Sizes of stack variables

Sizes of stack variables

[chuckw]

Hey All,
  I was just trying to minimize the size of a program the other day,
and I noticed that the size of the varibles created on the stack is
not necessarily the size they should be.  So, for instance, the 
following code: 

void main()
{
  char buffer[8];

  printf("%s\n", buffer);
}

produces the following assembly:
   ...
8  main:
9    pushl %ebp
10   movl  %esp, %ebp
11   subl  $24, %esp
12   andl  $-16, %esp
13   movl  $0, %eax
     ...

No problem right, the buffer variable is created on the stack, and
I am assuming that the number 24 being subtracted from the stack pointer
is the 8 bytes for the buffer + 16 bytes program overhead(environment, etc).
Now, if I change the source to the following:

void main()
{
  char buffer[16];
   
  printf("%s\n", buffer);
}

produces the following assembly:
   ...
   8  main:
   9    pushl %ebp
   10   movl  %esp, %ebp
   11   subl  $40, %esp
   12   andl  $-16, %esp
   13   movl  $0, %eax
   ...

Shouldn't the stack size only increase to 32 which is 16 bytes of overhead
plus the 16 bytes for the variable?

If someone has some insight into this I would be much abliged.

Chuck

Re: Sizes of stack variables

[nanakos]

Hi the stack size depends on the optimization of the compiler ( gcc )
.Just read the stack frame that is used from the compiler.You can find
such info in http://gcc.gnu.org or sth similar.

Best regards,
Chris.



> Hey All,
>   I was just trying to minimize the size of a program the other day,
> and I noticed that the size of the varibles created on the stack is
> not necessarily the size they should be.  So, for instance, the
> following code:
>
> void main()
> {
>   char buffer[8];
>
>   printf("%s\n", buffer);
> }
>
> produces the following assembly:
>    ...
> 8  main:
> 9    pushl %ebp
> 10   movl  %esp, %ebp
> 11   subl  $24, %esp
> 12   andl  $-16, %esp
> 13   movl  $0, %eax
>      ...
>
> No problem right, the buffer variable is created on the stack, and
> I am assuming that the number 24 being subtracted from the stack pointer
> is the 8 bytes for the buffer + 16 bytes program overhead(environment,
> etc).
> Now, if I change the source to the following:
>
> void main()
> {
>   char buffer[16];
>
>   printf("%s\n", buffer);
> }
>
> produces the following assembly:
>    ...
>    8  main:
>    9    pushl %ebp
>    10   movl  %esp, %ebp
>    11   subl  $40, %esp
>    12   andl  $-16, %esp
>    13   movl  $0, %eax
>    ...
>
> Shouldn't the stack size only increase to 32 which is 16 bytes of overhead
> plus the 16 bytes for the variable?
>
> If someone has some insight into this I would be much abliged.
>
> Chuck
>
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