How to change the value of a variable without the compiler knowing? - c

Question

I want to verify the role of volatile by this method. But my inline assembly code doesn't seem to be able to modify the value of i without the compiler knowing. According to the articles I read, I only need to write assembly code like __asm { mov dword ptr [ebp-4], 20h }, I think I write the same as what he did.
actual output:
before = 10
after = 123
expected output:
before = 10
after = 10
Article link: https://www.runoob.com/w3cnote/c-volatile-keyword.html
#include <stdio.h>
int main() {
int a, b;
// volatile int i = 10;
int i = 10;
a = i;
printf("before = %d\n", a);
// Change the value of i in memory without letting the compiler know.
// I can't run the following statement here, so I wrote one myself
// mov dword ptr [ebp-4], 20h
asm("movl $123, -12(%rbp)");
b = i;
printf("after = %d\n", b);
}

Answer 1

I want to verify the role of volatile ...
You can't.
If a variable is not volatile, the compiler may optimize; it does not need to do this.
A compiler may always treat any variable as volatile.
How to change the value of a variable without the compiler knowing?
Create a second thread writing to the variable.
Example
The following example is for Linux (under Windows, you need a different function than pthread_create()):
#include <stdio.h>
#include <pthread.h>
int testVar;
volatile int waitVar;
void * otherThread(void * dummy)
{
while(waitVar != 2) { /* Wait */ }
testVar = 123;
waitVar = 3;
return NULL;
}
int main()
{
pthread_t pt;
waitVar = 1;
pthread_create(&pt, 0, otherThread, NULL);
testVar = 10;
waitVar = 2;
while(waitVar != 3) { /* Wait */ }
printf("%d\n", testVar - 10);
return 0;
}
If you compile with gcc -O0 -o x x.c -lpthread, the compiler does not optimize and works like all variables are volatile. printf() prints 113.
If you compile with -O3 instead of -O0, printf() prints 0.
If you replace int testVar by volatile int testVar, printf() always prints 113 (independent of -O0/-O3).
(Tested with the GCC 9.4.0 compiler.)