I applied AFL to an old project to re-familiarize myself with the tool. I found that the project would not compile with AFL's afl-gcc-fast, but it would compile with AFL's afl-clang-fast.

Versions used:

american-fuzzy-lop-4.35c-5.20260206gitafbcb07.fc44.x86_64
gcc-16.0.1-0.10.fc44.x86_64
clang-22.1.1-2.fc44.x86_64

Reproducible: Always

Steps to Reproduce:
I downloaded an old version of flac (https://ftp.osuosl.org/pub/xiph/releases/flac/flac-1.0-src.tar.gz), and I configured it with:

CC="gcc -std=c89" ./configure --disable-shared

Next, I tried to compile it with:

make CC="/bin/afl-gcc-fast -std=c89"
Actual Results:
The build failed with the following error:

[...]
make[3]: Entering directory 'flac-1.0/src/libFLAC'
/bin/sh ../../libtool --mode=compile /bin/afl-gcc-fast -std=c89 -DPACKAGE=\"flac\" -DVERSION=\"1.0\" -DSIZEOF_SHORT=2 -DSIZEOF_INT=4 -DSIZEOF_LONG=8 -DSIZEOF_LONG_LONG=8  -I. -I.      -I../.. -I./include -I ../../include -Wall -W -O3 -DNDEBUG -fomit-frame-pointer -funroll-loops -finline-functions -Winline -DFLaC__INLINE=__inline__ -g -O2 -c bitbuffer.c
../../libtool: line 722: test: =: unary operator expected
/bin/afl-gcc-fast -std=c89 -DPACKAGE=\"flac\" -DVERSION=\"1.0\" -DSIZEOF_SHORT=2 -DSIZEOF_INT=4 -DSIZEOF_LONG=8 -DSIZEOF_LONG_LONG=8 -I. -I. -I../.. -I./include -I ../../include -Wall -W -O3 -DNDEBUG -fomit-frame-pointer -funroll-loops -finline-functions -Winline -DFLaC__INLINE=__inline__ -g -O2 -c bitbuffer.c -o bitbuffer.o
afl-cc++4.36a by Michal Zalewski, Laszlo Szekeres, Marc Heuse - mode: GCC_PLUGIN-DEFAULT

[-] PROGRAM ABORT : GCC and plugin have incompatible versions, expected GCC 16.0.1, is 16.0.1
         Location : plugin_init(), instrumentation/afl-gcc-pass.so.cc:482


Additional Information:
Replacing "gcc" with "clang" allowed the compile to complete. So, there seems to be a problem with the gcc plugin, but the clang plugin works.