C Strings Are Memory

The terminator is the string's length

char word[6] = {'h', 'e', 'l', 'l', 'o', '\0'};
char word2[] = "hello";              // same thing; size 6, terminator included
const char* msg = "hello";           // points at a read-only literal

Every string function finds the end by scanning for '\0'. No terminator = the scan runs off the array = undefined behavior. This is the single most important sentence in this module.

What's really stored

"hi" occupies 3 bytes: 'h', 'i', '\0'. strlen("hi") is 2 (terminator not counted); the array needs 3 slots.

Character work

char c = 'A';
if (c >= 'a' && c <= 'z') { /* lowercase letter */ }
char up = c - 'a' + 'A';             // 'b' -> 'B' — arithmetic on codes
char digit_val = '7' - '0';          // 7 — digit char to number

Characters are small integers. 'a'..'z', 'A'..'Z', '0'..'9' are each contiguous ranges (for these sets, in every toolchain you will meet here).

Literals vs arrays

A string literal ("hello") lives in read-only storage: writing through the pointer is undefined behavior. A char array you declared is writable. When you need to modify, copy into an array first.