System V IPC Suite: message queues
  discuss msgsnd(2) msgrcv(2) 

  SysV Message Queues need:
  
    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/msg.h>

   int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

   ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
 
  Message queues provide a way for a process to send or receive messages from 
  other processes either in order or by priority. 

  There is usually a system limit of 8K bytes per message. The systems 
  programmer must first decide on a structure for the messages, of which 
  the first element MUST be "long mtype" because every message must be 
  assigned a type. All the other fields are up to the programmer (subject 
  to the 8K byte limit). Typical example, 
  
  struct	priv_msgbuf { 
  		long	mtype;	/* message type mandatory > 0 */
  		char	buf[128]; } mesg;
  
  The message type (mtype) can be any value > 0. You can load mtype with the 
  sending process' PID and in this way only grab messages from that process. 
  The other fields were chosen based on what information needs to be sent.
  
  All System V IPC primitives (message queues, shared memory, semaphores)
  are persistent; they are not released upon exit of the creating process
  and must be manually removed. Typically, one uses the path to an existing 
  file together with a random integer to create a key:
  
  	static key_t ipckey;
    int PROJ = 1;
  	static int msqid;
  	ipckey = ftok(path_to_existing_file, PROJ);
  
  If you are only creating one instance PROJ is usually set to 1; if you 
  are creating many, PROJ could be an index or a process id. If the file
  doesn't exist the key will be invalid.  Permissions are like permissions 
  on files. The creating process calls
  
  	#define PERMS 0600	/* rw------- */
  	msqid = msgget(ipckey, IPC_CREAT | IPC_EXCL | PERMS);
  	if (msqid < 0) fprintf(stderr, "\n *** create failed ***");
  
  The creation could fail if the key was invalid or if the message queue
  already existed, perhaps because you ran the program before and forgot
  to delete the queue. If this happens you should use the command "ipcs"
  to show all ipc structures (message queues, semaphores, shared memory)
  and the command "ipcrm" to remove the ones you own. See the man pages
  for details on using these commands.
  
  A process which wants to use this existing message queue (and has
  permission) calls
  
  	msqid = msgget(ipckey, PERMS);
  
  The process which wants to send a message (with no special options) fills 
  in the structure mesg, making sure to set mesg.mtype, and sends via

   
    memset(mesg.buf, 0, strlen(mesg.buf)); /* clear out the space first */
    stcpy(mesg.buf,"is anyone out there?");
    mesg.type = 1;
    msgsnd(mqid, &mesg, sizeof(mesg), 0);
 
    mtype_wanted = 1; 
  	ret = msgrcv(msqid, &mesg, mtype_wanted, 0);
  
  blocking until a message of the desired type is available. If mtype_wanted 
  is set to zero the first message available will be returned regardless of 
  type. If mtype is a priority (low numbers have more priority) and if 
  mtype_wanted is negative then the first message of the lowest mtype which 
  is less or equal to the absolute value of mtype_wanted is returned. In this 
  way the queue can be type-driven, independent of type, or priority driven.
  
  When the processes are through using the message queue the creator
  SHOULD remove the queue in order to conserve system resources via
  
  	msgctl(msqid, IPC_RMID, (struct msqid_ds *)0);
  
  The last argument is ignored for the remove (IPC_RMID) command.