Google Analytics

Monday, January 16, 2006

API or Application Programming Inconsistencies

Got the mail out anyway now. Back to coding - or actually not so much coding as struggling with other issues. Latest is the issue to get Visual Studio to like WTL - the Windows Template Library that AxCrypt2Go will use for it's GUI. I haven't really succeeded, but I guess I'll just live with it, but it's a bit frustrating to implement a program with a library that does not support Intellisense - and lacks documentation to boot. But it is free... Here's an example of how fun it can be to program the Win32 API - it's an oldie and still goodie, but it serves as a nice example of why it's important to think about things before coding...

The Win32 API call GetModuleFileName is declared as:

DWORD GetModuleFileName(HMODULE hModule, LPTSTR lpFilename, DWORD nSize)

It gets a file name of a running executable, and places it in the buffer pointed to by lpFilename, restricted in number of chars by nSize. So far so good. But how do you figure out how large a buffer you need? Well... The function will return the number of chars copied. If there is not enough room, the result is truncated and nSize is returned. There is no way for the caller to determine if the result was truncated or just exactly fit! To make matters worse - it is definitely not bounded by MAX_PATH as some functions are, it may return an arbitrarily long path, when prefixed with \\?\, if that format was used when the executable was loaded.
All this leads us to ridiculous code like the following (truncated for brevity):

DWORD dwMaxLen = 0, dwLen;
std::auto_ptr<_TCHAR> szModuleFileName;
do {
    szModuleFileName = std::auto_ptr<_TCHAR>(new _TCHAR[dwMaxLen += MAX_PATH]);
    dwLen = ::GetModuleFileName(hModule, szModuleFileName.get(), dwMaxLen);
} while
(dwLen == dwMaxLen);
There are many variants on the above, but the point is we must use a trial-and-error loop! A much nicer version is a better API like, for example, GetTempPath. This function will instead return the number of characters required to hold the result, thus easily enabling the caller to determine if the provided buffer was large enough or the result was truncated. Even better, it allows the following pattern:

DWORD dwLen = GetTempPath(0, NULL);
std::auto_ptr<_TCHAR> szTempPath(new _TCHAR[dwLen]);
dwLen = GetTempPath(dwLen, szTempPath.get());

No loop, just the right amount of data etc. It makes a difference! It should be noted that as far as I can determine, both these APIs are the same age... So - the morale of the story - always think about how the caller can use your API, and never make assumptions about the size of data.

Ok, back to actually writing the program I'm supposed to be coding...

Back from 2 hours of coding - net result: 10 lines of code. Lean and mean, and using the basic API has it's price. All I'm trying to do is to enable my app to display icons from the system icon list plus one single private icon (the ugly one that's used to indicate AxCrypt-encrypted files). This turns out to be a major headache, since the list view control needs a single image list and an index into that to display an icon, and we the contents of the system icon list is dynamic, so we can't just copy that and be done with it. The strategy now is exceedingly complex, may it's time to re-evaluate and check if list views can be coerced into displaying icons some other way. Readers: any ideas?

No comments:

Post a Comment