The PID is returned to the parent process, enabling it to refer to the child in further function calls.
Some consider this to be a potential security vulnerability in that it allows information about the system to be extracted, or messages to be covertly passed between processes.
[7] On some systems, like MPE/iX, the lowest available PID is used, sometimes in an effort to minimize the number of process information kernel pages in memory.
The current process ID is provided by a getpid() system call,[8] or as a variable $$ in shell.
[13] On the Windows NT family of operating systems, process and thread identifiers are all multiples of 4, but it is not part of the specification.