Thread: Issue with TCP socket communication in C program.

Good afternoon. I have written a basic C program to exchange text between sockets. However, I have a issue where the program hangs at the point where I send a message from the client socket. I ran the program in GDB and saw no errors, the program just hung.
My code is included below.

Code:

#include <stdio.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdbool.h>
#define MAX 80
#define PORT 3636
#define SA struct sockaddr


void server_func(int connfd)
{
    char buff[MAX];
    int n;
    memset(buff, 0, MAX);
    read(connfd, buff, sizeof(buff));
    printf("From client: %s\t To client : ", buff);
    const char *response = "Response from server";
    write(connfd, response, strlen(response) + 1);
}


void client_func(int sockfd)
{
    char buff[MAX];
    int n;
    memset(buff, 0, MAX);
    printf("Enter the string : ");
    fgets(buff, MAX, stdin);
    write(sockfd, buff, strlen(buff) + 1);
    memset(buff, 0, MAX);
    read(sockfd, buff, sizeof(buff));
    printf("From Server : %s \n", buff);
}


int client()
{
    int sock_holder;
    int confd;
    // printf("Client called\n");
    printf("Hello  Client");
    struct sockaddr_in svraddcli, cliadd;
    sock_holder = socket(AF_INET, SOCK_STREAM, 0);
    if (sock_holder == -1)
    {
        perror("Client Socket creation failed\n");
    }
    else
        printf("Client Socket creation worked\n");
    memset(&svraddcli, 0, sizeof(svraddcli));
    svraddcli.sin_family = AF_INET;
    svraddcli.sin_addr.s_addr = inet_addr("127.0.0.1");
    svraddcli.sin_port = htons(PORT);


    if (connect(sock_holder, (SA *)&svraddcli, sizeof(svraddcli)) != 0)
    {


        printf("Connection with server failed \n");
    }
    else
        printf("Connected to server\n");


    client_func(confd);
    // close(sock_holder);
    return 0;
}


int main()
{
    int sock_holder;
    int confd;
    int len;
    struct sockaddr_in svradd, cliadd;
    sock_holder = socket(AF_INET, SOCK_STREAM, 0);


    printf("Hello  Server");
    if (sock_holder == -1)
    {
        printf("Socket creation failed\n");
    }
    else
        printf("Socket creation worked\n");


    memset(&svradd, 0, sizeof(svradd));
    svradd.sin_family = AF_INET;
    svradd.sin_addr.s_addr = htonl(INADDR_ANY);
    svradd.sin_port = htons(PORT);


    if ((bind(sock_holder, (SA *)&svradd, sizeof(svradd))) != 0)
    {


        printf("Socket bind failed \n");
        exit(0);
    }
    else
        printf("Socket binding worked \n");


    if ((listen(sock_holder, 5)) != 0)
    {


        printf("Listen failed\n");
    }
    else
        printf("Server listening \n");


    client();
    len = sizeof(cliadd);
    confd = accept(sock_holder, (SA *)&cliadd, &len);
    if (confd < 0)
    {
        printf("server accept failed\n");
        exit(0);
    }
    else
        printf("server has accepted client\n");
    client_func(confd);
    server_func(confd);


    // close(sock_holder);
}

No matter what compiler and IDE you are using, please turn on the warnings and turn up to the highest level!

Code:

bar.c: In function ‘server_func’:
bar.c:19:9: warning: unused variable ‘n’ [-Wunused-variable]
   19 |     int n;
      |         ^
bar.c: In function ‘client_func’:
bar.c:31:9: warning: unused variable ‘n’ [-Wunused-variable]
   31 |     int n;
      |         ^
bar.c: In function ‘client’:
bar.c:48:35: warning: unused variable ‘cliadd’ [-Wunused-variable]
   48 |     struct sockaddr_in svraddcli, cliadd;
      |                                   ^~~~~~
bar.c: In function ‘main’:
bar.c:125:48: warning: pointer targets in passing argument 3 of ‘accept’ differ in signedness [-Wpointer-sign]
  125 |     confd = accept(sock_holder, (SA *)&cliadd, &len);
      |                                                ^~~~
      |                                                |
      |                                                int *
In file included from /usr/include/netinet/in.h:23,
                 from /usr/include/arpa/inet.h:22,
                 from bar.c:2:
/usr/include/x86_64-linux-gnu/sys/socket.h:307:42: note: expected ‘socklen_t * restrict’ {aka ‘unsigned int * restrict’} but argument is of type ‘int *’
  307 |                    socklen_t *__restrict __addr_len);
      |                    ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
bar.c: In function ‘client’:
bar.c:72:5: warning: ‘confd’ is used uninitialized [-Wuninitialized]
   72 |     client_func(confd);
      |     ^~~~~~~~~~~~~~~~~~
bar.c:45:9: note: ‘confd’ was declared here
   45 |     int confd;
      |         ^~~~~

I don't do socket programming, so I defer other suggestions to others.

Pro tip..

Code:

printf("Socket creation failed\n");

If the socket failed.. You probably want to exit the program.

Code:

fputs("Socket creation failed\n", stderr);
exit(1);