Thoughts on a small program

One , Code ：

#include <stdio.h>

int main(int argc, char* argv[])
{
    
    int i = 0;
    int arr[3] = {
    0};
    for(; i <= 3; i++)
    {
    
        arr[i] = 0;
        printf("hello world\n");
    }
    return 0;
}

The running result of this program , On some machines , Not printing four lines "hello word", It will print in an infinite loop "hello world".

Two , Memory allocation problem

In memory allocation , It will be allocated by increasing or decreasing the memory address

3、 ... and ,Linux Memory layout

The local variables in the function body exist on the stack , And it's a continuous stack . The picture below is Linux Memory layout of the process ; Stack area in high address space , Address increases from high address to low address .

3、 ... and , Code parsing

I know from the front , Variable i Follow arr On adjacent addresses , And i The address of arr The address is big （ Low address value is large ）, From the memory layout ,i stay arr Below ; therefore ,arr Address assignment is from top to bottom , Array arr Cross the border just to visit i; namely arr[3] = 0; Think of i= 0; Cause infinite loop .

Four , Code validation

#include <stdio.h>


int main(int argc, char* argv[]){
    
    int i = 0;
    int arr[3] = {
    0};
    for(; i <= 3; i++){
    
	printf(" Variable i  The address for ：%p\n", &i);
        printf("m[0]  The address for ：%p\n", &arr[0]);
        printf("m[1]  The address for ：%p\n", &arr[1]);
        printf("m[2]  The address for ：%p\n", &arr[2]);
        printf("m[3]  The address for ：%p\n", &arr[3]);
        arr[i] = 0;
        printf("hello world\n");
    }
    return 0;
}

gcc -fno-stack-protector main.c -o test

 Variable i  The address for ：0x7ffde08ed5ec
m[0]  The address for ：0x7ffde08ed5e0
m[1]  The address for ：0x7ffde08ed5e4
m[2]  The address for ：0x7ffde08ed5e8
m[3]  The address for ：0x7ffde08ed5ec