Tracing code from user to hardware

Question

I'm curious if someone can point me in the right direction here. I'm learning about computer systems programming (the basics) and I'm trying to trace code through different levels to see how each interacts with the other. An example would be calling the fgets() function in C or getline() in C++ or similar. Both of those would make calls to the system right? Is there an easy way to look at the code that is called?

I'm working on Unix (Ubuntu). Is this something that is proprietary with Windows and Apple? Any good resources out there for this kind of thing? As always, thanks guys!

Answer 1

At least in the UNIX world, the answer is fairly easy: "Use the Source, Luke".

In your example, you would look at the sources for, say, fgetc(). That's in the C standard library, and the easiest way to find the source is google something like "C libraary fgets() source".

When you get that source, you'll see a bunch of code handling buffers etc, and a system call, probably to read(2). The "2" there tells you it is documented in Chapter 2 of the manual (eg, you can find it with man 2 read).

The system call is implemented in the kernel, so then you need to read the kernel source. Proceed from there.

Now, what you need to find this all without having to read randomly about in the sources (although that's the way a lot of people have learned it, it's not very efficient) is to get hold of a book on Linux like Kerrisk's The Linux Programming Interface, which explains some of these things at a somewhat higher level than just the source.

Answer 2

Something fgets is located within libc. That is, it's a userland library linked with most C binaries. Check out glibc, which is currently the most common implementation.

Eventually, libc will start making system calls to the kernel. You can get the source at kernel.org. Check out KGDB for kernel debugging. The simplest way to to do kernel debugging is to use a second machine connected via null model cable.