Pid file and process check Post: 303036935

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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

07-17-2019

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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 REDHAT

setsid

SETSID(2)						     Linux Programmer's Manual							 SETSID(2)

NAME

       setsid - creates a session and sets the process group ID

SYNOPSIS

       #include <unistd.h>

       pid_t setsid(void);

DESCRIPTION

       setsid() creates a new session if the calling process is not a process group leader.  The calling process is the leader of the new session,
       the process group leader of the new process group, and has no controlling tty.  The process group ID and session ID of the calling  process
       are set to the PID of the calling process.  The calling process will be the only process in this new process group and in this new session.

RETURN VALUE

       The session ID of the calling process.

ERRORS

       On  error,  -1  will be returned.  The only error which can happen is EPERM. It is returned when the process group ID of any process equals
       the PID of the calling process. Thus, in particular, setsid fails if the calling process is already a process group leader.

NOTES

       A process group leader is a process with process group ID equal to its PID. In order to be sure that setsid will succeed,  fork	and  exit,
       and have the child do setsid().

CONFORMING TO

       POSIX, SVr4.

SEE ALSO

       setpgid(2), setpgrp(2)

Linux 1.0.0							    1994-08-27								 SETSID(2)

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