CODEWITHSUNDEEP
clinuxlinkersystem-callsloader
user2277550
user2277550

Reputation: 621

What and where exactly is the loader?

I understand every bit of the C compilation process (how the object files are linked to create the executable). But about the loader itself (which starts the program running) I have a few doubts.

  1. Is the loader part of the kernel?

  2. How exactly is the ./firefox or some command like that loaded? I mean you normally type such commands into the terminal which loads the executable I presume. So is the loader a component of the shell?

I think I'm also confused about where the terminal/shell fits into all of this and what its role is.

Upvotes: 1

Views: 562

Answers (2)

Jeff Allen
Jeff Allen

Reputation: 1471

The format of an executable determines how it will be loaded. For example executables with "#!" as the first two characters are loaded by the kernel by executing the named interpreter and feeding the file to it as the first argument. If the executable is formatted as a PE, ELF, or MachO binary then the kernel uses an intrepter for that format that is built in to the kernel in order to find the executable code and data and then choose the next step.

In the case of a dynamically linked ELF, the next step is to execute the dynamic loader (usually ld.so) in order to find the libraries, load them, abd resolve the symbols. This all happens in userspace. The kernel is more or less unaware of dynamic linking, because it all happens in userspace after the kernel has handed control to the interprter named in the ELF file.

Upvotes: 2

Jean-Baptiste Yunès
Jean-Baptiste Yunès

Reputation: 36391

The corresponding system call is exec. It is part of the kernel and in charge of cleaning the old address space that makes the call and get a new fresh one with all materials to run a new code. This is part of the kernel because address space is a kind of sandbox that protect processes from others, and since it is critical it is in charge of the kernel.

The shell is just in charge of interpreting what you type and transform it to proper structures (list or arrays of C-strings) to pass to some exec call (after having, most of the time, spawned a new process with fork).

Upvotes: 1

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