A TCP client in C

socket, connect, write, read, close — the whole client in five calls.

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

int fetch(const char *ip, int port) {
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    if (fd < 0) {
        return 0;
    }

    struct sockaddr_in addr;
    memset(&addr, 0, sizeof addr);
    addr.sin_family = AF_INET;
    addr.sin_port = htons((unsigned short) port);
    inet_pton(AF_INET, ip, &addr.sin_addr);

    if (connect(fd, (struct sockaddr *) &addr, sizeof addr) != 0) {
        close(fd);
        return 0;
    }

    const char *request = "GET /healthz HTTP/1.0\r\n\r\n";
    write(fd, request, strlen(request));

    char buffer[256];
    ssize_t n = read(fd, buffer, sizeof buffer - 1);
    if (n > 0) {
        buffer[n] = '\0';
        printf("%s", buffer);
    }
    close(fd);
    return n > 0;
}

How it works

1. AF_INET with SOCK_STREAM means IPv4 TCP.
2. The port must be converted with htons.
3. Every descriptor is closed on the way out.

Keywords and builtins used here

• char
• const
• fetch
• if
• int
• return
• short
• sizeof
• sockaddr
• sockaddr_in
• ssize_t
• struct
• unsigned

The run, in numbers

Lines

35

Characters to type

713

Tokens

210

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 115 seconds.

Type this snippet

Step 1 of 1 in Sockets, step 13 of 14 in Systems programming.

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A TCP client in other languages

• Python