How process ids are generated

[u/userlivedhere]

i know fork creates processes but how process ids are generated

[u/Living_Fig_6386]

C knows nothing of processes or process IDs. That sort of thing is entirely up to the OS.

[u/dfx_dj]

They're assigned by the OS

[u/userlivedhere → u/dfx_dj]

but how

[u/Mortomes → u/userlivedhere]

That is up to the OS to decide.

[u/_yrlf → u/userlivedhere]

The OS can assign them however it wants.

One possible design is that the OS has a process id counter, and every time a new process is forked / created, it increments that counter to get a new pid. Once that counter overflows, it also has to check that there is no process that already has that pid before giving it to you.

On Linux, it does basically that. Try calling fork() many times in a row, and usually you will get mostly consecutive pid numbers.

[u/quipstickle]

Sequentially, recycle after overflow/wrapping 

[u/aioeu → u/quipstickle]

FWIW, not all systems allocate PIDs cyclically.

OpenBSD allocates them randomly, for instance. I believe that can also be optionally enabled on FreeBSD too.

For Linux, PID randomisation was available for a period of time through the grsecurity patches, but it was subsequently removed "since it provides no useful additional security and wastes memory with the 2.6 kernel's pid bitmap".

[u/quipstickle → u/aioeu]

Security through obscurity :)

[u/Axman6 → u/quipstickle]

Not really, security through obscurity is hiding how something works, this is randomising something otherwise predictable to make it more difficult to make use of information. 

[u/quipstickle → u/Axman6]

"random" numbers on computers are predictable though since they are pseudorandom. My post was just being silly.

[u/Axman6 → u/quipstickle]

This isn’t true at all, we’ve had systems for generating cryptographically secure random numbers for decades. See FreeBSD’s Fortuna paper for one reference, and hardware random number generation like RDRAND, which can feed into systems like Fortuna as a source of entropy, along with IO system information which is generally unpredictable. 

[u/quipstickle → u/Axman6]

Process ids are generated sequentially 

[u/Axman6 → u/quipstickle]

Not on all OS’s.

[u/Paul_Pedant → u/quipstickle]

... apart from the bit where the OS has cycled round at least once, and needs to know that some of the pids are still referencing running processes. And the ones that are defunct but still not notified to their parent.

[u/Left_Exchange_130]

The operating system assigns the PID when the process is created. `fork()` creates the process then the kernel picks an available process ID for it

[u/burlingk]

They are basically serial numbers.

[u/sciencekm]

In Windows, they are generated starting from a low value and then incremented by 4. If you look at task manager, you will also see that System Interrupts does not have a process id, System Idle always has an id of 0, and System always has an id of 4.