Question

The program

Reads out a textfile and emails it’s contents

The original code (with help of Sehe)

https://gist.github.com/1342118#file_test.cpp

The problem:

I am trying to run the application multiple times using threading, using a different textfile for each thread. This means I can no longer use a global vector because thread 3 may add to / alter this vector while thread 1 is still working on it. This means this line is becoming problematic:

curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);

This calls this function:

static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *userp)
{
  struct WriteThis *pooh = (struct WriteThis *)userp;

  if(size*nmemb < 1)
    return 0;

  if (pooh->counter < text.size())
  {
      const std::string& data = text[pooh->counter];

      memcpy(ptr, data.data(), data.length());
      pooh->counter++; /* advance pointer */
      return data.length();
  }
  return 0; /* no more data left to deliver */
}

As you can see it uses the globally defined vector “text”. I thought I could fix it by simply making the vector inside that function (read_callback) but since I’m using threading now, the filename is no longer hard coded so I end up having to pass a variable anyway.

Curl seems to fill in the variables that read_callback accepts automatically so how is this done?

Just to clarify, here’s what I have now:

std::vector<string> read_text(char* fname)
{
    std::ifstream myfile (fname);
    std::vector<string> text;

    std::string line;
    while (std::getline(myfile, line))
    {
        text.push_back(line + '\n');
    }

    return text;
}

static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *userp)
{

    std::vector<string> text;
    text = read_text(textfilename);

  struct WriteThis *pooh = (struct WriteThis *)userp;

  if(size*nmemb < 1)
    return 0;

  if (pooh->counter < text.size())
  {
      const std::string& data = text[pooh->counter];

      memcpy(ptr, data.data(), data.length());
      pooh->counter++; /* advance pointer */
      return data.length();
  }
  return 0; /* no more data left to deliver */
}

But read_text doesn’t work because textfilename hasn’t been passed to the function.

Answer 1

Read carefully again curl_easy_setopt‘s documentation. You might set the CURLOPT_READDATA to your textfilename and cast the userp to get it inside your read_callback

edit

Your userp is actually used; so you should add the textfilename in a new field of your WriteThis struct. Most callback functions take only one user data, but it should be enough (since you can pack arbitrary stuff in it).