From the fork() man page: “After a fork() in a multithreaded program, the child can safely call only async-signal-safe functions (see signal-safety(7)) until s

From the fork() man page: “After a fork() in a multithreaded program, the child can safely call only async-signal-safe functions (see signal-safety(7)) until such time as it calls execve(2)“ For example, you can't do any dynamic memory allocation or call a function that may allocate.