* Sizes of stack variables
@ 2003-10-29 20:30 chuckw
2003-11-03 16:12 ` nanakos
0 siblings, 1 reply; 2+ messages in thread
From: chuckw @ 2003-10-29 20:30 UTC (permalink / raw)
To: linux-c-programming
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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Sizes of stack variables
2003-10-29 20:30 Sizes of stack variables chuckw
@ 2003-11-03 16:12 ` nanakos
0 siblings, 0 replies; 2+ messages in thread
From: nanakos @ 2003-11-03 16:12 UTC (permalink / raw)
To: chuckw; +Cc: linux-c-programming
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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>
^ permalink raw reply [flat|nested] 2+ messages in thread
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2003-10-29 20:30 Sizes of stack variables chuckw
2003-11-03 16:12 ` nanakos
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