queue is full the sender is blocked. Throws interprocess_error on error.

2.     bool try_send(const void * buffer, std::size_t buffer_size,
                     unsigned int priority);

   Sends a message stored in buffer "buffer" with size "buffer_size" through the message queue with priority "priority". If the message queue is full the sender is not blocked and returns false, otherwise returns true. Throws interprocess_error on error.

3.     bool timed_send(const void * buffer, std::size_t buffer_size,
                       unsigned int priority,
                       const boost::posix_time::ptime & abs_time);

   Sends a message stored in buffer "buffer" with size "buffer_size" in the message queue with priority "priority". If the message queue is full the sender retries until time "abs_time" is reached. Returns true if the message has been successfully sent. Returns false if timeout is reached. Throws interprocess_error on error.

4.     void receive(void * buffer, std::size_t buffer_size, std::size_t & recvd_size,
                    unsigned int & priority);

   Receives a message from the message queue. The message is stored in buffer "buffer", which has size "buffer_size". The received message has size "recvd_size" and priority "priority". If the message queue is empty the receiver is blocked. Throws interprocess_error on error.

5.     bool try_receive(void * buffer, std::size_t buffer_size,
                        std::size_t & recvd_size, unsigned int & priority);

   Receives a message from the message queue. The message is stored in buffer "buffer", which has size "buffer_size". The received message has size "recvd_size" and priority "priority". If the message queue is empty the receiver is not blocked and returns false, otherwise returns true. Throws interprocess_error on error.

6.     bool timed_receive(void * buffer, std::size_t buffer_size,
                          std::size_t & recvd_size, unsigned int & priority,
                          const boost::posix_time::ptime & abs_time);

   Receives a message from the message queue. The message is stored in buffer "buffer", which has size "buffer_size". The received message has size "recvd_size" and priority "priority". If the message queue is empty the receiver retries until time "abs_time" is reached. Returns true if the message has been successfully sent. Returns false if timeout is reached. Throws interprocess_error on error.

7.     std::size_t get_max_msg() const;

   Returns the maximum number of messages allowed by the queue. The message queue must be opened or created previously. Otherwise, returns 0. Never throws

8.     std::size_t get_max_msg_size() const;

   Returns the maximum size of message allowed by the queue. The message queue must be opened or created previously. Otherwise, returns 0. Never throws

9.     std::size_t get_num_msg();

   Returns the number of messages currently stored. Never throws

10.     bool do_receive(block_t block, void * buffer, std::size_t buffer_size,
                        std::size_t & recvd_size, unsigned int & priority,
                        const ptime & abs_time);

11.     bool do_send(block_t block, const void * buffer, std::size_t buffer_size,
                     unsigned int priority, const ptime & abs_time);

### `message_queue` public static functions

1.     static bool remove(const char * name);

   Removes the message queue from the system. Returns false on error. Never throws

2.     static std::size_t
       get_mem_size(std::size_t max_msg_size, std::size_t max_num_msg);

   Returns the needed memory size for the shared message queue. Never throws