when the target forks, this breaks all normal fuzzing runs. Sometimes, though, it is enough to just run the child process. If this is the case, then this LD_PRELOAD library will always return 0 on fork, the target will belive it is running as the child, post-fork.
This is defork.c from the amazing preeny project https://github.com/zardus/preeny
It is altered for AFL++ to work with its fork-server: the initial fork will go through, the second fork will be blocked.