C client socket does not output/connect to tcp server-CodePudding

Why does this header file go into a freeze state or it does not output anything even if the server is accepting connection form other scripts/tools

tcpclient.h

#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#pragma once

int socket_desc;
struct sockaddr_in server_addr;

int init_client()
{
    socket_desc = socket(AF_INET, SOCK_STREAM, 0);  

    if (socket_desc < 0 )
    {
        printf("Failed to create socket\n");
        return -1;
    }
    return 0;
}

int connection(char *host,int port )
{

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    server_addr.sin_addr.s_addr = inet_addr(host);

    if (connect(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr))<0)
    {
        printf("Failed to connect @ %s:%d\n",host,port);
        return -1;
    }
    printf("Connected to [%s:%d]\n",host,port);
    return 0;
}

using a main.c like so

int main()
{
    int soc = init_client();
    int con = connection("192.168.0.12",6666);
    return 0;
}

the only warning I get is that the variable soc is not used

CodePudding user response：

Your program works correctly in my computer.... It's not clear what do you claim on.

Anyway, after tweaking it a bit, I have some points to comment on it. I changed the ip address to 127.0.0.1 and port to 80 and started an apache server locally to check that the server is, indeed ready to accept connections.

First I checked that the connection opened locally with a telnet(1) command (you will probably have to install it in your machine as it is not installed now by default)

$ telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
^]
telnet> close
Connection closed.
$ _

then I changed several things in your code (they should not affect the run, but they will make you trouble in the future if you don't take this path)

I first created a socket.h file to put prototypes for both functions, so you don't run in trouble if you later change the prototypes.

socket.h

#ifndef _SOCKET_H
#define _SOCKET_H

int init_client(void);
int connection(const char *host, int port);

#endif /* _SOCKET_H */

As you see, I changed the parameter `host` into a `const char *` type, so the compiler knows that the string is not changed inside the function and can optimize calls, based on that.

Second, I #included the file in both modules, so if you change the interface, both modules (provider and consumer) know about the interface change.

socket.c

#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <errno.h>
#include <string.h>

I used the last two includes to use strerror(errno), that gives you the reason for the connection error (mainly being a connection refused error)


#include "socket.h"

int socket_desc = -1;
struct sockaddr_in server_addr;

this is not good practice. A better solution would be to return the socket and pass it as parameter to the connection call below. Using global variables will run you into trouble later if you want to use these functions in different threads, as the socket_desc global variable is common to all the threads you will have.


int init_client(void)
{
    socket_desc = socket(AF_INET, SOCK_STREAM, 0);

    if (socket_desc < 0 )
    {
        printf("Failed to create socket, %s\n",
            strerror(errno));
        return -1;
    }
    return 0;
}

int connection(const char *host, int port)
{

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    server_addr.sin_addr.s_addr = inet_addr(host);

    printf("trying connect to %s:%d\n", host, port);

    if (connect(socket_desc, (struct sockaddr*) &server_addr, sizeof(server_addr))<0)
    {
        printf("Failed to connect @ %s:%d, %s\n",
                host, port, strerror(errno));
        return -1;

printing here the reason of the non connection is a good idea to know what is happening.

    }
    printf("Connected to [%s:%d]\n",host,port);
    return 0;
}

A better solution would be the following:

socket.c

#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <errno.h>
#include <string.h>

#include "socket.h"

int init_client(void)
{
    int socket_desc = socket(AF_INET, SOCK_STREAM, 0);

    if (socket_desc < 0 )
    {
        printf("Failed to create socket: %s\n",
            strerror(errno));
    }
    return socket_desc;
}

int connection(int sd, const char *host, int port)
{

    struct sockaddr_in server_addr;

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    server_addr.sin_addr.s_addr = inet_addr(host);

    printf("trying connect to %s:%d\n", host, port);

    if (connect(sd, (struct sockaddr*) &server_addr, sizeof(server_addr)) < 0) {
        printf("Failed to connect @ %s:%d, %s\n",
                host, port, strerror(errno));
        return -1;
    }
    printf("Connected to [%s:%d]\n", host, port);
    return 0;
}

this modifications, made that calling the program to localhost:80 resulted in:

$ a.out
trying connect to 127.0.0.1:80
Connected to [127.0.0.1:80]
$ _

while using 79 as the port resulted in:

$ a.out
trying connect to 127.0.0.1:79
Failed to connect @ 127.0.0.1:79, Connection refused
Failed connection(), Connection refused
$ _