How do you dereference a pointer to get/set the value it points to?

Put * in front of a pointer: *p reads the value it points at, and *p = … writes through it to the original variable.

int x = 42;
int *p = &x;

// READ through pointer
// value = 42
int value = *p;

// WRITE through pointer
// x is now 100
*p = 100;

Dereferencing NULL crashes:

int *p = NULL;
// SEGFAULT! Dereferencing null pointer
*p = 5;

Always check before dereferencing:

if (p != NULL) {
    *p = 5;
}
// Or shorter:
if (p) {
    *p = 5;
}

Double dereference (pointer to pointer):

int x = 42;
int *p = &x;
int **pp = &p;

// x is now 100
**pp = 100;