I implemented a custom mutator, and it takes about 1~5 mins for one mutate cycle.

When I run wtf with the custom mutator, I get the following error:

Could not receive size (-1)
Receive failed

Is there any part of the code that sets the timeout or flag such as O_NONBLOCK?

Actually, I already checked O_NONBLOCK.

int is_block = !(fnctl(Fd, F_GETFL, 0) & O_NONBLOCK);
// Result: 1