A Wstringop-overflow Warning in GCC

[nkh0472]

There is a warning when compiling fdkaac 1.0.2 using the latest MinGW with MYSYS2:

The compile script is basically as follows:

``` cd fdkaac-1.0.2 autoreconf -i #warning happens in next command CC="gcc -pipe -static-libgcc" CXX="g++ -pipe -static-libgcc" ./configure --prefix=$MINGW_PREFIX/$MINGW_CHOST/ CFLAGS="-O3 -flto -march=native" make -j8 make install ```

gcc -pipe -static-libgcc  -O3 -flto -march=native   -o fdkaac.exe src/aacenc.o src/caf_reader.o src/extrapolater.o src/limiter.o src/lpc.o src/m4af.o src/main.o src/metadata.o src/parson.o src/pcm_float_converter.o src/pcm_native_converter.o src/pcm_readhelper.o src/pcm_sint16_converter.o src/progress.o src/wav_reader.o  src/compat_win32.o fdkaac.o  -liconv -lfdk-aac -lm -lfdk-aac
In function 'apple_chan_chunk',
    inlined from 'caf_parse' at src/caf_reader.c:190:17,
    inlined from 'caf_open' at src/caf_reader.c:230:9,
    inlined from 'open_input' at src/main.c:754:27,
    inlined from 'main' at src/main.c:802:19:
src/pcm_readhelper.c:231:24: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  231 |             mapping[i] = i;
      |                        ^
src/main.c: In function 'main':
src/caf_reader.c:29:13: note: at offset 0 to object 'chanmap' with size 8 declared here
   29 |     uint8_t chanmap[8];
      |             ^

(full logs in https://github.com/kekyo/fdk-aac-win32-builder/issues/9#issuecomment-852979506)

Code related to the problem

https://github.com/nu774/fdkaac/blob/347e995cfc44941f6d6dfe8d4e09362a71ea8d26/src/pcm_readhelper.c#L207-L208 https://github.com/nu774/fdkaac/blob/347e995cfc44941f6d6dfe8d4e09362a71ea8d26/src/pcm_readhelper.c#L230-L231

---

According to a similar problem in another project (https://github.com/tarantool/tarantool/issues/5564), this warning is due to a zero-lenght array, and happened when using GCC version >= 10.

I don't know how to fix this, but this info might be helpful https://github.com/tarantool/tarantool/commit/8915ae203f873198caf76fbad4bf9edce76a6153.

Zero-lenght arrays are GNU C extension. There's ISO C99 flexible array member, which is preffered mechanism to declare variable-length types.

Flexible array member allows us to avoid applying sizeof operator cause it's incomplete type, so it will be an error at compile time. There're any moments else why it's better way to implement such structures via FAM: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

In this issue it fixed gcc 10 warning: "warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]"

---

Version info of GCC

``` $ gcc -v Using built-in specs. COLLECT_GCC=E:\Etc\HUGE\msys64\mingw64\bin\gcc.exe COLLECT_LTO_WRAPPER=E:/Etc/HUGE/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/lto-wrapper.exe Target: x86_64-w64-mingw32 Configured with: ../gcc-10.3.0/configure --prefix=/mingw64 --with-local-prefix=/mingw64/local --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --with-native-system-header-dir=/mingw64/x86_64-w64-mingw32/include --libexecdir=/mingw64/lib --enable-bootstrap --enable-checking=release --with-arch=x86-64 --with-tune=generic --enable-languages=c,lto,c++,fortran,ada,objc,obj-c++,jit --enable-shared --enable-static --enable-libatomic --enable-threads=posix --enable-graphite --enable-fully-dynamic-string --enable-libstdcxx-filesystem-ts=yes --enable-libstdcxx-time=yes --disable-libstdcxx-pch --disable-libstdcxx-debug --enable-lto --enable-libgomp --disable-multilib --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-libiconv --with-system-zlib --with-gmp=/mingw64 --with-mpfr=/mingw64 --with-mpc=/mingw64 --with-isl=/mingw64 --with-pkgversion='Rev2, Built by MSYS2 project' --with-bugurl=https://github.com/msys2/MINGW-packages/issues --with-gnu-as --with-gnu-ld --with-boot-ldflags='-pipe -Wl,--dynamicbase,--high-entropy-va,--nxcompat,--default-image-base-high -Wl,--disable-dynamicbase -static-libstdc++ -static-libgcc' 'LDFLAGS_FOR_TARGET=-pipe -Wl,--dynamicbase,--high-entropy-va,--nxcompat,--default-image-base-high' --enable-linker-plugin-flags='LDFLAGS=-static-libstdc++\ -static-libgcc\ -pipe\ -Wl,--dynamicbase,--high-entropy-va,--nxcompat,--default-image-base-high\ -Wl,--stack,12582912' Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 10.3.0 (Rev2, Built by MSYS2 project) ```

[nkh0472]

Add -Wstringop-overflow=0 to CFLAGS could hide the warning.

[nu774]

It seems that you somehow specify custom CFLAGS. As far as I can see, -flto switch triggers it. Try without it. Since you have added -flto, it's your problem. Not mine.

[nkh0472]

You are right. Will close is issue soon.