Is it possible implementing a C program which does not perform any system calls?

Question

My teacher asked me to write a C function which does not perform any syscalls. It doesn't matter that the function accomplishes nothing.

Does the following function perform any syscall?

int func() {
  return 0;
}

If it does, can you give me an exemplar function like the one I'm looking for?

Thanks a lot.

This isn't clear; you're asking whether this code, which does nothing, does something? — Oliver Charlesworth, Jun 14 '13 at 16:56
Is this some kind of trick question? Ultimately, yes, your code is going to call _exit(0). — Nicholas Wilson, Jun 14 '13 at 16:57
Or to put in another way, any program that terminates must have a syscall because there's no instruction a (not-undefined) program can use to make the processor stop executing instructions! — Nicholas Wilson, Jun 14 '13 at 16:59
@nikita_em add this line in your question first, always explain your question good you one can understand and respond back. — Grijesh Chauhan, Jun 14 '13 at 17:01
He said "I know how to write func, that doesn't have sys call", so I'm in trouble :( — nemelianov, Jun 14 '13 at 17:02
Sure, but the example is `main`, which is special, and all of the other comments appear to be talking baout whole programs. — Carl Norum, Jun 14 '13 at 17:05

score 6 · Accepted Answer · edited May 23 '17 at 11:45

Even if the code itself does not contain system calls (what could meet the requirements), there are some implied system calls to actually initialize, run, stop and cleanup the process, even if they're not part of your binary. Which system calls are performed is platform dependent. Furthermore, at least the exit status will be set according to how you shut down your process: return statement vs exit() in main()

I guess, your teacher will be happy with that code, it doesn't use the standard library, which itself contains many system calls for different purposes (just like most other libraries). You won't be able to read/write from/to stdin/out and files/sockets, etc.. So you can't do IO, process creation & multithreading, synchronization, etc. since all that requires system calls (things like user threads and spinlocks may be a notable exception here). One cannot write useful userland programs without system calls, except for programs taking some args, with a result returned as an int (e.g. command line tools). You can also implement 'fully quiet' CPU heating stuff.

Yep, it does't use libc. I'll check it tomorrow and respond. Thx for your answer ;-) — nemelianov, Jun 14 '13 at 18:05

score 2 · Answer 2 · answered Jun 14 '13 at 17:07

No, your example function doesn't make any system calls. You could just compile and disassemble it to be sure:

$ cc -O3 -c example.c 
$ objdump -d example.o 

example.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <func>:
   0:   31 c0                   xor    %eax,%eax
   2:   c3                      retq

Or without optimizations, if that's important:

$ cc -c example.c 
$ objdump -d example.o 

example.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <func>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   b8 00 00 00 00          mov    $0x0,%eax
   9:   5d                      pop    %rbp
   a:   c3                      retq

I'm curious about this answer as I believe a program can't run without syscalls. Have you seen the other answers? — Matheus Santana, Mar 14 '18 at 20:05

score 1 · Answer 3 · answered Jun 14 '13 at 17:04

1

Yes the above has system calls. You can use ptrace() to stop every time there is a system call, and you'll see that there are system calls. How else would the program be loaded and unloaded from memory without the operating system?

answered Jun 14 '13 at 17:04

KrisSodroski

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The original question had an empty `main` function. Does it have a system call? Depends on what you mean by "have". The calling environment undoubtedly performs system calls, but the function itself doesn't. The revised question has an empty function called `func`. That function clearly does not explicitly make any system calls -- but for all we know the implementation could require a system call to allocate a function call activation record. If it's possible at all to have a function that doesn't "have" any system calls, then an empty non-main function certainly qualifies. – Keith Thompson Jun 14 '13 at 18:52

Is it possible implementing a C program which does not perform any system calls?

3 Answers3