CODEWITHSUNDEEP
c
user986424
user986424

Reputation: 11

How many bytes does the function need?

int main(){
   int i;
}

I did $ebp-$esp in gdb after calling the function and it gave me 4. But what about old ebp? Can I consider this function needs 8 bytes on the stack? 4 bytes for an int and 4 bytes for an old ebp.

note- I built it by using -mpreferred-stack-boundary=2 -march=i386

Upvotes: 1

Views: 2391

Answers (3)

AusCBloke
AusCBloke

Reputation: 18492

How much memory that function requires is going to depend entirely upon the compiler you're using. This is the disassembly of the above code using gcc 4.6.2 with the compile command:

gcc -mpreferred-stack-boundary=2 -march=i386 -m32 -o test test.c

   0x08048394 <+0>: push   ebp
   0x08048395 <+1>: mov    ebp,esp
   0x08048397 <+3>: pop    ebp
   0x08048398 <+4>: ret

What's fairly obvious is that i is ignored, since it's not used at all. Therefore you have 4 bytes on the stack for the return address pushed by call, and another 4 for preserving ebp.

As you'd expect, if you enabled slight optimisations with the -O1 compile flag it becomes:

   0x08048394 <+0>: ret

Since there's no need for a stack frame at all, main does nothing.

Upvotes: 0

Neil
Neil

Reputation: 55392

Assuming you're compiling without optimisations, you're not counting the old ebp, because esp gets copied to ebp after the old ebp is pushed on the stack. I think what you're seeing is gcc saving ecx although offhand I'm not sure why it does.

Upvotes: 1

user149341
user149341

Reputation:

Depends entirely on how the compiler decides to optimize the function. Some optimizing compilers might build this function as something along the lines of "main: ret", causing it to use no stack space at all.

Upvotes: 1

Related Questions