Part 2 - TCP Client

Once we have a process listening at a TCP port, we can send a message to the server with a client process.

The client process will need to

1. create a socket with the same socket() and get the fd, same as the server
2. initiate and establish connection with connect()
3. write data to the socket after connect() with write()
4. retrieve the server’s response with read()

Connecting to a server socket

We need to connect the client socket to the destination using connect(2).

#include <sys/socket.h>

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

We have encountered the sockaddr type before, we will use the sockaddr_in type for internet socket.

struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr("127.0.0.1");
addr.sin_port = htons(PORT); 

if (connect(sockfd, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
    perror("Failed to connnect to server socket");
    return 1;
};

Socket I/O

After connecting to the server, we can do read() and write() on the socket. For now, we will be using a 1024 (1kb) buffer.

#define BUFFER_SIZE 1024

// hard coded data for now
char * data = "(hard coded) Hi!";
if (write(sockfd, data, strlen(data)) < 0) {
    perror("failed to write data");
    return 1;
}
log_with_time("wrote data (%db) to server", strlen(data));

char * recv_data[BUFFER_SIZE];
if (read(sockfd, recv_data, BUFFER_SIZE) < 0) {
    perror("failed to read server response");
}
log_with_time("server response:%s", recv_data);

The final client program will be (the log.h file is in the previous document)

#define PORT 8080

#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>

#include "log.h"

#define BUFFER_SIZE 1024

int main() {
    int sockfd;
    sockfd = socket(AF_INET, SOCK_STREAM, 6);
    if (sockfd == -1) {
        perror("Error opening socket");
        return 1;
    }
    log_with_time("openned socket with fd:%d", sockfd);

    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    addr.sin_port = htons(PORT); 
    if (connect(sockfd, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
        perror("Failed to connnect to server socket");
        return 1;
    };

    // hard coded data for now
    char * data = "(hard coded) Hi!";
    if (write(sockfd, data, strlen(data)) < 0) {
        perror("failed to write data");
        return 1;
    }
    log_with_time("wrote data (%db) to server", strlen(data));
    
    char * recv_data[BUFFER_SIZE];
    if (read(sockfd, recv_data, BUFFER_SIZE) < 0) {
        perror("failed to read server response");
    }
    log_with_time("server response:%s", recv_data);
    
    return 0;
}