Question

I researched the Linux kernel code (2.6.11) about the creation of a process/thread, and followed do_fork()->alloc_pidmap()

It seems that alloc_pidmap always returns pid > 300 once the previous pid’s ever reached the max pid, while actually daemon’s pid is always < 300 (Is this correct?).

Does a daemon obtain its pid using a function other than alloc_pidmap()? If so, does it imply the daemon process is not created using do_fork?

Answer 1

AFAIK pid are allocated by the kernel; the limit of 300 (i.e. #define RESERVED_PIDS 300 private inside kernel/pid.c) you are seeing is perhaps because on most systems, several processes have been forked early in the bootstrap (e.g. from initrd perhaps).

You could test by booting from GRUB directly into a kernel with init=/bin/sh

Some processes are kernel processes (without userland code, e.g. kworker or kauditd), which are not started by fork from init or descendants. They are probably started with kthread_create inside the kernel (and without any syscall).

And you should explain why are you asking that. Is your question about determining if a process is a deamon or not?