Question

I am currently trying to make an updater for my software project. I need it to be able to download multiple files, I don’t mind if they download in sync or one after each other, whatever is easier (file size is not an issue). I followed the example from the libcurl webpage and a few other resources and came up with this:

#include <iostream>
#include <stdio.h>
#include <curl/curl.h>
#include <string.h>
size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) {
    size_t written;
    written = fwrite(ptr, size, nmemb, stream);
    return written;
}
int main(void){
    for (int i = 0; i < 2;){        //download 2 files (loop twice)
        CURL *curl;
        FILE *fp;
        CURLcode res;
        char *url = "http://sec7.org/1024kb.txt";  //first file URL
        char outfilename[FILENAME_MAX] = "C:\\users\\grant\\desktop\\1024kb.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);
            curl_easy_cleanup(curl);
            fclose(fp);
        }
         url = "http://sec7.org/index.html"; //I want to get a new file this time
         outfilename[FILENAME_MAX] = "C:\\users\\grant\\desktop\\index.html";

    }
    return 0;
}

The first issue is if i remove the new file assignments (*url = "http://...") and just try to loop the download code twice, the program simply stops responding. This occurs in any combination of the download being called more than once in the program. The other issue is that I am unable to change the value of the character array outfilename[FILENAME_MAX]. I feel like this is just some silly error I am making but no solution comes to mind. Thank you!

Answer 1

1. Why not put this in a function and call it twice?

2. Your syntax for the arrays is all wrong, plus all the variables inside the loop are local, which means they are destroyed after each loop iteration.

3. What Conspicuous Compiler said. That’s what’s causing your program to freeze; it’s stuck in an infinite loop because i is never > 2.

Put your code into a function like so:

void downloadFile(const char* url, const char* fname) {
    CURL *curl;
    FILE *fp;
    CURLcode res;
    curl = curl_easy_init();
    if (curl){
        fp = fopen(fname, "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);
        curl_easy_cleanup(curl);
        fclose(fp);
    }
}

And call it twice with the relevant file names and urls:

downloadFile("http://sec7.org/1024kb.txt", "C:\\users\\grant\\desktop\\1024kb.txt");
downloadFile("http://sec7.org/index.html", "C:\\users\\grant\\desktop\\index.html");

The example function is very bad though, it’s just an example. You should alter it to return error codes/throw exceptions, and stuff like that.