How to handle 0 Content length in POST request in CGI?

In my CGI application something happens when I pass a 0 content length POST request. Here is my code:

char* len_ = getenv("CONTENT_LENGTH");
char* type_ = getenv("REQUEST_METHOD");
if(len_ != NULL)
{
    // The code crashes somewhere here
    long int len = strtol(len_, NULL, 10);
    char* postdata = (char*)malloc(len + 1);
    if (!postdata) { exit(EXIT_FAILURE); }
    //fgets(postdata, len + 1, stdin);
    string temp = "";
    fstream ff;
    string fileName = string(XML_DATA_DIRECTORY) + string("data.xml");
    ff.open(fileName.c_str(), ios::in | ios::out | ios::trunc);
    // ff.open(fileName.c_str());

    if(ff)
    {
        // Modified: To handle new line in the Xml request
        while(fgets(postdata, len + 1, stdin) != NULL)
        {
            temp.append(postdata);
        }
        ff << temp;
    }
    else
    {
        // Error on the ifstream
    }
    ff.close();
    //free(postdata);
}
else
{
    // No Data
}

I test my application using the Http-Requester plugin for FireFox, when I pass a POST request with no data, the application seems like it enters a loop and no response. If I pass a GET request, the code works fine because len_ becomes NULL and it exits the if statement. If I pass a POST request with data, it works fine and receives the data well and saves it.

The case I can’t figure out is POST with CONTENT_LENGTH = 0. How to figure this case ? I tried strlen(len_) but it did not work. Thanks for the help.

Answer

Check if getenv returns NULL, e.g.:

char* len_;
long int len;

len_ = getenv("CONTENT_LENGTH");
if (len_ && sscanf(len_, "%ld", &len) == 1) {
   if (len > 0) {
      ...
   }
}

Note that (as pointed out by @Deduplicator) is better to declare len as unsigned long or size_t because CONTENT_LENGTH (the number of bytes being sent by the client) is always positive.