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Full Discussion: Pid file and process check
Top Forums UNIX for Beginners Questions & Answers Pid file and process check Post 303036935 by Don Cragun on Wednesday 17th of July 2019 10:41:44 PM
Old 07-17-2019
I don't know of any OS that randomly picks the next PID to use when creating a new process. But, it is theoretically possible to do it that way.

Most of the systems I have worked on have a maximum PID value that is a configuration parameter. They start out assigning PIDs from 1 and increment the PID they want to assign by one every time a new process is created (e.g., by fork()), but before assigning that value, they check to see if that PID is currently in use. If it is in use, the OS tries again with the next possible PID until it finds one that is not in use. Using this method, you won't run into problems with PID recycling unless there are so many processes running on your system that you don't have many possible PIDs available that aren't in use.

Furthermore, if you have a ps utility that conforms to POSIX requirements, you can ask ps to just print the fields you want to see and not have to worry about some unexpected value in a field you aren't interested in containing what looks like a PID. (Look at your man page for the ps -o format option.)
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LEARN ABOUT HPUX

process_id_min

process_id_min(5)						File Formats Manual						 process_id_min(5)

NAME

       process_id_min - specify a minimum value for process IDs (PIDs)

VALUES

   Failsafe
   Default
   Minimum
   Maximum
       must be less than or equal to If the difference between and inclusive is less than is effectively limited to this difference.

DESCRIPTION

       The  tunable  specifies the minimum process ID (PID) value to generate for new processes.  It allows application developers to duplicate an
       environment where PID values are large for the purpose of software validation.

       This tunable is primarily intended for development environments.

   Who is Expected to Change This Tunable?
       Anyone interested in software program validation in an environment where PIDs have large values.

   Restrictions on Changing
       Changes to the tunable are effective immediately.  However, any existing processes and their PIDs are not changed.

       It is recommended that the system be rebooted after specifying this tunable so that all PIDs in the system will have values in the selected
       range.

   When Should the Value of This Tunable Be Raised?
       Raise this tunable to validate software for tolerance of large PID values in a development environment.

   What Are the Side Effects of Raising the Value?
       If the difference between and is less than the number of processes allowed to exist simultaneously will be lower than the value of

   When Should the Value of This Tunable Be Lowered?
       Lower this tunable when the system is meant to be moved from a development to a production environment.

   What Are the Side Effects of Lowering the Value?
       If the difference between and is less than the number of processes allowed to exist simultaneously will be lower than the value of

   What Other Tunable Values Should Be Changed at the Same Time?
       It  may	be  desirable  to change The tunable should, ideally, be set for the maximum value when validating software which uses PIDs.  When
       changing it is recommended that the system be rebooted to ensure all processes have PIDs within the desired range.

WARNINGS

       All HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases	of
       HP-UX.

       The HP-UX kernel may silently round the selected values for and/or (for example, to the nearest power of 2) in order to accommodate the PID
       generation algorithm.

       It is recommended that validation of software be carried out with the largest configurable value (1,073,741,567) for the tunable,  in  con-
       junction with the tunable set to something of the same magnitude (for example, 1,000,000,000).

       The manpage for (see process_id_max(5)) should be consulted for information about potential application issues with large PID values.

       Because	session IDs (SIDs) and process group IDs (PGIDs) are the same as the process ID of the session or group leader, an increase in the
       minimum PID also increases the minimum SID and PGID.

       The system may assign process ID values less than to certain special system processes.

       Booting the system with set to a non-zero value may cause the initialization process (aka init) to have a PID of something  other  than	1.
       However,  most  system  utilities  and  commands will, by default, report the value of the initialization process's PID as 1.  For example,
       returns 1 if the caller's parent is the initialization process, regardless of its actual PID.

       Installation of optional kernel software, from HP or other vendors, may cause changes to tunable  parameter  values.   After  installation,
       some  tunable  parameters may no longer be at the default or recommended values.  For information about the effects of installation on tun-
       able values, consult the documentation for the kernel software being installed.	For information about optional kernel  software  that  was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

SEE ALSO

       init(1M), fork(2), getppid(2), nproc(5), process_id_max(5).

       whitepaper, available on

							     Tunable Kernel Parameters						 process_id_min(5)
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