On Thursday, 21 April 2022 at 05:49:12 UTC, Alain De Vos wrote:
Following program:
```
import std.stdio;
void main() @trusted
{
int *p=null;
void myfun(){
int x=2;
p=&x;
writeln(p);
writeln(x);
}
myfun();
*p=16;
writeln(p);
writeln(*p);
}
```
outputs :
7FFFFFFFDFAC
2
7FFFFFFFDFAC
32767
I don't understand why. Would it be possible to explain ?
See my comment for some ELI5 of what's going on.
Of course it's a bit more complicated than that, but I hope it
gets the point across.
```
void main() @trusted
{
int *p=null; // P is null obviously
void myfun(){
int x=2;
p=&x; // Sets the address of p to the address of x, which is on
the stack of myfun
writeln(p); // Writes the address of p
writeln(x); // Writes the value of x (same as value of p)
}
myfun(); // We call myfun
// Any memory that was in the stack of myfun is invalid here,
returning to the stack of main.
*p=16; // Sets the value of p to 16, but p points to the address
of an invalid memory location, since x was on the stack within
myfun and thus isn't valid outside of myfun
writeln(p); // Writes the address of p, which is the same since
the address of p was stored on the stack of main
writeln(*p); // Attempts to write the value of p, which points to
an "invalid" memory address, or at least it's now something
completely different than it was before, considering the stack of
myfun is gone, so it writes out a garbage value
}
```
