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Those Compiler Warning Blues
It's always instructive to crank the warning level up on compilers. It also
helps to use different compilers since they tend to warn about different
things. With GCC (GNU Compiler Collection) [1], I use -Wall (which sadly,
isn't all possible warnings [2]) but today I learned that clang [3] (the
default compiler on Mac OS-X these days) has a -Weverything option, so hey,
why not try it?
Wow!
It's not kidding—it warns about everything! Missing prototypes, gratuitous
use of packed structures [4], added padding to structures, signed conversions
[5] (not only unsigned to signed, which I can see possibly losing
information, but signed to unsigned, which doesn't), loss of interger
precision, relying on auto-conversion of function calls (in my case,
assigning the result of a function that returns a double [6] to an unsigned
long long [7] variable), alignment changes in unions, even “default label in
switch which covers all enumeration values.”
It's a lot of output to pour through. And this is for code that passes
cleanly through GCC.
But in the ton of “legal, even if a bit questionable C” it still managed to
find a real bug in my code:
> In file included from common/XXXXlib.c:11:
> third_party/uuid/src/uuid.h:34:17: warning: 'SHORT_MAX' is not defined, evaluates to 0 [-Wundef]
> #if RAND_MAX == SHORT_MAX
> ^
>
It's a typo—it should be SHRT_MAX (apparently, there was a severe shortage of
vowels in 70s computing, which is why C got stuck with a bunch of vowel-
impaired identifiers—sheesh!) but at the same time, it's perfectly legal C,
which is why I never noticed this until now.
[1]
https://gcc.gnu.org/
[2]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31573
[3]
http://clang.llvm.org/
[4]
http://www.catb.org/esr/structure-packing/
[5]
http://en.wikipedia.org/wiki/Signedness
[6]
http://en.wikipedia.org/wiki/Double-precision_floating-point_format
[7]
http://en.wikipedia.org/wiki/C_data_types#Basic_types
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