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I have an Angular front end with a C++ powered back end. I want the C++ back end to grab a file from a blob URL created by the front end (with URL.CreateObjectURL).

I have tried using URLDownloadToFile:

HRESULT hr = URLDownloadToFile(NULL, theBlobURL, outfilename, 0, NULL);

As well as curl:

                CURL* curl;
                FILE* fp;
                CURLcode res;

                curl = curl_easy_init();
                if (curl) {
                    fp = fopen(outfilename2, "wb");
                    curl_easy_setopt(curl, CURLOPT_URL, theBlobURL);
                    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, NULL);
                    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);

                    res = curl_easy_perform(curl);
                    if (res != CURLE_OK)
                        fprintf(stderr, "curl_easy_perform() failed: %s\n",
                            curl_easy_strerror(res));

                    curl_easy_cleanup(curl);
                }

Both of these methods work with a traditional file URL (I tested with this public file), but fail with my blob URL. URLDownloadToFile gives "the specified protocol is unknown" as the HRESULT and curl saus "CURLE_COULDNT_RESOLVE_HOST".

I've confirmed via the browser that the blob URL is still available at the time I am trying to open it.

Do I need to do anything different to get a blob?

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  • Why not use PHP to handle the upload? Then the PHP script can call your C++. – moonman239 Jun 24 at 19:01
  • Can you explain a little more how this might work? I'm not great with PHP. – psOneOneOne Jun 24 at 19:06
  • What does the value of theBlobURL actually look like? If it does not begin with http: or https: then traditional HTTP libraries will not be able to access it. Sounds like theBlobURL contains a URL that is private to the browser, so only the browser can access its content – Remy Lebeau Jun 24 at 19:28
  • @psOneOneOne first, the client sends the server the URI of the file to be uploaded, then in PHP you use something like move_uploaded_file($_FILE['(the form field of the file)'],"(a new path for the file)") and then you can have the C++ program reference the file at that new location. – moonman239 Jun 24 at 19:54
  • @moonman239 but the URI of the file to be uploaded is the root problem. When user drops or selects a file in JS, I cannot see the file path, I only have the resulting file. This is why I was using Blob URL to begin with. – psOneOneOne Jun 24 at 20:00
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Remy Lebeau was correct, the Blob URL is private to the browser. It cannot be downloaded outside the browser.

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