怀疑堆栈指针

Question

Conside a program:

#include <stdio.h>

void function(int a, int b, int c){
    char buffer1[5];
    char buffer2[10];
}

void main(){
        function(1,2,3);
}

compiling this with
gcc test.c -m32 -g -o test -fno-stack-protector
and doing
objdump -S test > test.dis

I get the following dump for the function "function"

void function(int a, int b, int c){
 80483ed:       55                      push   %ebp
 80483ee:       89 e5                   mov    %esp,%ebp
 80483f0:       83 ec 10                sub    $0x10,%esp
        char buffer1[5];
        char buffer2[10];
}

Consider another variant of the same program:

#include <stdio.h>

void function(int a, int b, int c){
        char buffer1[7];
        char buffer2[10];
}

void main(){
        function(1,2,3);
}

on compiling and generating the dump with same commands i get:

void function(int a, int b, int c){
 80483ed:       55                      push   %ebp
 80483ee:       89 e5                   mov    %esp,%ebp
 80483f0:       83 ec 20                sub    $0x20,%esp
        char buffer1[7];
        char buffer2[10];
}

my question is what causes the stack pointer to be decremented by 16 in first case and 32 in second case while only 2 more bytes are required in 2nd case?

I am running 64 bit ubuntu 14.04 on Intel processor

Answer 1

From the GCC documentation :

-mpreferred-stack-boundary = num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 ( 16 bytes or 128 bits), except when optimizing for code size ( -Os ), in which case the default is the minimum correct alignment (4 bytes for x86, and 8 bytes for x86-64).

So as space is allocated on the stack for the arrays, the stack is also being aligned on the default boundary of 16 bytes. In the first case you've got 15 bytes of data which is less than 16, so 16 bytes are allocated. In the second case you've got 17 bytes of data which is more than 16 but less than (the next multiple of 16) 32, so 32 bytes of space is allocated.