The standard function library has a rich and varied set of strings and character handling functions.
The string functions operate on null-terminated arrays of characters and require the header <string.h>. The character functions use the header <ctype.h>.
Because C has no bounds checking on array operations, it is the programmer's responsibility to prevent an array overflow. Neglecting to do so many cause your program to crash.
The header <string.h> defines the size_t type, which is the result of the sizeof operator and is some form of unsigned integer.
You will learn only those functions that operate on characters of type char. These are the functions originally defined by Standard C, and they are, by far, the most widely used and supported.