executing-file.md

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Here I'm trying to understand what happens when I run

./hello

#include <stdio.h>

int main() {
    printf("Hello!\n");
}

a simple "Hello World" program written in C, in Unix -- what I'd have to do if I wanted to write an OS that could execute it.

I'm going to assume that ./hello is statically linked, because that sounds simpler to deal with. It's worth noting that a statically linked hello is 868K on my machine. Eep.

I compiled it using

gcc -static hello.c -o hello

Any (nice!) comments or clarifications are appreciated.

Read from the filesystem

To run a program, I have to be able to find the program. So there would need to be some kind of filesystem and I would need to read the file from somewhere.

Copy the text into memory

In a Unix system, executables are in the ELF format.

So I would need to copy the "text" of the program somewhere.

Copy the data into memory

There is a string in the program. It needs to go somewhere.

Give the program a stack pointer

This program doesn't actually allocate memory, so perhaps it does not need a heap and it doesn't matter where the heap pointer is. It does need a stack. stack overflow question on how the stack works in assembly

Implement system calls

hello has some system calls in it. I found this out by running

objdump -d -M intel hello | grep 'syscall'

syscall is an assembly instruction for making a system call. That looks like

  401385:       b8 03 00 00 00          mov    eax,0x3
  40138a:       0f 05                   syscall 

The number stored in eax is the system call that is called. In this case, 3

There are 119 instances of syscall, and it's using several different system calls. This is worrying.

(Explained more in this stackoverflow question)

Making sure a stack overflow doesn't happen

I have no idea how the OS would check up on the program. I guess it doesn't just let the program run, but takes away control periodically and makes sure the stack pointer hasn't moved too far. How would it take away control? Hmm.

When there is a stack overflow I guess it sends a signal to the program, which is a POSIX thing.

I do not understand this.

???

There are no mallocs in the program, so I would not need to allocate memory for it or anything.

What else?!??

Outstanding questions

• How long would this take for a human (where human = me) to write from scratch?
• Is there a way to write a smaller program with less system calls and magic? There are like 50 system calls and what are they even doing?
• Do I need a heap if I never use malloc?
• Could I write my own printf in assembly that does less and is simpler? Just printing a string is pretty easy...
• How do I kill a program?

Useful links

• How statically linked programs run on Linux, via @steveklabnik
• A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux , via @leftside
• long Reddit comment on what to do if you are stuck alone in a room with a computer and no OS, by kragen