This is a simple and quick C language reference with focus on embedded systems!
assert.h – Assertion
ctype.h – Character operations
math.h – Mathematical declarations
stdarg.h – Variable argument list handling
stdbool.h – Boolean type and values
stddef.h – Standard definitions
stdint.h – Posix-compliant integer types
stdio.h – Standard buffered input/output
stdlib.h – C standard library
time.h – Time-related types and functions
string.h – Memory and string manipulation