Communication between C programs across a adhoc network

Question

I have the following server code

    #include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include <string.h>
#include <stdio.h>


#define HELLO_PORT 12345
#define HELLO_GROUP "255.0.0.37"

main(int argc, char *argv[])
{
    struct sockaddr_in addr;
        struct sockaddr_in client,server;
        int s,n;
        char index;
    int f; 
        char b1[100];
        int fd, cnt,i=0;;
        struct ip_mreq mreq;
        //char *message="Hello, World!";
    char *message=NULL;
        /* create what looks like an ordinary UDP socket */
        if ((fd=socket(AF_INET,SOCK_DGRAM,0)) < 0) 
    {
        perror("socket");
        exit(1);
        }
        s=socket(AF_INET,SOCK_DGRAM,0);
        server.sin_family=AF_INET;
        server.sin_port=2000;
        server.sin_addr.s_addr=inet_addr("10.42.0.47");
        bind(s,(struct sockaddr *)&server,sizeof(server));
        printf("\nServer ready,waiting for client....\n");
        n=sizeof(client);
        /* set up destination address */
        memset(&addr,0,sizeof(addr));
        addr.sin_family=AF_INET;
        addr.sin_addr.s_addr=inet_addr(HELLO_GROUP);
        addr.sin_port=htons(HELLO_PORT);

        /* now just sendto() our destination! */
        while (1) 
        {
        recvfrom(s,b1,sizeof(b1),0,(struct sockaddr *) &client,&n);
        printf("client : %s\n",b1);
        //sleep(5);
        index=b1[0];
        f=b1[0];
        //printf("b1[1] is %c\n",index);
        //printf("b1[0] is %c\n",f);
        f=32;
        b1[0]=f;
        //printf("new b1[0] is %c\n",b1[0]);
        //printf("%s\n",b1);
        if(index!='C')
        {
            if (sendto(fd,b1,sizeof(b1),0,(struct sockaddr *) &addr,sizeof(addr)) < 0)
            {
                    perror("sendto");
                    exit(1);
            }
            //i++;
            /*if(i>=4)
            {
                i=0;
            }*/
            sleep(1);
        }
        }
}

And the following client code which runs on a different system

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include <string.h>
#include <stdio.h>
#include <stdio.h> 
#include <mysql.h>

#define HELLO_PORT 12345
#define HELLO_GROUP "225.0.0.37"
#define MSGBUFSIZE 1024

main(int argc, char *argv[])
{
    MYSQL mysql; 
    MYSQL_ROW row; 
    MYSQL_RES *result; 
        unsigned int num_fields; 
    unsigned int i; 
    mysql_init(&mysql); 
        struct sockaddr_in server;
        int s,n;
    char b1[100],b2[100],a[100];
    char re[100]={0};
    char g=" ";
    printf("%c\n",re);
    strcpy(b2,"1");
    char message[20],ch='n';
        struct sockaddr_in addr;
        int fd, nbytes,addrlen,j=0;
        struct ip_mreq mreq;
        char msgbuf[MSGBUFSIZE];
        u_int yes=1;            /*** MODIFICATION TO ORIGINAL */

     /* create what looks like an ordinary UDP socket */
        if ((fd=socket(AF_INET,SOCK_DGRAM,0)) < 0) 
    {
        perror("socket");
        exit(1);
        }
    s=socket(AF_INET,SOCK_DGRAM,0);
        server.sin_family=AF_INET;
        server.sin_port=2000;
        server.sin_addr.s_addr=inet_addr("10.42.0.1");
        printf("\nClient ready....\n");
        n=sizeof(server);

    if (!mysql_real_connect(&mysql,"localhost","root","nidhi","project",0,NULL,0)) 
    {    
            fprintf(stderr, "Failed to connect to database: Error: %s\n", 
            mysql_error(&mysql)); 
    } 
    /**** MODIFICATION TO ORIGINAL */
        /* allow multiple sockets to use the same PORT number */
        if (setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(yes)) < 0) 
    {
            perror("Reusing ADDR failed");
            exit(1);
        }
    /*** END OF MODIFICATION TO ORIGINAL */
        /* set up destination address */
        memset(&addr,0,sizeof(addr));
        addr.sin_family=AF_INET;
        addr.sin_addr.s_addr=htonl(INADDR_ANY); /* N.B.: differs from sender */
        addr.sin_port=htons(HELLO_PORT);  
        /* bind to receive address */
        if (bind(fd,(struct sockaddr *) &addr,sizeof(addr)) < 0) 
    {
        perror("bind");
        exit(1);
        }

