Whats wrong with my UDP Client Server?

Question

I have a UDP Server that should sit and wait for a client to connect to it, and send it a string of a filename. Right now I just want it to echo back the filename to the client. Here is my code

Server

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

#define MAXBUFLEN 1024

// Usage: ./server PORT
int main(int argc, char *argv[])
{
    int sockfd;
    struct sockaddr_in server;
    size_t clientSize;
    struct sockaddr_storage client;
    char buf[MAXBUFLEN];
    int portno = atoi(argv[1]);
    int numbytes;

    printf("Port: %d\n", portno);

    // Create UDP Socket
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("Can't create socket");
        exit(-1);
    }

    // Configure socket
    memset(&server, 0, sizeof server);
    server.sin_family = AF_INET; // Use IPv4
    server.sin_addr.s_addr = INADDR_ANY; // My IP
    server.sin_port = htons(portno); // Server Port

    // Bind socket
    if ((bind(sockfd, (struct sockaddr *) &server, sizeof(server))) == -1) {
        close(sockfd);
        perror("Can't bind");
    }

    while (1) {
        printf("Waiting for data...\n");

        // Receive data from Client
        clientSize = sizeof(client);
        numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1,0,
                (struct sockaddr *) &client, &clientSize);

        buf[numbytes] = '\0';

        printf("client sent: %s\n", buf);

        // Rely to client
        sendto(sockfd, buf, MAXBUFLEN-1, 0,
                (struct sockaddr *) &client, &clientSize);
    }

    printf("Closing");
    close(sockfd);

    return 0;
}

Client

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

#define MAXBUFLEN 1024

//Usage: ./client PORT IP FILE
int main(int argc, char *argv[]) {
    int sockfd;
    struct sockaddr_in server;
//  struct sockaddr_storage client;
    char buf[MAXBUFLEN];
    int portno = atoi(argv[1]);
    char *serverIP = argv[2];
    char *filename = argv[3];
    int numbytes;

    printf("Port: %d, IP:%s, File:%s\n", portno, serverIP, filename);

    // Create UDP Socket
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("Can't create socket");
        exit(-1);
    }

    // The server IP and Port
    memset(&server, 0, sizeof server);
    server.sin_family = AF_INET; // Use IPv4
    server.sin_port = htons(portno); // Server Port
    struct hostent *hp = gethostbyname(serverIP);
    memcpy((char*)&server.sin_addr, (char*)hp->h_name, hp->h_length);

    printf("Sent %s\n", filename);

    // No need to bind, just send request for file
    int serverSize = sizeof(server);
    sendto(sockfd, argv[3], strlen(argv[3]), 0,
            (struct sockaddr *) &server, serverSize);

    printf("Waiting for reply\n");

    recvfrom(sockfd, buf, MAXBUFLEN-1, 0,
            (struct sockaddr *) &server, serverSize);

    buf[numbytes] = '\0';

    printf("server sent: %s\n", buf);

    close(sockfd);
}

So far the Server just prints out

Waiting for data...

only once... so its never getting a request.

and the Client prints out:

./client 8083 127.0.0.1 hello
Port: 8083, IP:127.0.0.1, File:hello
Sent hello
Waiting for reply
server sent: 

Please help, I'm new to the world of C and UDP Sockets.

Oh yeah, and I already read Beej's Guide... please don't just post that as an answer.

Answer 1

Fixed the bug! Thank You Tommy. Here is what I am running right now.

Server:

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>

#define MAXBUFLEN 1024

void error(char *msg) {
    perror(msg);
    exit(0);
}

int main(int argc, char *argv[]) {
    int sockfd;
    struct sockaddr_in server, client;
    int serverSize, clientSize, numbytes;
    char buf[MAXBUFLEN];
    int portno = atoi(argv[1]);

    if (argc < 2) {
        fprintf(stderr, "ERROR, no port provided\n");
        exit(0);
    }

    printf("Running on Port: %d\n", portno);

    // Create UDP Socket
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        error("Can't create socket");
    }

    // Configure Server Info
    serverSize = sizeof(server);
    bzero(&server, serverSize);
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(portno);

    // Bind
    if (bind(sockfd, (struct sockaddr *) &server, serverSize) < 0) {
        error("binding");
    }

    clientSize = sizeof(struct sockaddr_in);

    while (1) {
        printf("Waiting for Client Request...\n");
        numbytes = recvfrom(sockfd, buf, MAXBUFLEN, 0,
                (struct sockaddr *) &client, &clientSize);
        if (numbytes < 0) {
            error("recvfrom");
        }
        buf[numbytes] = '\0';

        printf("Received request for File: %s\n", buf);
        numbytes = sendto(sockfd, buf, sizeof(buf), 0,
                (struct sockaddr *) &client, clientSize);
        if (numbytes < 0) {
            error("sendto");
        }
        sendFile(sockfd, client);
    }
}

sendFile(int sockfd, struct sockaddr_in client) {

}

Client:

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

#define MAXBUFLEN 1024

void error(char *msg) {
    perror(msg);
    exit(0);
}

int main(int argc, char *argv[]) {
    int sockfd;
    struct sockaddr_in server, client;
    int serverSize;
    struct hostent *hp;
    char buf[MAXBUFLEN];
    int numbytes;
    char *serverIP = argv[1];
    int portno = atoi(argv[2]);
    char *filename = argv[3];
    int done = 1;

    if (argc != 4) {
        printf("Usage: SERVER PORT FILENAME\n");
        exit(1);
    }

    // Create Socket
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        error("Can't create socket");
    }

    // Configure Server Info
    server.sin_family = AF_INET;
    hp = gethostbyname(serverIP);
    if (hp == 0)
        error("Unknown host");
    bcopy((char *) hp->h_addr, (char *) &server.sin_addr, hp->h_length);
    server.sin_port = htons(portno);
    serverSize = sizeof(struct sockaddr_in);

    // Send Message
    strcat(buf, filename);
    numbytes = sendto(sockfd, buf, strlen(buf), 0, &server, serverSize);
    if (numbytes < 0)
        error("Sendto");
    bzero(buf, MAXBUFLEN);

    // Build file then set Done = 0
    while (done) {
        numbytes = recvfrom(sockfd, buf, MAXBUFLEN, 0, &client, &serverSize);
        if (numbytes < 0)
            error("recvfrom");

        done = 0;
    }

    printf("Received the packet: %s\n", buf);
}

I also found a great example here: http://www.linuxhowtos.org/C_C++/socket.htm

Thanks for everyone's help!

Answer 2

In the first place, there is a bug in the call to sendto of your server. clientSize should be a value, not a pointer. Also, as Tom said, numbytes should be set to the result of recvfrom.

The big problem is likely that you are copying hp->h_name into the sin_addr element of the server. h_name is the name of the host, not the IP address. Try replacing

    memcpy((char*)&server.sin_addr, (char*)hp->h_name, hp->h_length);

with

    memcpy((char*)&server.sin_addr, (char*)hp->h_addr_list[0], hp->h_length);

in your client.

Answer 3

numbytes in your client is not initialised nor is it used! Check again in your code....

Hope this helps, Best regards, Tom.

Answer 4

Silly question, but is your firewall blocking port 8083.

Bob.