Here's a preview from my zine, Bite Size Linux! If you want to see more comics like this, sign up for my saturday comics newsletter or browse more comics!
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read the transcript!
The Linux kernel has code to do a lot of things
- read from a hard drive
- make network connections
- create new processes
- kill process
- change file permissions
- keyboard drivers
Your program doesn’t know how to do those things
program, blithely: TCP? dude I have no idea how that works.
program: NO, I do not know how the ext4 filesystem is implemented. I just want to read some files!
Programs ask Linux to do work using system calls
program: please write to this file (switch to running kernel code)
Linux: done! I wrote 1097 bytes! (program resumes)
Every program uses system calls
Python program: I use the ‘open’ syscall to open files Java program: me too! C program: me three!
And every system call has a number (e.g. chmod is 390 on x86.64)
so what’s actually going on when you change a file’s permissions is:
program: run syscall #90 with these arguments Linux: ok!
You can see which system calls a program is using with strace
$ strace ls /tmp
will show you evey system call ’ls’ uses! it’s really fun!
warning: strace has high overhead so don’t run it on your production database