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posts/when-a-program-is-run.md

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+---
+title: "When a program is run"
+
+description: "but nobody observes it does it have side effects?"
+
+date: "2025-05-14"
+
+draft: true
+
+tags: [how things work]
+---
+
+I don't know how much of what I know would be considered "interesting" or "enlightening" but in terms of what happens when a program stars on Linux I at least have some knowledge that a beginner wouldn't have.
+
+So yeah if this article is boring or confusing apologies for that.
+
+Also there's a lot of asterisks which I'm not going to elaborate on, just know that when there's an asterisk that means something is a bit of a half truth.
+
+## What runs a program?
+
+There are a couple of plausible sounding answers to this question.
+
+1. Another program
+2. The operating system
+
+Both answers of course beg more questions, namely "what runs the first program?" and "what does that even mean?" respectively.
+
+Both answers relate to something called the kernel.
+
+## What's the kernel?
+
+The (Linux) kernel is some\* of the code which allows for "normal" programs to not worry about various hard problems.
+Hard problems including
+
+- interfacing with the hardware
+- storing data in a filesystem
+- and virtualization/concurrency aka the subject of this post
+
+To answer our 2 questions then, the kernel starts the first\* program and is realistically the only\* part of the operating system which can be differentiated from a normal program.
+
+## Running a program
+
+Lets say a program wants to run a program then.
+There are a couple of ways for a program to do this depending on semantics.
+
+### `exec` syscall
+
+First off a program can just replace itself with a different program with the `exec` syscall.
+This isn't very useful most of the time but there are some cases where this behavior is desirable, suid/sgid programs like `sudo` and `doas` and if I remember correctly [s6](https://skarnet.org/software/s6/) makes use of it when it's setup as the first program the system runs but I don't remember specifics.
+
+In any case most of the time when a program runs another program it has other stuff it wants to do afterwards.
+
+### `fork` syscall and then `exec` syscall
+
+This is generally the model taught in operating systems classes where a process makes a duplicate of itself and then the duplicate morphs into the different program.
+For real world usage though this isn't what's generally done.
+
+### `clone` syscall
+
+Instead the `clone` syscall is used for effieciency reasons alongside having more options for configuring the process that's spawned.
+`clone` is also used for creating threads because for the kernel threads are just weird looking processes.
+
+## Stepping through an example
+
+