So why not walk through this and see if we can understand what is happening.
First of all, to help with clarity, I'm going to rename the
x and
y variables in
main to be
mx and
my. That way there's no ambiguity about what a variable name is referring to.
int main (void) {
int mx=4,my=5;
stuff(&mx,my);
printf("%d %d\n",mx,my);
}
void stuff(int *a,int b){
*a=3;b=2;
morestuff(*a,&b);
}
void morestuff( int x,int *y) {
x=*y;
*y=10;
}
So when
main calls
stuff(&mx, my), the
value of
my (==5) is passed as the second argument (
b);
stuff has no idea where that value came from, no pointer to it, so there's no way that
my can be changed and
my will still be 5 at the
printf.
Now to determine what the value of
mx will end up as.
stuff takes a pointer to int as
a and
main passes a pointer to
mx.
The
*a=3; says to set the value 3 into the location pointed to by
a.
a is pointing to
mx, so this sets
mx to 3.
But you say
What about "b=2;"?
Remember that the value passed as the
b parameter is the value 5, not a pointer to where the 5 is stored, so this assignment does not change anything outside of
stuff. In fact, at this point,
b behaves
exactly as if it were declared as a local variable to
stuff. It can be changed locally, but the changes don't
escape from
stuff.
So what happens when
morestuff is called?
morestuff has a value as its first parameter (
x) and a pointer to int as its second parameter (
y). The
stuff passes
&b (the address of
b) as
y. We've already determined that changes to
b don't escape from
stuff, so nothing that happens to
y in
morestuff can change anything visible outside of
stuff.
Also, like the
b parameter to
stuff, the
x parameter of
morestuff is a
value parameter. In this case, the
*a when
stuff is calling
morestuff, is
dereferencing the
a pointer. It is taking what
a points to, which is actually
mx and retrieving its value, which had just been set to 3. Again, as a
value parameter, changes to
x cannot escape from
morestuff.
So finally at the end we see that
mx was set (by
stuff) to the value 3 and my was left unchanged as the value 5.
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