4. Standard Library Headers

The standard library headers define macros and declare functions - cppreference.com/w/c/header

Sample from stdio.h:

  #define NULL   0		// see notes on NULL
  #define BUFSIZ 8192		// see notes on BUFSIZ and EOF
  #define EOF    (-1)

  int printf(const char *format, ...);
  int scanf(const char *format, ...);

Sample from errno.h:

  extern int errno;

Sample from math.h:

  double sqrt(double x);
  float sqrtf(float x);
  long double sqrtl(long double x);

  double sin(double x);