UNIX for Dummies Questions & Answers

Hi i am very new to programming in UNIX and don't understand the difference between a system call and a normal function call. Also can I implement system calls from within a program? If so could someone please give me an example of a system call from within a program. Lastly, when creating a program related to systems programming is it necessary to be in a specific shell? Sorry if these questions are pretty basic but I am completely lost here. Any help would be greatly appreciated.

When you make a function call, your program jumps to a different location in memory. It's just a program jump with a little extra stack work, nothing magic, just running around in its own private little universe (process).

To make a system call, you set registers to some particular values, call INT 0x20, and bang, it's done. Your own program doesn't jump anywhere or do anything -- the system call just happens, like magic, then leaves your program right where it started.

Code does execute, of course, but not in your program. INT 0x20 just passes a message into the operating system. When that happens, the OS literally stops your program, rearranges that program's private universe in the manner requested. Once it's finished, the kernel starts your program running again. This isn't necessarily instant. If you do a read() on a pipe or socket with no data in it, your program might be sleeping entire seconds for whatever's on the other end to write into it.

So. Function calls: Runs instructions inside your program. These instructions can't do anything except read or write memory or alter certain unprivileged registers.

System calls: Instantly stops your program and sends a message to the operating system, asking it to do something. The OS decides what to do with your request and does it, in kernel mode, with much higher privileges. Once finished, starts your program again.