Added source file, updated README

Author: kyildirim

README.md

@@ -1,2 +1,2 @@
 # whispers
-Chinese Whispers implemented in C.
+Chinese Whispers implemented in C to demonstrate System V IPC.

whispers.c

@@ -0,0 +1,89 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/ipc.h>   
+#include <sys/msg.h>  
+#include <errno.h> 
+#include <time.h>
+
+#define INVALID_ARG_COUNT -31
+
+int main(int argc, char **argv){
+
+	if(argc!=2)return INVALID_ARG_COUNT;
+
+	//Seed random
+	srand(time(NULL));
+
+	int len = strlen(argv[1]);
+
+	//Create message queue
+	int mq = msgget(IPC_PRIVATE, 0600|IPC_CREAT|IPC_EXCL);
+	if (mq == -1) {
+    	perror("msgget");
+    	exit(-1);
+	}
+
+	//Message buffer struct
+	struct msgbuf {
+    	long mtype;     
+    	char mtext[1];
+	};
+
+	char* msg = argv[1];
+	struct msgbuf *mb = (struct msgbuf*)malloc(sizeof(struct msgbuf)+len);
+	mb->mtype = 1;
+
+	strcpy(mb->mtext, msg);
+
+	//Send to first process to start the cycle
+	int rc = msgsnd(mq, mb, len+1, 0);
+	if (rc == -1) {
+	    perror("msgsnd");
+	    exit(-2);
+	}
+
+	int rands[len];
+	int r;
+	//Generate random numbers for probability calculations.
+	for(r = 0; r<len; r++)rands[r]=rand();
+	//Fork
+	int i;
+	for(i = 0; i < len; i++) {
+	    int pid = fork();
+	    if(pid < 0) {
+	        printf("Error");
+	        exit(1);
+	    } else if (pid == 0) {
+	    	int p = i+1;
+	    	//printf("%d alive\n", p);
+	        struct msgbuf* rMsg = (struct msgbuf*)malloc(sizeof(struct msgbuf)+len);
+	        int rc = msgrcv(mq, rMsg, len+1, p, 0);
+	        //printf("%d recieved\n", p);
+			if (rc == -1) {
+			    perror("msgrcv");
+			    exit(1);
+			}
+			if((rands[i]%2)==0)rMsg->mtext[p-1]++;
+			rMsg->mtype=p+1;
+			if(p==len){
+				rMsg->mtype=1;
+			}
+			rc = msgsnd(mq, rMsg, len+1, 0);
+			//printf("%d sent\n", p);
+			if(p==1){ //First child waits for last message.
+				rc = msgrcv(mq, rMsg, len+1, p, 0);
+				printf("%s\n", rMsg->mtext);
+			}
+	        return 0;
+	    } else  {
+
+	    }
+	}
+
+	return 0;
+
+}