Pointers, arrays and strings in C

Pointers are the topic that separates confident C candidates from the rest. Most tricky output questions combine a pointer, an array and a little arithmetic, so it pays to learn exactly how these pieces relate.

What a pointer holds

A pointer stores an address. &x gives the address of x, and *p reads or writes the value at the address held in p.

int x = 10;
int *p = &x;
*p = 20;
printf("%d\n", x);   // 20

Changing *p changes x because both refer to the same memory. This is also how a function can modify a caller's variable: pass its address, as in swap(&a, &b). Passing plain values only gives the function copies.

Arrays and pointer arithmetic

In most expressions, an array name converts to a pointer to its first element. Adding 1 to a pointer moves it by one element, not one byte, so arr[i] and *(arr + i) mean the same thing.

int arr[] = {5, 10, 15};
int *q = arr;
printf("%d %d\n", *(q + 1), q[2]);   // 10 15

Watch precedence: *q + 1 reads the first element and adds 1, giving 6, while *(q + 1) reads the second element. The array name itself cannot be reassigned, but a pointer variable can.

Strings are char arrays

A C string is an array of characters that ends with the null character '\0'. The literal "hi" therefore takes three bytes.

• sizeof on an array gives its total size in bytes, including the null character.
• strlen counts characters up to, but not including, the null character.
• Compare strings with strcmp, not ==, which compares addresses.

char name[] = "code";
printf("%zu %zu\n", sizeof(name), strlen(name));   // 5 4

Never write past the end of an array. C does not check bounds, so the program may print garbage, crash, or seem fine until much later.

Tracing habit

For pointer questions, draw a box for each variable with its address and value, and move the pointer arrow as the code runs. It takes a minute and prevents most mistakes.