Why would fopen fail to open a file that exists?

Question

I'm on Windows XP using Visual Studio 6 (yes I know it's old) building/maintaining a C++ DLL. I'm encountered a problem with fopen failing to open an existing file, it always returns NULL.

I've tried:

• Checking errno and _doserrno by setting both to zero and then checking them again, both remain zero, and thus GetLastError() reports no errors. I know fopen isn't required to set errno when it encounters an error according to a C standard.
• Hardcoding the file path, which are not relative.
• Tried on another developers machine which the same result.

The really strange thing is CreateFile works and the file can be read with ReadFile. We believe this works in a release build, however we are also seeing some very odd behaviour in other areas of the application and we're not sure if this is related.

The code is below, I don't see anything odd it looks quite standard to me. The source file hasn't changed for just under half a year.

HRESULT CDataHandler::LoadFile( CStdString szFilePath )
{
    //Code
    FILE* pFile;
    if ( NULL == ( pFile = fopen( szFilePath.c_str(), "rb") ) )
    {
        return S_FALSE;
    }
    //More code
}

Answer 1

The Answer:

I found the cause, too many open file handles cause by some recent updates to the application. These where not code changes though so this bug has been present for a while. I stepped into the fopen function down to a function called _getstream. This attempts to find a stream not in use, the function searches a table of 512 streams Sure enough all 512 where in use and other calls to fopen where failing. I used the handle tool from sysinternals to see the number of used handles.