advanced Visual Studio kung-fu test -- Calling functions from the Immediate Window during debugging

Question

I see some related questions have been asked, but they're either too advanced for me to grasp or lacking a step-by-step guide from start to finish (most of them end up being insider talk of their own experiment results). OK here it is, given this simple program:

#include <stdio.h>
#include <string.h>

int main()
{
    FILE * f;
    char buffer[100];

    memset(buffer, 0, 100);

    fun();

    f = fopen("main.cpp", "r");
    fread(buffer, 1, 99, f);
    printf(buffer);
    fclose(f);

    return 0;
}

What it does is basically print itself (assume file name is main.cpp).

Question

How can I have it print another file, say foobar.txt without modifying the source code? It has something to do with running it through VS's, stepping through the functions and hijacking the FILE pointer right before fread() is called. No need to worry about leaking resources by calling fclose().

I tried the simple f = fopen("foobar.txt", "r") which gave

CXX0017: Error: symbol "fopen" not found

Any ideas?

Edit

I found out the solution accidentally on Debugging Mozilla on Windows FAQ. The correct command to put into the Immediate Window is

f = {,,MSVCR100D}fopen("foo.txt", "r")

However, it doesn't really answer this question:

• I still don't understand what is going on here.
• How to systematically find out the {,,MSVCR100D} part for any given method? I know the MSVCR version changes from system to system. How can I find that out?
• Could anyone explain the curly brace syntax, especially, what are those two commas doing there? Are there more hidden gems using this syntax?

Answer 1

The curly-braces syntax is the VS Context Operator, which is mostly broken - as elaborated at the link (or at least, hardly a hidden gem). It is used anywhere the VS expression-evaluation-engine needs to resolve functions at runtime, e.g. evaluating compound expressions in the watch window, setting breakpoints, or (as you witnessed) in the immediate/command windows.

Its 3rd argument is the binary which includes the implementation of the function you wish to call. For a CRT function such as fopen, that usually amounts to -

MSVCR[VS version][D].dll

(I seem to recall the extension was needed - but maybe that changed in VS2010). The 'D' indicates the debug dll version. The VS version number is decoded as: VS2005 -> 8, VS2008 -> 9, VS2010 -> 10.

Also note the evaluation-engine has various other gotchas to watch out for.