The system identifies processes by a unique process identification value or PID. The PID is a numerical value represented by the opaque type4
pid_t, which is typically an
int. Because of backward compatibility with earlier Unix and Linux versions, however, the default maximum value is only 32,768 (that of a
short int), although the value optionally can be increased as high as four million (this is controlled in
<linux/threads.h>. The kernel stores this value as
pid inside each process descriptor.
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