Is there anyway from a C prog to find whether the OS is currently running in 32bit or 64bit mode. I am using a simple program as below

int main(void){
     switch(sizeof(void*)){
        case 4: printf("32\n");
        break;
        case 8: printf("64\n");
        break;
    }
}

Is this a correct approach ? Would this code work in all the scenarios like, If the hardware is 64bit and the OS is 32bit what would it return ? I don't have machine to test this in diff configurations.

Thanks for the advice.