        /* use setsockopt() to request that the kernel join a multicast group */
        mreq.imr_multiaddr.s_addr=inet_addr(HELLO_GROUP);
        mreq.imr_interface.s_addr=htonl(INADDR_ANY);
        if (setsockopt(fd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq)) < 0) 
    {
        perror("setsockopt");
        exit(1);
        }

        /* now just enter a read-print loop */
        while (1) 
    {
        /*if ((nbytes=recvfrom(fd,msgbuf,MSGBUFSIZE,0,(struct sockaddr *) &addr,&addrlen)) < 0) 
        {
                perror("recvfrom");
                exit(1);
        }
        printf("the server says: %s\n",msgbuf);*/
        printf("would you like to send?(y/n)\n");
        scanf("%c",&ch);
        fflush(stdin);
        if(ch=='y')
        {
        printf("\nClient:");
            gets(a);
        printf("%c\n",a);
        strcat(b2,a);
        printf("%s\n",b2);
            sendto(s,b2,sizeof(b2),0,(struct sockaddr *) &server,n);
        b2[100]=NULL;
        strcpy(b2,"1");
        }
        addrlen=sizeof(addr);
        if ((nbytes=recvfrom(fd,msgbuf,MSGBUFSIZE,0,(struct sockaddr *) &addr,&addrlen)) < 0) 
        {
                perror("recvfrom");
                exit(1);
        }
        printf("the server says: %s\n",msgbuf);
        if(mysql_query(&mysql,msgbuf));
        else
        {
            strcpy(re,"Client 1: ");
            sendto(s,re,sizeof(re),0,(struct sockaddr *) &server,n);
            //printf("inside else case\n");
            result=mysql_store_result(&mysql);
            //printf("\n after getting the result...\n");
            num_fields = mysql_num_fields(result); 
            //printf("\n after getting numfields...\n");
                while ((row = mysql_fetch_row(result))) 
                {    
                        unsigned long *lengths; 
                //printf("\n inside while....\n");
                        lengths = mysql_fetch_lengths(result); 
                        for(i = 0; i < num_fields; i++) 
                        { 
                                printf("[%.*s] \t", (int) lengths[i], row[i] ? row[i] : "NULL"); 
                    strcat(re,row[i]);
                    j++;
                    //printf("%d\n",j);
                    if(j==4)
                    {
                        //printf("the result is .....%s\n",re);
                        sendto(s,re,sizeof(re),0,(struct sockaddr *) &server,n);
                        memset(re,0,100);
                        strcpy(re,"Client 1: ");
                        j=0;
                    }

                        }    
                        printf("\n"); 
                } 
            } 
    }
}

I created an adhoc network and it assigned two ip address to the server and the client. 10.42.0.1/24 to the server and 10.42.0.47/24 to client

however the unicast operation from the client to the server is not functioning correctly neither is the multicast from the server to client.

secondly how do i determine whether both the programs are part of the same multicast group. The error on the client is either

setsockopt:invalid argument

or

setsockopt:no such device

Any help on the same is greatly appreciated..!!

Answer 1

There are a lot of potential bugs in your code:

Server:

1.You are not checking if the 's' socket was successfully created. However, your example will work without it since there is enough memory for socket creation.

2.You are not checking the return code from bind(), maybe you can't bind the socket to the port and that's why you get 'invalid argument' error

3.You are not checking the return code from recvfrom() ....

4.The IP 255.0.0.37 is reserved for future use, I am not sure it is valid, why not use a valid ip range?

5.For multicasting you have to use ip addresses in range: 224.0.0.0/4, not 10.0.0.0/8 since it is reserved for local networks.

And regarding the client, you should first make working udp protocol code, and then add mysql to it. You also have to tell us at which line you have the error, otherwise it is difficult to analyse the problem.