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By default, the maximum PID number is 32,767 (PID_MAX_DEFAULT - 1); the system administrator may reduce this limit by writing a smaller value into the /proc /sys/kernel/pid_max file (/proc is the mount point of a special filesystem, see the section "Special Filesystems" in Chapter 12). In 64-bit architectures, the system administrator can enlarge the maximum PID number up to 4,194,303.


Cover of Understanding the Linux Kernel, 3rd Edition


Linux stores pid_max with short int and short int varies in different architectures.
x32 x64
short int 16-bit 32-bit