The linker is not happy:

$ ninja -C out/coverage/ -j20 zlib_uncompress_fuzzer
ninja: Entering directory `out/coverage/'
[1/1] LINK ./zlib_uncompress_fuzzer
FAILED: zlib_uncompress_fuzzer zlib_uncompress_fuzzer.dSYM zlib_uncompress_fuzzer.dSYM/Contents/Info.plist zlib_uncompress_fuzzer.dSYM/Contents/Resources/DWARF/zlib_uncompress_fuzzer 
export DEVELOPER_DIR=/Users/mmoroz/projects/chromium/src/build/mac_files/Xcode.app;  TOOL_VERSION=1504131674 ../../build/toolchain/mac/linker_driver.py ../../third_party/llvm-build/Release+Asserts/bin/clang++  -Wcrl,dsym,.  -stdlib=libc++ -arch x86_64 -segprot PROTECTED_MEMORY rw r -Werror -Wl,-dead_strip -isysroot ../../build/mac_files/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -mmacosx-version-min=10.9.0 -fprofile-instr-generate -fsanitize-coverage=trace-pc-guard -Wl,-ObjC -Wl,-U,_LLVMFuzzerCustomMutator -Wl,-U,_LLVMFuzzerInitialize -Wl,-U,_sanitizer_options_link_helper -fsanitize-coverage=trace-pc-guard -o "./zlib_uncompress_fuzzer" -Wl,-filelist,"./zlib_uncompress_fuzzer.rsp"  
ld: file not found: /Users/mmoroz/projects/chromium/src/third_party/llvm-build/Release+Asserts/lib/clang/6.0.0/lib/darwin/libclang_rt.ubsan_osx_dynamic.dylib
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Traceback (most recent call last):
  File "../../build/toolchain/mac/linker_driver.py", line 229, in <module>
    Main(sys.argv)
  File "../../build/toolchain/mac/linker_driver.py", line 79, in Main
    subprocess.check_call(compiler_driver_args)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['../../third_party/llvm-build/Release+Asserts/bin/clang++', '-stdlib=libc++', '-arch', 'x86_64', '-segprot', 'PROTECTED_MEMORY', 'rw', 'r', '-Werror', '-Wl,-dead_strip', '-isysroot', '../../build/mac_files/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk', '-mmacosx-version-min=10.9.0', '-fprofile-instr-generate', '-fsanitize-coverage=trace-pc-guard', '-Wl,-ObjC', '-Wl,-U,_LLVMFuzzerCustomMutator', '-Wl,-U,_LLVMFuzzerInitialize', '-Wl,-U,_sanitizer_options_link_helper', '-fsanitize-coverage=trace-pc-guard', '-o', './zlib_uncompress_fuzzer', '-Wl,-filelist,./zlib_uncompress_fuzzer.rsp']' returned non-zero exit status 1
ninja: build stopped: subcommand failed.



I think the reason is that we use "-fsanitize-coverage=trace-pc-guard" (we need that for fuzz targets), but don't use any other sanitizer. In that case, clang wants to use ubsan runtime, which is not included in Mac toolchain.

Sounds like one more reason to finally switch to -fsanitize=fuzzer.