I'm working on a project that needs a licensing system. So I decided to use the licensing system of my sales platform Gumroad.

It seems pretty straightforward, the only thing is that I can't seem to add custom data to a cURL request in C .

This is the example posted in the documentation:

curl https://api.gumroad.com/v2/licenses/verify \
  -d "product_permalink=QMGY" \
  -d "license_key=YOUR_CUSTOMERS_LICENSE_KEY" \
  -X POST

This works very well from the Ubuntu Terminal, but when I try to implement it in C , it doesn't work.

This is the C code:

std::stringstream get_response()
{
   std::stringstream str;
   curl::curl_ios<std::stringstream> writer(str);

   curl::curl_easy easy(writer);

   easy.add<CURLOPT_URL>("https://api.gumroad.com/v2/licenses/verify");
   easy.add<CURLOPT_POSTFIELDS>("product_permalink=QMGY");
   easy.add<CURLOPT_POSTFIELDS>("license_key=YOUR_CUSTOMERS_LICENSE_KEY");

   try
   {
      easy.perform();
   }
   catch (curl::curl_easy_exception error)
   {
      auto errors = error.get_traceback();
      error.print_traceback();
   }

   return str;
}

It is able to get a response from the server, but it doesn't recognize the license key.

I assume that this is because CURLOPT_POSTFIELDS is not the right field to replace -d.

Does anyone know the right field to use?

There are like 300 different ones, and I have no clue which one is the right one.

Can someone please point me in the right direction?

CodePudding user response：

Per the curl documentation for the -d option:

-d, --data

(HTTP MQTT) Sends the specified data in a POST request to the HTTP server, in the same way that a browser does when a user has filled in an HTML form and presses the submit button. This will cause curl to pass the data to the server using the content-type application/x-www-form-urlencoded. Compare to -F, --form.

--data-raw is almost the same but does not have a special interpretation of the @ character. To post data purely binary, you should instead use the --data-binary option. To URL-encode the value of a form field you may use --data-urlencode.

If any of these options is used more than once on the same command line, the data pieces specified will be merged with a separating &-symbol. Thus, using '-d name=daniel -d skill=lousy' would generate a post chunk that looks like 'name=daniel&skill=lousy'.

If you start the data with the letter @, the rest should be a file name to read the data from, or - if you want curl to read the data from stdin. Posting data from a file named 'foobar' would thus be done with -d, --data @foobar. When -d, --data is told to read from a file like that, carriage returns and newlines will be stripped out. If you do not want the @ character to have a special interpretation use --data-raw instead.

Examples:

 curl -d "name=curl" https://example.com
 curl -d "name=curl" -d "tool=cmdline" https://example.com
 curl -d @filename https://example.com

See also --data-binary, --data-urlencode and --data-raw. This option overrides -F, --form and -I, --head and -T, --upload-file.

Note the part about specifying -d multiple times.

CURLOPT_POSTFIELDS is the correct field to use. You are simply using it the wrong way. You need to set CURLOPT_POSTFIELDS only once, providing it with all of the data to be posted, in the correct format the server is expecting. Since you are trying to post a application/x-www-form-urlencoded format, the data needs to be name=value pairs separated by &, eg:

std::stringstream get_response()
{
   std::stringstream str;
   curl::curl_ios<std::stringstream> writer(str);

   curl::curl_easy easy(writer);

   easy.add<CURLOPT_URL>("https://api.gumroad.com/v2/licenses/verify");
   easy.add<CURLOPT_POSTFIELDS>("product_permalink=QMGY&license_key=YOUR_CUSTOMERS_LICENSE_KEY");

   try
   {
      easy.perform();
   }
   catch (curl::curl_easy_exception error)
   {
      auto errors = error.get_traceback();
      error.print_traceback();
   }

   return str;
}

If any of the name or value data may contain reserved or non-ASCII characters, you will have to url-encode that data, such as with curl_easy_escape().

CodePudding user response：

pretty sure you forgot easy.add<CURLOPT_POST>(1);