Fetch a web page using C

walking

This C program fetches a web page using the HTTP protocol and Unix sockets. It's based on getaddrinfo and socket.

#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <err.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <stdarg.h>

/* "BSIZE" is the size of the buffer we use to read from the socket. */

#define BSIZE 0x1000

/* Quickie function to test for failures. It is actually better to use
   a macro here, since a function like this results in unnecessary
   function calls to things like "strerror". However, not every
   version of C has variadic macros. */

static void fail (int test, const char * format, ...)
{
    if (test) {
	va_list args;
	va_start (args, format);
	vfprintf (stderr, format, args);
	va_end (args);
	// exit ok
	exit (EXIT_FAILURE);
    }
}

/* Get the web page and print it to standard output. */

static void get_page (int s, const char * host, const char * page)
{
    /* "msg" is the request message that we will send to the
       server. */

    char * msg;

    /* "format" is the format of the HTTP request we send to the web
       server. */

    const char * format =
        "GET /%s HTTP/1.0\r\nHost: %s\r\nUser-Agent: fetch.c\r\n\r\n";

    /* This holds return values from functions. */

    int status;

    /* I am using non-standard function "asprintf" for convenience. If
       you don't have "asprintf", use "snprintf" with a fixed-size
       buffer and check its return value against the length of the
       buffer after the call. */

    status = asprintf (& msg, format, page, host);

    /* Check that "asprintf" succeeded. */

    fail (status == -1 || ! msg, "asprintf failed.\n");

    /* Send the request. */

    status = send (s, msg, strlen (msg), 0);

    /* Check it succeeded. The FreeBSD manual page doesn't mention
       whether "send" sets errno, but
       "http://pubs.opengroup.org/onlinepubs/009695399/functions/send.html"
       claims it does. */

    fail (status == -1, "send failed: %s\n", strerror (errno));

    while (1) {
	/* The number of bytes received. */
        int bytes;
	/* Our receiving buffer. */
        char buf[BSIZE+10];
	/* Get "BSIZE" bytes from "s". */
        bytes = recvfrom (s, buf, BSIZE, 0, 0, 0);
	/* Stop once there is nothing left to print. */
        if (bytes == 0) {
            break;
        }
        fail (bytes == -1, "%s\n", strerror (errno));
	/* Nul-terminate the string before printing. */
        buf[bytes] = '\0';
	/* Print it to standard output. */
        printf ("%s", buf);
    }
    free (msg);
}

int main ()
{
    struct addrinfo hints, *res, *res0;
    int error;
    /* "s" is the file descriptor of the socket. */
    int s;
    /* Get one of the web pages here. */
    const char * host = "www.lemoda.net";

    memset (&hints, 0, sizeof (hints));
    /* Don't specify what type of internet connection. */
    hints.ai_family = PF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    error = getaddrinfo (host, "http", & hints, & res0);
    fail (error, gai_strerror (error));
    s = -1;
    for (res = res0; res; res = res->ai_next) {
        s = socket (res->ai_family, res->ai_socktype, res->ai_protocol);
        fail (s < 0, "socket: %s\n", strerror (errno));
        if (connect (s, res->ai_addr, res->ai_addrlen) < 0) {
            fprintf (stderr, "connect: %s\n", strerror (errno));
            close (s);
	    // exit ok
            exit (EXIT_FAILURE);
        }
        break;
    }
    if (s != -1) {
        get_page (s, host, "c/");
    }
    freeaddrinfo (res0);
    return 0;
}

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