UDP Sockets in C under Linux

Problem:
The server receives the correct number and prints it, prints correctly the IP address but when it comes to port, it prints rubbish:
What went wrong ?


This is the client, it reads a number from standard input and sends it to the server.

Code:

---

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

int main() {
        int sock, n, nr;
        socklen_t length;
        struct sockaddr_in server, from;
        sock = socket(AF_INET, SOCK_DGRAM, 0);
        if(sock < 0) {
                printf("Can not create socket in client!\n");
        }
        memset(&server, 0, sizeof(struct sockaddr_in));
        server.sin_family = AF_INET;
        server.sin_port = htons(5913);
        server.sin_addr.s_addr = inet_addr("127.0.0.1");
        length = sizeof(struct sockaddr_in);
        scanf("%d",&nr);
        nr = htonl(nr);
        n = sendto(sock, &nr, sizeof(nr), 0, (const struct sockaddr *)&server, length);
        if(n < 0) {
                printf("Can not send from client");
        }
        close(sock);
        return 0;
}

---

This is the server

Code:

---

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define N 100
#define M 50
#define K 100

char IP[N][M];
int Arr1[N][K];
int Arr2[N][K];

int main() {
        int sock, n, nr;
        socklen_t fromlen;
        struct sockaddr_in server;
        struct sockaddr_in from;

        sock = socket(AF_INET, SOCK_DGRAM, 0);
        if (sock < 0)
          printf("Can not create socket in server\n");

        memset(&server, 0, sizeof(struct sockaddr_in));
        server.sin_family = AF_INET;
        server.sin_port = htons(5913);
        server.sin_addr.s_addr = INADDR_ANY;

        if(bind(sock, (struct sockaddr *)&server, sizeof(server)) < 0)
          printf("Can not bind in server!\n");
       
        fromlen = sizeof(struct sockaddr_in);
        while(1) {
                unsigned int gen;
                n = recvfrom(sock, &nr, sizeof(nr), 0, (struct sockaddr *) &from, &fromlen);
                if (n < 0)
                  printf("Error when receiving in server!\n");
                nr = ntohl(nr);
                gen = ntohs(from.sin_port);
                printf("I have received %d from IP %s and port %u \n",nr,inet_ntoa(from.sin_addr), gen);
        }
}

---

Show us what you actually saw.

Rubbish like "6423" perhaps?
If it is, then you have a byte swapping problem somewhere.

Perhaps memset your 'from' struct as well. At least you would always get a consistent answer if the function is not updating your struct.

Rubbish in range 50 000 - 60 000, even if I memset my from struct.

I get different rubbish each time a new client connects to the server.

What can be wrong ? It seems coded book-like and it's a very simple example !

Code:

---

nr = htonl(nr);

---

Since you are pretending you have a long, why aren't you actually using a long:

Code:

---

int sock, n, nr;

---

There's no guarantee that int and long are the same thing.


Quzah.

It would probably be a good idea to have some
if ( n == sizeof(nr) )
tests as well, to really check for success.

What you are saying makes sense. Now my code looks like this:

Server

Code:

---

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define N 100
#define M 50
#define K 100

int pars;

int main() {
        int sock, n, nr;
        socklen_t fromlen;
        struct sockaddr_in server;
        struct sockaddr_in from;

        sock = socket(AF_INET, SOCK_DGRAM, 0);
        if (sock < 0)
          printf("Can not create socket in server\n");

        memset(&server, 0, sizeof(struct sockaddr_in));
        server.sin_family = AF_INET;
        server.sin_port = htons(5913);
        server.sin_addr.s_addr = INADDR_ANY;

        if(bind(sock, (struct sockaddr *)&server, sizeof(server)) < 0)
          printf("Can not bind in server!\n");
        memset(&from, 0, sizeof(struct sockaddr_in));
        fromlen = sizeof(struct sockaddr_in);
        while(1) {
                int n, l1;
                fflush(stdout);
                n = recvfrom(sock, &l1, sizeof(l1), 0, (struct sockaddr*) &from, &fromlen);
                if (n < 0) {       
                        printf("Can not receive in server!\n");
                }
                if (n == sizeof(nr)) printf("\n\nSuccess\n\n");
                l1 = ntohl(l1);
                printf("I have received %d from IP: %s, Port: %hu",l1,inet_ntoa(from.sin_addr), ntohs(from.sin_port));
        }
}

---

Client

Code:

---

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

#define N 100
int sock, n, nr, nr2;
socklen_t length;
struct sockaddr_in server, from;

void snd() {
        int i,j;
        length = sizeof(struct sockaddr_in);
        nr = -1;
        nr = htonl(nr);
        n = sendto(sock, &nr, sizeof(nr), 0, (struct sockaddr *)&server, length);
        if(n < 0) {
          printf("Can not send from client");
        }
        nr = -2;
        nr = htonl(nr);
        n = sendto(sock, &nr, sizeof(nr), 0, (struct sockaddr *)&server, length);
        if(n < 0) {
          printf("Can not send from client");
        }

}
void receive() {
}
int main() {
        sock = socket(AF_INET, SOCK_DGRAM, 0);
        if(sock < 0) {
                printf("Can not create socket in client!\n");
        }
        srand(time(0));
        memset(&server, 0, sizeof(struct sockaddr_in));
        server.sin_family = AF_INET;
        server.sin_port = htons(5913);
        server.sin_addr.s_addr = inet_addr("127.0.0.1");
        snd();
        receive();       
        close(sock);
        return 0;
}

---

The server prints "Success" each time he gets a number(the number received is the number sent).
I am using Ubuntu and compiling with gcc.

The port is still rubbish(range 50 000 - 60 000) and isn't consistent across different clients connecting to the same port.

1) you are not binding the socket in your client (thus no valid return address)
2) you are setting the client IP to "localhost" which will only work on the same machine.
3) you are asking both client and server to use the same port, which will fail on the same machine.