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What is the maximum value of the PID of a process?

Also, is it possible to change the PID of a running process?

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1 Answer 1

up vote 22 down vote accepted

On Linux, you can find the maximum PID value for your system with this:

$ cat /proc/sys/kernel/pid_max

This value can also be written using the same file, however the value can only be extended up to a theoretical maximum of 32768 for 32 bit systems or 4194304 for 64 bit:

$ echo 32768 > /proc/sys/kernel/pid_max

It seems to be normative practice on most 64 bit systems to set this value to the maximum 32 bit value, but this is by convention rather than a requirement.

From man 5 proc:

/proc/sys/kernel/pid_max  
  This file (new in Linux 2.5) specifies the value at which PIDs wrap around
  (i.e., the value in this file is one greater than the maximum PID). The
  default value for this file, 32768, results in the same range of PIDs as
  on earlier kernels. On 32-bit platfroms, 32768 is the maximum value for
  pid_max. On 64-bit systems, pid_max can be set to any value up to 2^22
  (PID_MAX_LIMIT, approximately 4 million).

And no, you cannot change the PID of a running process. It gets assigned as a sequential number by the kernel at the time the process starts and that is it's identifier from that time on. The only thing you could do to get a new one is have your code fork a new process and terminate the old one.

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and we can be sure of kernel bits with this –  Aquarius Power Jun 12 at 22:40

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