Pointer Parameters
beginner15 min readLesson 39 of 148
C passes arguments by value; pointers are how a function reaches back.
Why plain parameters can't
C function arguments are copies:
void bump(int n) { n = n + 1; } // bumps the COPY
int x = 5;
bump(x);
// x is still 5 — the function changed its own copy
Passing an address changes the object
Give the function the address, and it can dereference its way back to the original:
void bump(int *n) { *n = *n + 1; } // follows the pointer to the caller's int
int x = 5;
bump(&x); // pass the ADDRESS
// x is now 6
This is the single most important pointer pattern in C: out-parameters.
scanf("%d", &n) works for exactly this reason — you hand scanf the address
so it can deposit the value.
Two outputs from one function
void minmax(int a, int b, int *lo, int *hi) {
if (a <= b) { *lo = a; *hi = b; }
else { *lo = b; *hi = a; }
}
...
int lo, hi;
minmax(3, 8, &lo, &hi);
A function returns one value; pointers give it many.
The contract
A pointer parameter is a promise: "this points at a valid int". Passing
NULL or an uninitialized pointer breaks the contract — the function will
dereference garbage. Module 14 introduces checking.