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.
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.
In a Unix system, executables are in the ELF format.
So I would need to copy the "text" of the program somewhere.
There is a string in the program. It needs to go somewhere.
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
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)
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?!??
- 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?
On your query about interrupts...
Interrupts can be called by any hardware that has a IRQ line. Interrupts are sent directly to one of the CPUs and they have a location in memory setup to jump to handle that interrupt quickly and get back to what they were doing.
Network interfaces, USB, disk controllers usually do interrupts when they have something ready to send, i.e. a new packet has come in the network interface (though sometimes they batch them which is called interrupt mitigation). There is also a programable timer chip on the motherboard, which in Linux OS is typically programmed to interrupt a set number of times a second (e.g. 1000 on my box) which calls the scheduler to run.
Processes run until they block (by doing a system call) or run out of their timeslice (~20ms). When a process blocks, the system handles the system call and typically that has some wait in it, e.g. it asks for something from disk that might take 1-10ms. The OS then does something else in that 1-10ms. When the data comes back from the disk an interrupt is raised and the system puts that into memory. The original process is now runable again and will be ran by the scheduler based on it's algorithm. If the process runs over it's timeslice (yours wouldn't - a but one doing some CPU intensive stuff would) some thing else can be scheduled for a bit before it get a chance to run again.