#ifdef considered harmful (1992) [pdf]

https://usenix.org/legacy/publications/library/proceedings/sa92/spencer.pdf

by Annatar • 8 years ago

39 46 8 years ago

beefhash• 8 years ago

It bears noting that at some point, you will likely need to #ifdef. There's no way around it for non-trivial code. Simple example: Getting the size of a file if said file is greater than the maximum size of long, meaning you can't fseek to the end and use ftell(); you'll need platform-specific APIs like stat(2) or GetFileSizeEx().

You can also do the platform checks in the build system instead and have multiple per-platform C files implement APIs for portability shims -- but given that the platform check is likely unportable (at least in make as specified by POSIX), you're just making different portability assumptions.

dvfjsdhgfv• 8 years ago

You can also do the platform checks in the build system instead and have multiple per-platform C files implement APIs for portability shims -- but given that the platform check is likely unportable (at least in make as specified by POSIX), you're just making different portability assumptions.

And yet, it seems that's what the authors of the article did:

The best method of managing system-specific variants is to follow those same basic principles: define a portable interface to suitably-chosen primitives, and then implement different variants of the primitives for different systems. The well-defined interface is the important part: the bulk of the software, including most of the complexity, can be written as a single version using that interface, and can be read and understood in portable terms.

avar• 8 years ago

I may be wrong, but I was under the impression that this was portable on all POSIX systems:

    uname_S := $(shell sh -c 'uname -s 2>/dev/null || echo not')
    ifeq ($(uname_S),Linux)
        HAVE_LINUX_SPECIFIC_STUFF = Yes
    endif

This is how git does its conditional compilation purely powered by Makefile logic: https://github.com/git/git/blob/master/config.mak.uname

LukeShu• 8 years ago

That requires GNU Make; neither $(shell) nor ifeq are in POSIX Make.

jstanley2• 8 years ago

Makefile logic is basically just an ifdef one level up.

jwilk3• 8 years ago

$(shell sh -c 'uname -s 2>/dev/null || echo not')

Is there a reason they don't use simpler:

  $(shell uname -s 2>/dev/null || echo not)

?

comex• 8 years ago

Ack… please don’t do this. It will break if you’re cross-compiling binaries for a different OS than the OS you’re building on. If you want to do per-OS configuration in a Makefile or some type of configure step, that’s fine – but you should do it either based on a target triple (--host in autoconf-speak) which can be overridden by the user, or by running the given compiler on test programs and checking the output (like autoconf does, but ideally with one or two tests rather than a bazillion of them).

Annatar3• 8 years ago

No; this is GNUmake specific syntax.

The simplest and cleanest thing to do is to write a Makefile per uname(1).compiler, like so:

  Makefile.Linux.SunStudio
  Makefile.Linux.icc
  Makefile.Linux.PGI
  Makefile.Linux.GCC
  Makefile.AmigaOS.vbcc
  Makefile.AmigaOS.DiceC
  Makefile.AtariTOS.gcc
  Makefile.Spearmint.gcc
  Makefile.illumos.gcc
  Makefile.illumos.SunStudio

...and so on. The source code and generic targets like clean: would go into a central Makefile.core which would then be included by the OS.compiler specific Makefile. The OS specific Makefile would only set $CC, $CXX, $FC, $CFLAGS, $CXXFLAGS, $FFLAGS and so on.

This way, no configure system would be needed and adding support for a new $OS.compiler suite would be trivial.

The code would have to be so clean as to allow compilation by any compiler of course.

ape4• 8 years ago

Yes, nobody likes using #ifdef but if you need it, its great that its there

pjmlp3• 8 years ago

Hence, why the abstraction translation units should be named according to their abstraction level.

Meaning module_posix would be for all POSIX platforms, whereas module_aix would be Aix only.

Avamander3• 8 years ago

Let's not forget microcontrollers, the moment one wants to write something more universal (different MCU support) using ifdefs is basically unavoidable.

pjmlp3• 8 years ago

Not when using one translation unit per MCU and letting the Makefile pick the right one.

Back on my C days, I was able to write portable code between Windows NT, Windows 2000, Red-Hat Linux, Debian Linux, Aix, HP-UX, Solaris where the #ifdef were only used as include guards, regarding my own code.

yzmtf2008• 8 years ago

Similarly, if you’re writing an native extension targeting multiple versions of ruby, python, etc., ifdefs are unavoidable.

JdeBP4• 8 years ago

If someone wants to fix Wikipedia's article on Henry Spencer, a full citation just waiting for {{cite conference}} is at https://news.ycombinator.com/item?id=17610233 .

arkj• 8 years ago

Every construct can be harmful if one doesn’t understand its purpose.

fanf2• 8 years ago

I’ve been maintaining unifdef since about 2001. One of the rules I have set myself is to never use #if in unifdef. It’s an easy program to port, so this isn’t very hard :-) but Microsoft’s appalling C libraries make it more painful than it should be.

The main technique is to set the include path depending on the behaviour of the target platform, and have per-platform variant header files. For unifdef, this basically means standard vs windows.

The other thing I don’t have to deal with is build-time configuration options or feature switches. It’s easy to get into a combinatorial explosion with #if but kinda hard if you force yourself to reify each option as a pair of include files!

snarfy• 8 years ago

Isn't this just moving the goal posts? Now all of your #ifs and the complexity they bring are in your build system.

tedunangst• 8 years ago

Perhaps there are fewer conditionals in the build than there would be in the code.

JdeBP4• 8 years ago

It already was in the build system. The macros that control conditional compilation when one is doing it with the preprocessor do not appear out of thin air. A build system has to set them appropriately in the first place.

All of the mechanism for auto-detecting (or manually specifying) the capabilities of the target have to be there. It is simply a case of whether they set preprocessor macros via command-line options or tell the build system to use alternative source files.

The latter has the advantage of only creating build dependencies against the modules that actually use the library functionality in question, rather than against the compiler command line and potentially forcing a re-build of everything (when an honest dependencies mechanism is being used).

pandaman2• 8 years ago

The article does not seem to mention any uses of conditional compilation outside of portability. But consider the most common case: debug and release versions of the same program.

You want the extra checks and diagnostics in the debug version and fastest code in the release. Using assert() will only get you so far because you might need to compute values to assert on and if those require function calls then the calls will still be there when asserts are eliminated in the release. And if you want additional room in your data structures to store debug info then assert() is of no use.

eesmith• 8 years ago

When you say "the most common case", I assume you mean "the most common case now", and not in 1992 when this paper was written.

The abstract does start "We believe that a C programmer's impulse to use #ifdef in an attempt at portability is usually a mistake" so it's fair to assume that they were not trying to cover other uses of #ifdef.

That said, your points weren't completely ignored. Page 193 mentions #ifdef use in debug builds in the text starting "An optional feature such as debugging assistance ...".

The part about "additional room in your data structures" follows under the guideline "Such declarations, in turn, preferably should be confined to header (.h) files, to minimize the temptation to introduce #ifdef into main-line code".

Annatar3• 8 years ago

“Debug” and “Release” only pertains to Windows. You are always supposed to build with -g on UNIX and UNIX like operating systems because besides a few miliseconds of startup time, including the source code into the binary doesn’t affect anything but file size. The runtime linker skips those sections and links only code and data into memory, but the source code is there if you load it into the debugger. Kernel engineers taught me this.

pandaman2• 8 years ago

There are much more differences between debug and release than having debug info included in the executable so your reasoning makes little sense. Many people, for example, build games for PS4, which runs a FreeBSD Unix-like OS, unawares that their debug and release builds only pertain to Windows...

#ifdef considered harmful (1992) [pdf]

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