Download file using libcurl in C/C++

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The example you are using is wrong. See the man page for easy_setopt. In the example write_data uses its own FILE, *outfile, and not the fp that was specified in CURLOPT_WRITEDATA. That’s why closing fp causes problems – it’s not even opened.

This is more or less what it should look like (no libcurl available here to test)

#include <stdio.h>
#include <curl/curl.h>
/* For older cURL versions you will also need 
#include <curl/types.h>
#include <curl/easy.h>
*/
#include <string>

size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) {
    size_t written = fwrite(ptr, size, nmemb, stream);
    return written;
}

int main(void) {
    CURL *curl;
    FILE *fp;
    CURLcode res;
    char *url = "http://localhost/aaa.txt";
    char outfilename[FILENAME_MAX] = "C:\\bbb.txt";
    curl = curl_easy_init();
    if (curl) {
        fp = fopen(outfilename,"wb");
        curl_easy_setopt(curl, CURLOPT_URL, url);
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
        res = curl_easy_perform(curl);
        /* always cleanup */
        curl_easy_cleanup(curl);
        fclose(fp);
    }
    return 0;
}

Updated: as suggested by @rsethc types.h and easy.h aren’t present in current cURL versions anymore.

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