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Comment by humbledrone | Hacker News Reader

The thing that shocks me about this bug is that it should have been caught when the compiler issued a warning about an unused function parameter. Either the warning was not enabled, or the programmer just ignored it; either of those options indicates a pretty severe lack of professionalism.

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"Pretty severe" lack of professionalism? It's not unprofessional not to turn the warnings all the way up and sift through each and every one. Some of us work code inherited from other people with different coding guidelines, and some of the warnings will generate loads of messages for code that is not only technically correct but stylistically correct as well. The unused parameter warning is one of the biggest offenders in this area. And if you have a hundred warnings for unused parameters, which ones do you pay attention to?

Another potential issue is that the code in the patch might almost never be called in the first place. Most libc implementations (three that I know of, where I've examined the "memset" source) have a "memset" like that as a fallback only for platforms without an assembly version.

I'd say the problem is that there are several different ways this error could have been detected sooner: warnings, test, static analysis tools, and review, but none of them worked. But again, it's also possible that the code is never called because assembly versions are always available.

| The unused parameter warning is one of the biggest offenders in this area. And if you have a hundred warnings for unused parameters, which ones do you pay attention to?

Well, so then you're exposed to this error. Do:

    void foo(int x)
    {
       (void)x;
    }

to silence unused parameter warnings.

Go look at Google code search, which includes vast quantities of code written by professional teams and (often later) open sourced. Very little of it casts expressions to (void). When I started at Arbor, I got rid of all the (void) casts. What an eyesore.

Very little C code is const-correct either. Most of the C devs I know make fun of const-correctness.

| What an eyesore.

It's only an eyesore if you don't acknowledge the benefits that strict warnings can bring to the table.

| Most of the C devs I know make fun of const-correctness.

I for one don't mind putting in a tiny bit of upfront effort so that my compiler can double-check my work. Do the C devs you know also make fun of assert?

I don't acknowledge the benefits of casting things to (void). Therefore: definitely an eyesore.

And, no. Nobody makes fun of assert. Which rather makes my point for me.

If assertions disappear in certain optimized builds then what do you do, or recommend, to deal with warnings related to unused parameters or values if you don't believe that casting to void has value?

I don't understand how the question you ask even follows from the premise you forwarded. The fact that memset is probably always dead code here is why it's not really a bug. The fact that warnings about unused return values are useless 100-1000 times for every time they're potentially useful is why I don't litter my code with pointless casts.

As I pointed out above, in gcc you can explicitly mark variables as potentially unused.

And what do you suggest for MSVC, which doesn't support attribute unused or have any comparable declspecs?

If what you say is true, then you have to either put up with the warnings, compile at a level that doesn't have the warnings, or figure out a hack around them - I don't like any of those options.

I'm not as dogmatic as some others are in the thread. I just think it's a good idea to be warned when a variable is unused, enough that I'll mark my very few that might be #ifdef'ed out. If you're using a compiler that doesn't let you mark variables as such... well that sucks. You do what you can.

Your compiler may offer a more formal way to indicate that it's acceptable for a parameter to be unused. In gcc it's an attribute:

int foobar(int foo, int bar __attribute__((unused))) { /* Code that does not use bar */}

If you're using gcc, you can say __attribute__((unused)): http://gcc.gnu.org/onlinedocs/gcc-3.3.1/gcc/Variable-Attribu...

(void)x does not work on all compilers to silence the warning. gcc + cl != C.

So what if the code doesn't get called? That doesn't prevent it from being built. Considering that memset() is part of libc, which is a shared library, presumably it's going to get compiled regardless of whether it's called.

Current codebase: 9734 warnings.

As a point of pride, none of them were created by me, but it is what it is. Some of them come from since deprecated functions and nobody wants to go back and mess with old code; some of them come from sloppiness; some of them come from java's collection handling.

And yet this codebase gets a lot of work accomplished anyway.

The "unused parameter" error is one of the most useless of C compiler errors. There are dozens of perfectly valid reasons why a parameter would go unused, so the occasional error just gets lost in the noise.

It's more helpful in C++, which can omit parameter names to indicate that it's intentionally unused. But then you're using C++; not worth the tradeoff.

It's not hard to type a statement like "(void)v;" to explicitly say, "v is intentionally unused." I fail to see the drawback of this approach. Of course you should only do this when (1) you can't remove the parameter from the function's prototype and (2) there is a good reason for it to be unused. But once you develop that habit, the compiler can help you catch bugs that are otherwise easy to miss. Sounds like a worthwhile tradeoff to me.

What you want is __attribute__((unused)), assuming you're compiling with gcc.

Then you get a "statement with no effect" warning, which is usually more serious. I'll take a bogus warning in real code over a bogus warning in contrived code any day.

