Downloading remote firmware payloads or API data over HTTP/HTTPS in C requires interacting with socket streams. Rather than implementing raw POSIX TLS sockets manually, libcurl provides a production-tested library for performing robust HTTP file downloads in C.

Production Implementation: libcurl File Downloader

curl_download.cc
#include <stdio.h>
#include <stdlib.h>
#include <curl/curl.h>
 
// Callback function executed as incoming data chunks arrive from server
static size_t write_data_callback(void *ptr, size_t size, size_t nmemb, void *stream) {
    FILE *file_handle = (FILE *)stream;
    size_t written = fwrite(ptr, size, nmemb, file_handle);
    return written;
}
 
int download_file(const char *url, const char *out_filepath) {
    CURL *curl_handle = curl_easy_init();
    if (!curl_handle) return 1;
 
    FILE *fp = fopen(out_filepath, "wb");
    if (!fp) {
        perror("Failed to open local destination file");
        curl_easy_cleanup(curl_handle);
        return 1;
    }
 
    // Configure cURL options
    curl_easy_setopt(curl_handle, CURLOPT_URL, url);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, write_data_callback);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, fp);
    curl_easy_setopt(curl_handle, CURLOPT_FOLLOWLOCATION, 1L); // Follow 301/302 redirects
 
    // Perform HTTP GET transfer
    CURLcode res = curl_easy_perform(curl_handle);
 
    fclose(fp);
    curl_easy_cleanup(curl_handle);
 
    if (res != CURLE_OK) {
        fprintf(stderr, "Download failed: %s\n", curl_easy_strerror(res));
        return 1;
    }
 
    printf("Successfully downloaded: %s -> %s\n", url, out_filepath);
    return 0;
}
 
int main(void) {
    curl_global_init(CURL_GLOBAL_DEFAULT);
    download_file("https://lynxbee.com/favicon.ico", "downloaded_icon.ico");
    curl_global_cleanup();
    return 0;
}

Key Callback & Option Mechanics:

  • `CURLOPT_WRITEFUNCTION`: Callback function called by libcurl whenever network buffer data is ready to be flushed to disk.

  • `CURLOPT_FOLLOWLOCATION`: Automatically follows HTTP 301 and 302 redirects to target final URLs.