Linux System Calls – What Did the System Call ‘tuxcall’ Do?

The software has to be portable. If you compile your C/C++ sources, then you don't need to know if you running i386/x86_64/arm/mips or what ever. The headers are linked in such way the software compiles.

All other header files exist because they were implemented a lot of different standards, there are ports from BSD and so on. So many of them are historically based. Where each one comes from and why they are there has many different reasons and will surely blow answers.

And an answer for asm-generic: stackoverflow

System calls per se are a concept. They represent actions that processes can ask the kernel to perform.

Those system calls are implemented in the kernel of the UNIX-like system. This implementation (written in C, and in asm for small parts) actually performs the action in the system.

Then, processes use an interface to ask the system for the execution of the system calls. This interface is specified by POSIX. This is a set of functions of the C standard library. They are actually wrappers, they may perform some checks and then call a system-specific function in the kernel that tell it to do the actions required by the system call. And the trick is that those functions which are the interface are named the same as the system calls themselves and are often referred directly as "the system calls".

You could call the function in the kernel that perform the system call directly through the system-specific mechanism. The problem is that it makes your code absolutely not portable.

So, a system call is:

• a concept, a sequence of action performed by the kernel to offer a service to a user process
• the function of the C standard library you should use in your code to get this service from the kernel.