I've never seen a compiler issue a warning about "(void)foo;". Even compilers which issue 'statement with no effect' warnings normally recognize deliberate casts to void as a way of saying "shut up, I know what I'm doing".

>There are dozens of perfectly valid reasons why a parameter would go unused, so the occasional error just gets lost in the noise.

Such as?

I can only think of a few, but they're valid:

1. An API-defined callback signature where you need to use (for example) the first and third parameters, and not the second.

2. A public API function that has become deprecated and the previous behavior is emulated in a way that doesn't require all the parameters.

3. Stub/wrapper functions.

4. Init/cleanup functions for modules written to a plugin API, when all parameters passed are not needed in all cases.

5. Language binding closure/thunk functions.

Most of the uses, I think, boil down to backwards compat and fixed callback/plugin interfaces. In those cases I think an __attribute__((unused)) or void cast aren't a huge burden. I usually turn on -Wunused-parameter just in case.

That sounds like a severe lack of perspective.

Huh? Any halfway decent software shop will have a policy of compiling with all warnings enabled, and fixing all sources of warnings (with only very rare exceptions). It is extremely unprofessional to disable or ignore warnings such as the "unused function parameter" warning. If you disagree, I respectfully submit that I would never let you near a C project that I was working on.

Any halfway decent software shop will have a policy of compiling with all warnings enabled

FWIW, your definition of "halfway decent" would exclude most software shops in the world.

It is extremely unprofessional to disable or ignore warnings such as the "unused function parameter" warning

Unused function parameter warnings are often noise (e.g., due to #ifdefs, unimplemented APIs, backward compatible APIs, etc). I agree that code should compile w/o warnings, but the benefits of tracking down and squelching unused function argument warnings are pretty marginal in my experience.

All I can say is than in 15 years (fuck.) I've never worked on a team that would have caught a bug like this in dead code. -Wall has been the gold standard on the teams I've worked on.

-Wall unfortunately leaves a lot of warnings disabled. We've used:

"-W -Wall -Wcast-align -Wstrict-prototypes -Wmissing -prototypes -Wpointer-arith -Wshadow -Wsign-compare -Wformat=2 -Wno-format-y2k -Wimplicit -Wmissing-braces -Wnested-externs -Wparentheses -Wtrigraphs"

Update: Forgot the most important one ... -Werror. Making warnings equivalent to errors is the only way to ensure that they're always taken care of.

-Wall doesn't enable -Wunused-parameter. You need -Wall -Wextra for that.

I would probably agree that most software shops in the world are not halfway decent.

The benefits of tracking down and squelching such warnings would have very likely caused this bug to never exist. The point is that you make "compiles without warnings" a policy, and you build a habit of maintaining that policy -- after a while it begins to take almost no effort at all.

Do you write C code?

(and for more than one platform?)

The unused function parameter warning is usually one of the most annoying and least useful. I routinely compile with "-Wall -Wno-unused" because I don't want to litter my code with casts to void.

What's with all the unused function parameters? Shouldn't that be an edge case?

Just about every callback interface takes a function pointer where the function expects a void * argument. Half the time it's not needed.

If the void * is the last parameter, just leave it off your implementation of the callback function.

You have to know that -Wextra exists, which I for one didn't until just now when I ran a sample program. All my projects just compiled with -Wall, which doesn't catch that error.

Also, unless every programmer on the project is being scrupulous about finding and correcting such errors, it quickly becomes line noise, and a lot of it completely unimportant stuff.

-Wextra is total insanity. I just compiled some C code I've been working on with that flag, and got the following: "warning: signed and unsigned type in conditional expression." I was testing if a variable I declared as an integer was less than 0. To my knowledge, every number literal in C is assumed to be of a signed type unless followed by a 'u'. But now I know better. 0 is the exception.

Are you sure?

    $ echo 'int main(int argc, char **argv) { if (argc < 0) { return 1; } else {return 0;}}' > test.c
    $ gcc -v
    Using built-in specs.
    Target: x86_64-linux-gnu
    Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.3-4ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i486 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
    Thread model: posix
    gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) 
    $ gcc -Wall -Wextra test.c
    test.c: In function ‘main’:
    test.c:1: warning: unused parameter ‘argv’
    $

gcc-4.3.4 behaved the same way in my test.

Yeah, that's why every programmer on the project has to be scrupulous about finding and correcting such errors. That's professionalism 101 -- if your team is not keeping the build warning free, things like this slip through.

Things like what, a blatantly obvious bug that would crash virtually every C program running on the system were it not for the fact that it was dead code?

This is practically a documentation bug.

[edit]

I kind of take it back, after mahmud's bzero() joke... out of oniguruma, Python, hexfiend, Apache, nginx, openssl, redis, regexkit, saxon, memcached, subversion, and valgrind, this hypothetical bug would have broken Python, redis, subversion, several apache modules, and memcached. The contents of my codebase/3p directory, FWIW.

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