System Calls

AlbertJJ · 10-26-2010, 11:10 PM

What is the real difference between system calls and normal function calls. Ultimately function calls too would be passed to kernel for some or the other work.

Sergei Steshenko · 10-26-2010, 11:43 PM

Quote:

Originally Posted by AlbertJJ

... Ultimately function calls too would be passed to kernel for some or the other work.

Why do you think so ?

If I write a function:

Code:

int add_2_ints(int a, int b)
  {
  return a + b;
  }
...
// in 'main':

int sum = add_2_ints(2, 3);

, where exactly is the OS kernel involved ?

paulsm4 · 10-26-2010, 11:54 PM

Hi -

Quote:

Q: What is the real difference between system calls and normal function calls?

A: There is a HUGE difference.
The latter operates in the context of the user process. The former jumps to a completely different context: from "user space" into "kernel space".

This link describes it in more detail:
http://en.wikipedia.org/wiki/System_call

Quote:

Q: Ultimately function calls too would be passed to kernel for some or the other work?

Wrong: not every function call ends up as a system call. Not by a long shot

Sergei's example is one of many illustrating this fact.

The converse, however, is accurate. Every system call you make (for example, the syscall to "_open") is likely to start out as a function call from your program. See the section on "The library as an intermediary" in the above link.

'Hope that helps

Aquarius_Girl · 10-26-2010, 11:59 PM

If I write a code as follows:

Code:

int main ()
{
   return 0;
}

Will this return 0 statement not call any system call in return ?

Sergei Steshenko · 10-27-2010, 12:48 AM

Quote:

Originally Posted by anishakaul

If I write a code as follows:

Code:

int main ()
{
   return 0;
}

Will this return 0 statement not call any system call in return ?

'main' is a special function, upon its completion the kernel is notified that the task is no more needed, so it should be deleted from the task queue(s), etc.

Aquarius_Girl · 10-27-2010, 01:07 AM

Quote:

Originally Posted by AlbertJJ

Ultimately function calls too would be passed to kernel for some or the other work.

Then I think after reading the Sergei's post above, the above quote should have been re-framed as follows to make more sense:

Quote:

Ultimately function calls (which do not include the user defined functions but do include the C/C++ etc standard library functions) too would be passed to kernel for some or the other work.

Correct me if I am wrong.

paulsm4 · 10-27-2010, 01:47 AM

Quote:

Ultimately function calls (which do not include the user defined functions but do include the C/C++ etc standard library functions) too would be passed to kernel for some or the other work.

Correct me if I am wrong.

You're wrong

No offense - but Sergei's example is a great.

And if you wanted another example (an example in the Standard C library), then look no further than "strlen()"

.

I repeat what I said above:
Q: What is the real difference between system calls and normal function calls?
A: There is a HUGE difference. By definition, a "system call" changes context from user space to system space. A function call, on the other hand, is executed from user space.

Q: Ultimately function calls too would be passed to kernel for some or the other work?
A: Not every function call ends up as a system call.
In fact, probably FEW ever do.

PS:
"return", as I'm sure you know, is a C/C++ *operator*. As such, it's outside the scope of this discussion.

If you meant "function main()", you can easily create a function that makes NO system calls (e.g. "void main () { ; }"). The PROGRAM might make system calls (in both the preamble and exit code) ... but the FUNCTION doesn't.

Sergei Steshenko · 10-27-2010, 09:45 AM

Quote:

Originally Posted by paulsm4

...
PS:
"return", as I'm sure you know, is a C/C++ *operator*. As such, it's outside the scope of this discussion.
...

It's actually moot, and I'm not in the mood to dig the C99 standard at the moment, but my vague memories from the late eighties (and from used then Borland Turbo-C) tell me that the compiler may in reality compile 'main' quite differently from other functions. I.e. seeing 'return' in 'main' the compiler might decide not to translate 'return' the usual (for other functions) way, but rather to directly make system 'exit' call.

Again, it's not a "hard claim", but a possibility to consider.