Download file using libcurl in C/C++

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.

Just for those interested you can avoid writing custom function by passing NULL as last parameter (if you do not intend to do extra processing of returned data).
In this case default internal function is used.

Details
http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTWRITEDATA

Example

#include <stdio.h>#include <curl/curl.h>int main(void){    CURL *curl;    FILE *fp;    CURLcode res;    char *url = "http://stackoverflow.com";    char outfilename[FILENAME_MAX] = "page.html";    curl = curl_easy_init();                                                                                                                                                                                                                                                               if (curl)    {           fp = fopen(outfilename,"wb");        curl_easy_setopt(curl, CURLOPT_URL, url);        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, NULL);        curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);        res = curl_easy_perform(curl);        curl_easy_cleanup(curl);        fclose(fp);    }       return 0;}