Why does getenv("QUERY_STRING") return null in a FastCGI C++ program?

Question

So I have a working FastCGI application written in C++ using Light HTTPd, however I'm unable to retrieve the querystring using getenv("QUERY_STRING"). Everything works fine if I take out the querystring request (or add a check for null), but with that in place it fails:

#include <stdlib.h>
#ifdef _WIN32
#include <process.h>
#else
#include <unistd.h>
extern char ** environ;
#endif

#include "fcgio.h"
#include "fcgi_config.h"  // HAVE_IOSTREAM_WITHASSIGN_STREAMBUF
#include "redisclient.h"

....

 while (FCGX_Accept_r(&request) == 0)
 {        
    fcgi_streambuf cin_fcgi_streambuf(request.in);
    fcgi_streambuf cout_fcgi_streambuf(request.out);
    fcgi_streambuf cerr_fcgi_streambuf(request.err);

...

    cout << "Content-type: text/html\r\n"
                "\r\n"
                "<TITLE>^_^</TITLE>\n"
                "<H1>echo-cfpp</H1>\n"
                "<H4>PID: " << pid << "</H4>\n"
                "<H4>Request Number: " << ++count << "</H4>\n";

    // If I make this conditional on getenv("QUERY_STRING") not returning null,
    // then the program behaves reliably.
    cout <<getenv("QUERY_STRING");
 }

I've verified that I'm passing a querystring in the request, so why then is getenv("QUERY_STRING") returning null? And what should I be doing to retrieve it?

Answer 1

I don't have extensive experience with the reference FastCGI library for C/C++, but I've implemented both CGI and FastCGI libraries for Windows in the past, so the following might help.

Basically, as per the FastCGI specification, the CGI environment variables are passed through the FCGI_PARAMS stream, which are normally decoded by the FastCGI library. Now, FastCGI doesn't specify much about what is required and what isn't, and it's assumed that the rules are basically the same as for CGI. The CGI specification section 4.1.7 says the following about the QUERY_STRING environment variable:

The server MUST set this variable; if the Script-URI does not include a query component, the QUERY_STRING MUST be defined as an empty string ("").

Now, this basically means that your FastCGI library is decoding a QUERY_STRING parameter in the FCGI_PARAMS stream (or else the gateway server is not following the specification).

Since the reference library tries to abstract both CGI and FastCGI library in the same program and supports multi-threading, I strongly doubt that you would find the result in an environment variable (or there would be a race condition).

Basically, this means getenv() always returns NULL and you are passing a null const char* to an std::ostream through operator<< is illegal. This probably crashes your application since NULL is not a special value designating the end of stream.

---

TL; DR: You cannot access the QUERY_STRING value through the process environment because you are using FastCGI, not CGI. You need to read the library's documentation for the standard way to access the request's query string.

---

Edit: I've got some more info on this situation.

The documentation for FCGX_Accept_r() in fcgiapp.h says`:

Creates a parameters data structure to be accessed via getenv(3) (if assigned to environ) or by FCGX_GetParam and assigns it to *envp.

Using the following after FCGX_Accept_r would fix the issue:

environ = request.envp;

However, this is not safe for multi-threaded applications (environ is not in thread-local storage), so I recommend that you use the other documented method instead:

const char * query_string = FCGX_GetParam("QUERY_STRING", request.envp);