What is the maximum value of the Process ID?

Also, is it possible to change a Process ID?

up vote 53 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 same value as found on 32 bit systems, 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.

  • and we can be sure of kernel bits with this – Aquarius Power Jun 12 '14 at 22:40
  • 2
    A curious semi-counterexample to the "cannot change the PID" story is the case of a multithreaded program, where one of the auxiliary threads does an exec(). It overlays the whole process, but changes the PID (to the TID of the exec'ing thread). – fche Apr 12 '15 at 22:11
  • 3
    The Maximum value of a PID for 32-bit machine is 32767, not 32768. 0 is the scheduler and 1 is init, and then user processes get 2 through 32767. – user129258 Aug 19 '15 at 7:07
  • 2
    user129258 is right, and in agreement with both the manual page and the kernel doco. The maximum allowed value of a PID, as requested by the question, is one less than the value in that pseudo-file. – JdeBP Sep 20 '16 at 9:06
  • 1
    @user129258, this only holds up if you are using init. There is always the possibility to boot directly to a shell, let's say busybox and do thing manually from there without job control. Or even have some startup scrips and drop back to shell. In any case, the shell will be PID 1 in such a case. – Tim Jun 26 at 11:01

Other answers have explained

  • /proc/sys/kernel/pid_max for Linux and
  • 99999 for FreeBSD

But the question didn't specify an operating system. So here are some others:

  • On Solaris, the maximum value of a process ID is a kernel tunable parameter — pidmax in /etc/system — that defaults to 30,000 and that can be set anywhere between 266 and 999,999. Note that this is not max_nprocs, which is a kernel tunable parameter with a subtly different function.
  • On HP-UX 10 the kernel tunables named process_id_min and process_id_max prescribe the range of allowable process IDs.
  • On AIX, process IDs comprise several fields, including a "process slot" and a "generation count" field. The maximum possible value is 0x03FFFFFE, because the fields occupy only the bottom 26 bits of an integer, and bit #0 is always zero except for process #1.
  • On OpenBSD the maximum is 32766.
  • On NetBSD the maximum is 30000.

On FreeBSD the value of PID is between 0 and 99999 according to intro(2) (link). Here's a quote from the manual:

Process ID.
Each active process in the system is uniquely identified by a non-negative integer called a process ID. The range of this ID is from 0 to 99999.

If you want to read the source code on your own then PID_MAX is defined in sys/sys/proc.h (link).

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