gcc stack problem

scoban · 08-14-2005, 05:52 AM

when i create two buffers in C as below:
char buf1[5];
char buf2[10];

it will take up 8 bytes for buf1 and 12 bytes for buf2 totally 20 bytes from the stack.However on my athlon xp 2000 it takes 40 bytes why is this happening?? OS is slackware compiler gcc3.2.

Mara · 08-14-2005, 04:50 PM

How to do you calculate the amount of memory the program uses?

exvor · 08-14-2005, 05:20 PM

yea exactly. I think I brought up that question and was told there is not a way

primo · 08-14-2005, 09:05 PM

Buffers are usually allocated to the multiple of the word size == sizeof(int) on most machines.
In the first case, sizeof(int) == 4 so sizeof(buf1) == 8 && sizeof(buf2) == 12.
The reason for this is efficiency.

Quote:

How to do you calculate the amount of memory the program uses?

With size(1), ie: "size /bin/ls"
The size of individual buffers may be calculated as sizeof(buf) if it wasn't declared as "char *buf"

The second case puzzles me. Is sizeof(char) == 2 ?

scoban · 08-14-2005, 11:23 PM

Here is a sample C program:

Code:

void f(int a, int b) {
char x[5];
//char y[10];
}

void main() {
f(1,2);
}

And the GCC generated assembly code of the program:

Code:

	.file	"x.c"
	.text
.globl f
	.type	f, @function
f:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$16, %esp
	leave
	ret
	.size	f, .-f
.globl main
	.type	main, @function
main:
	pushl	%ebp
	movl	%esp, %ebp
	pushl	$2
	pushl	$1
	call	f
	addl	$8, %esp
	leave
	ret
	.size	main, .-main
	.section	.note.GNU-stack,"",@progbits
	.ident	"GCC: (GNU) 3.3.5"

When a function is called from the main, needed space is reserved from stack by decreasing the esp (subl $16, %esp)
Here there is only one local which is char[5] and it must be reserved 8 bytes (2 words) But as you see gcc allocates 16 bytes. For some reason it is doing some alligning but i dont understand what it is doing and why it is doing? However everything is OK if i use a char with a size of multiple of 4. For example if i use char [4] instead of char [5] than exactly 4 bytes allocated by gcc. Anyone can help??

jlliagre · 08-15-2005, 04:23 AM

Try "gcc -mpreferred-stack-boundary=2 ..."

scoban · 08-15-2005, 05:46 AM

Above code is already using -mpreferred-stack-boundary=2

Mara · 08-15-2005, 04:42 PM

It allocates more than it needs. For future use, probably. To see it yourself make something like

Code:

void f(int a, int b) {
char x[5];
//char y[10];
char z[4];
}

void main() {
f(1,2);
}

Still 16B of stack allocated.

scoban · 08-15-2005, 11:44 PM

I know that..when you do char [5] and char [12] it allocates 32 bytes...somehow it is alligning by multiple of 16...but there must be some configuration not to allocate more just needed...gcc version 2.95 and before does not do that only newer versions...