Question

I am learning the Sockets networking API. In this process, I have written a simple Echo server that uses TCP. I wrote the code in such a way that, as long as the server is running, anything typed on the client’s console should be echoed back to it. However, I am unable to achieve this. Although, for the first input, I get the echo, from next time onwards, I do not get any message.
I know, we can implement it to run for many clients using fork(), but I want to know the reason behind the blocking of the client, and if possible ways to correct it.
Here is the code for the client:

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

#define MAXCOUNT 1024

int main(int argc, char* argv[])
{
    int sfd;
    char msg[MAXCOUNT];
    char blanmsg[MAXCOUNT];
    struct sockaddr_in saddr;

    memset(&saddr,0,sizeof(saddr)); 
    sfd = socket(AF_INET,SOCK_STREAM,0);
    saddr.sin_family = AF_INET;
    inet_pton(AF_INET,"127.0.0.1",&saddr.sin_addr);
    saddr.sin_port = htons(5004);

    connect(sfd,(struct sockaddr*) &saddr, sizeof(saddr));
    for(; ;) {
        memset(msg,0,MAXCOUNT);
        memset(blanmsg,0,MAXCOUNT);
        fgets(msg,MAXCOUNT,stdin);
        send(sfd,msg,strlen(msg),0);
        recv(sfd,blanmsg,sizeof(blanmsg),0);
        printf("%s",blanmsg);
        fflush(stdout);
    }
    exit(0);
}   

Here is the code for the server:

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

#define MAXCOUNT 1024

int main(int argc, char* argv[])
{
    int sfd,nsfd,n,i,cn;
    char buf[MAXCOUNT];
    socklen_t caddrlen;
    struct sockaddr_in caddr,saddr; //Structs for Client and server Address in the Internet 

    sfd = socket(AF_INET,SOCK_STREAM,0);
    memset(&saddr,0,sizeof(saddr)); //Clear the Server address structure

    saddr.sin_family = AF_INET; //Internet Address Family
    saddr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    saddr.sin_port = htons(5004);

    bind(sfd, (struct sockaddr*) &saddr,sizeof(saddr));
    listen(sfd,1);

    for(; ;) {
        caddrlen = sizeof(caddr);
        nsfd = accept(sfd,(struct sockaddr*) &caddr,&caddrlen);
        cn = recv(nsfd,buf,sizeof(buf),0);
        if(cn == 0) {
            exit(0);
        }
        buf[cn] = '\0';
        send(nsfd,buf,strlen(buf),0);
        }
    close(nsfd);
    exit(0);
}       

Answer 1

You shouldn’t call accept within the loop on the server. Move the accept before the for loop on the server and it should work how you expect.

Calling accept in the loop like that will make the server block until a new connection comes in. Your client is only opening a single connection, so the server will block on the second call to accept.