Wednesday, January 28, 2015
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, I use -Wall (which
sadly, isn't all
possible warnings) but today I learned that clang (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, added padding to structures, signed conversions (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 to an unsigned long long 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.
