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 ULTRIX
setpgid
setpgid(3) Library Functions Manual setpgid(3)
Name
setpgid - set process group (POSIX)
Syntax
#include <sys/types.h>
int
setpgid(pid, pgrp)
pid_t pid, pgrp;
Description
The function is used to either join an existing process group or create a new process group within the session of the calling process (see
Upon successful completion, the process group ID of the process that has a process ID which matches pid is set to pgrp. If pid is zero,
then the call applies to the current process. In addition, if pgrp is zero, the process ID of the indicated process is used.
This function is available only in the POSIX environment.
Return Values
The function returns 0 when the operation is successful. If the request fails, -1 is returned and the global variable indicates the rea-
son.
Diagnostics
The function fails and the process group is not altered if one of the following occurs:
[EACCES] The value of the pid argument matches the process ID of a child process of the calling process and the child process has
successfully executed an function.
[EINVAL] The value of the pgrp argument is less than zero or is not a supported value.
[EPERM] The process indicated by the pid argument is a session leader.
The value of the pid argument matches the process ID of a child process of the calling process and the child process is not
in the same session as the calling process.
The value of the pgrp argument does not match the process ID of the process indicated by the pid argument and there is no
process with a process group ID that matches the value of the pgrp argument in the same session as the calling process.
[ESRCH] The value of the pid argument does not match the process ID of the calling process of a child process of the calling
process.
See Also
getpgrp(2), setsid(2)
setpgid(3)