Bugs I Encountered (Week 1)

Four steps when you build and run your code:

1. Pre-processing
2. Compiling
3. Assembling
4. Linking Mostly bugs may happen in step 2 and 4.

Why Mac treat warning as ERROR when compiling ?

When I did Week 1 homework, there was a bug that only Mac users met.

Source code:

switch (type) {
    case kNoCompression:
      block_contents = raw;
      break;
    default:
      assert(false);
      break;
  }

Normally, this code may cause a warning:

warning: enumeration value 'kSnappyCompression' not handled in switch [-Wswitch]

However, when I tested on Mac, it caused an error:

error: enumeration value 'kSnappyCompression' not handled in switch [-Werror,-Wswitch]

Everything went right when I tested on Ubuntu. Therefore, I went to look up [Werror], and found the bug in CMakeLists.txt:

Original file:

if(HAVE_CLANG_THREAD_SAFETY)
  target_compile_options(leveldb
    PUBLIC
      -Werror -Wthread-safety)
endif(HAVE_CLANG_THREAD_SAFETY)

Therefore the bug couldn’t be clearer any more:

Mac uses clang to compile files, and here according to the CMakeLists, once you have clang to compile, the compiler will treat some warnings as ERROR !

Once the bug was found, CMakeLists needed to be modified:

if(HAVE_CLANG_THREAD_SAFETY)
  target_compile_options(leveldb
    PUBLIC
      -Wthread-safety)
endif(HAVE_CLANG_THREAD_SAFETY)
## Delete -Werror, which means closing Werror

We could see that everything went right then.

Mac has clang only, the so-called ’gcc’ or ’g++’ are actually a pointer to clang. When you install gcc through HomeBrew, you can use gcc command, which meets most programmers’ taste, but what is working is still clang!!!  That’s why every trial was a failure on Mac even though you’d installed gcc.

$ gcc --version
Apple clang version 13.1.6 (clang-1316.0.21.2.5)
Target: arm64-apple-darwin21.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin