Difference between revisions of "Code Hygiene"

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Revision as of 23:44, 13 March 2016

In the past, submitting code to wine had a reputation for being difficult. We're trying several changes to make it easier for people to contribute code, but one thing we don't want to let go of are high standards for patches. Although we can't list all of the issues that might come up, here are a few that drew a lot of attention in the past.

Tango-style info icon.svg Many of these could theoretically be checked automatically, and there are already basic scripts floating around for some of them. It hasn't been discussed yet, but having our testbot check for these down the road could be a worthwhile project.

Set Read-Only Data to const

This task actually isn't too difficult. After compiling whatever files you're working on, you can use objdump -x on the resulting object files, then cross-reference the .data and .rodata segments from the dump. Any variables in .data that shouldn't be written to can be set as const in your source, which should place them in .rodata once you rebuild the files. Of course, be sure to fix any new compile warnings your changes generate too.

Dialog-warning.svg After you've made your changes, rebuild and actually run your modified version of wine to test. It's important you do some real-world testing because a variable incorrectly set as const can often cause segfaults.

Listen to Your Functions

Although not every function returns a value that is critical to your function, the return values of some functions should always be used. For example, it is dangerous to not check the return value of a write() or system() call, and it's practically useless to throw away the one from strtoul().

Fortunately, since passing functions and parameters is one thing compilers are built to understand, you can automatically check during the build process for functions you should be listening to more carefully. All you need to do is pass a few warning flags to your compiler when you build:

CFLAGS="-O1 -Wp,-D_FORTIFY_SOURCE=1"    (minimum)
CFLAGS="-O2 -Wp,-D_FORTIFY_SOURCE=2"    (preferred)

This should give a warning for every important function call whose return value is ignored. And don't just nod your head as you pass the return value to a dummy variable; you really have to listen and process the return value with the proper tests.

Avoid Superfluous Casts

Although clarity is a boon most of the time, it can actually be counter-productive to explicitly cast variables from one type to another when the compiler (and developers) implicitly understand it. Not only does it clutter up your lines of code, but more importantly, it can suppress compiler warnings that will otherwise warn you about casts you probably shouldn't be making.

The most flagrant versions of this revolve around... nothing, as in void functions, NULL values, etc. Here's a quick table of what these look like and how to fix them...

Description Solution Before After
Cast void * return value to another pointer type Let the compiler cast it implicitly (WCHAR *)HeapAlloc(GetProcessHeap(), 0, size) HeapAlloc(GetProcessHeap(), 0, size)
Cast NULL to a pointer or handle type Let the compiler cast it implicitly (HWND)NULL NULL
Cast NULL to an integer value or handle Just use zero instead (LRESULT)(HTREEITEM)NULL 0

Handling Duplicate Code

When working on a problem that's similar to one addressed elsewhere, "cut-and-paste" coding can be a kludgy but very effective way to prototype. The goal should always be to minimize duplication in your patches though, bundling common code into distinct modules whenever possible. That said, you can have situations where the current structure of the source or the build process would make this kind of refactoring a significant project.

In developing wine, we've come across a few situations like this, where refactoring still isn't expedient. So in the meanwhile, we try to at least keep the code in sync. Here is a list of current "cut-and-paste" relationships in the wine source tree:

Purpose File A File B Current Status
Demangle VC++ symbols into C function prototypes dlls/msvcrt/undname.c tools/winedump.msmangle.c Memory allocation needs to be rewritten to allow for a complete sync
Read dwarf2 info from ELF modules dlls/dbghelp/dwarf.c dlls/ntdll/signal_x86_64.c 100% synced since 27 Mar 2010
Use the minidump method for debug info tools/winedump/minidump.c programs/winedbg/tgt_minidump.c 100% synced since 29 Aug 2015

Remember that these duplications are seen as necessary evils, but if you sincerely believe something similar is the best solution for your problem at the moment, definitely discuss it with the other developers on the wine developers' mailing list. Also, if you do get your patch submitted, please add an entry for your duplication to the above table.

Debug Info in Critical Sections

If you ever need to implement or improve a CRITICAL_SECTION in wine, you should definitely set some debug info from the very get-go. This applies to both static CRITICAL_SECTION instances ...

static CRITICAL_SECTION foo_cs;
static CRITICAL_SECTION_DEBUG foo_cs_debug =
    0, 0, &foo_cs,
    { &foo_cs_debug.ProcessLocksList,
      &foo_cs_debug.ProcessLocksList },
    0, 0, { (DWORD_PTR)(__FILE__ ": foo_cs") }
static CRITICAL_SECTION foo_cs = { &foo_cs_debug, -1, 0, 0, 0, 0 };

... and dynamic ones (which happen to look much neater)

if (name.DebugInfo) name.DebugInfo->Spare[0] = (DWORD_PTR)(__FILE__ ": cs");

Of course, don't forget to do the right thing for your dynamic variables and unset DebugInfo before deleting the variable

if (name.DebugInfo) name.DebugInfo->Spare[0] = 0;

Tango-style info icon.svg Keep in mind that not all CRITICAL_SECTION objects use these exact constructors and destructors

16-bit Separation

Wine supports old 16-bit executables, but since many users might not be interested in this feature, we have a build option to disable it: --disable-win16. However, especially in its early versions, wine had a significant amount of overlap between its 16 and 32-bit functions.

This shouldn't be nearly as much of a problem now (it might be resolved entirely), but any changes should respect the separation so be sure that your submission:

  • Places code for 16-bit support in its own file in the appropriate 16-bit dll
  • Doesn't call 16-bit DLLs from a 32-bit one (with LoadLibrary16 or GetModuleHandle16)
  • Doesn't use a 16-bit include file for any 32-bit code
  • Doesn't implement a 16-bit function in a file for 32-bit code

Tango-style info icon.svg If you come across what you believe is a necessary exception, you can always discuss it on the wine developer's mailing list.

See Also