Question

My problem is quite infuriating, actually. I’ll show you the code first.

/*
** listener.c -- a datagram sockets "server" demo
*/
//Original Code: Brian Hall ([e-mail])
//Commented and modified by Vishal Kotcherlakota (PID A07124450)

#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 MYPORT "4960"   // the port users will be connecting to
#define YOURPORT "4961"
#define MAXBUFLEN 10000 

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
    //If the socket address struct says it's an IPv4...
    if (sa->sa_family == AF_INET) {
        //...return the IPv4 variable.
        return &(((struct sockaddr_in*)sa)->sin_addr);
    }
    //otherwise, assume it's IPv6, and get the IPv6 variable
    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int argc, char *argv[])
{
    int sockfd, sockfdAck; //socket file descriptor (handle)
    struct addrinfo hints, *servinfo, *p, *q;
    int rv;
    int numbytes;
    unsigned int seqNum, stateNum=0, ackNum;
    struct sockaddr_storage their_addr;
    struct timeval recvTime, timeStamp, latency;
    char buf[MAXBUFLEN], junk[MAXBUFLEN];
    size_t addr_len;
    char *ackBack;
    char s[INET6_ADDRSTRLEN];

    if (argc != 2)
    {
        fprintf(stderr, "usage: <executable> <hostname>\n");
        exit(0);
    }

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4
    hints.ai_socktype = SOCK_DGRAM;
    hints.ai_flags = AI_PASSIVE; // use my IP

    if ((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    // loop through all the results and bind to the first we can
    for(q = servinfo; q != NULL; q = q->ai_next) {
        if ((sockfd = socket(q->ai_family, q->ai_socktype,
                q->ai_protocol)) == -1) {
            perror("listener: socket");
            continue;
        }

        if (bind(sockfd, q->ai_addr, q->ai_addrlen) == -1) {
            close(sockfd);
            perror("listener: bind");
            continue;
        }

        break;
    }

    if (q == NULL) {
        fprintf(stderr, "listener: failed to bind socket\n");
        return 2;
    }

    freeaddrinfo(servinfo);

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4
    hints.ai_socktype = SOCK_DGRAM;

    if ((rv = getaddrinfo(argv[1], BACKPORT, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    // loop through all the results and bind to the first we can
    for(p = servinfo; p != NULL; p = p->ai_next) {
        if ((sockfdAck = socket(p->ai_family, p->ai_socktype,
                p->ai_protocol)) == -1) {
            perror("listener: socket");
            continue;
        }

        break;
    }

    if (p == NULL) {
        fprintf(stderr, "listener: failed to bind socket\n");
        return 2;
    }

    freeaddrinfo(servinfo);
        printf("listener: waiting to recvfrom...\n");
    while (1)
    {

        addr_len = sizeof their_addr;
        if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1 , 0,
            (struct sockaddr *)&their_addr, &addr_len)) == -1) {
            perror("recvfrom");
            exit(1);
        }

        /*printf("listener: got packet from %s\n",
            inet_ntop(their_addr.ss_family,
                get_in_addr((struct sockaddr *)&their_addr),
                s, sizeof s));
        printf("listener: packet is %d bytes long\n", numbytes);
        */
        buf[numbytes] = '\0';
        sscanf(buf,"%u %s",&seqNum, junk);
        if (seqNum == stateNum + 1)
        {
            stateNum = seqNum;
            printf("Ok, state is now %u.\n", stateNum);
        }
        ackBack = malloc(20*sizeof(char));
        sprintf(ackBack, "%u acknowledged\0", stateNum);
        numbytes = sendto(sockfdAck, ackBack, strlen(ackBack), 0, p->ai_addr, p->ai_addrlen);
        if (numbytes == -1);
        {
            perror("sendto");
            exit(1);
        }
        free(ackBack);
    }

    return 0;
} 

Please forgive the sloppy code; I’m desperately trying to finish this assignment on time. The goal is to develop an ARQ protocol using datagram sockets. This code should work, but when I run it, I get an error sendto: Success, meaning that my sendto() call failed. I can’t find documentation for this anywhere, and I’m getting to be extremely desperate.

Answer 1

It has nothing to do with having to bind() – in fact take a look at this syntax:

if (numbytes == -1) ;        // semicolon !
{
        perror("sendto");
        exit(1);
}

You have a condition without the body, and then the body without the condition, which always executes (as you can observe).

Add the printf of numbytes and you will see it is set correct, there is no error.

To avoid this kind of hard-to-see errors, I generally put the opening brace immediately after the condition – then you would have spotted this immediately. But of course this is a matter of the coding convention for the company/project.