ENUMERATION DATA TYPE IN C LANGUAGE:

Enumeration data type consists of named integer constants as a list.
It start with 0 (zero) by default and value is incremented by 1 for the sequential identifiers in the list.
Enum syntax in C:
- enum identifier [optional{ enumerator-list }];
Enum example in C:

enum month { Jan, Feb, Mar }; or
/* Jan, Feb and Mar variables will be assigned to 0, 1 and 2 respectively by default */
enum month { Jan = 1, Feb, Mar };
/* Feb and Mar variables will be assigned to 2 and 3 respectively by default */
enum month { Jan = 20, Feb, Mar };
/* Jan is assigned to 20. Feb and Mar variables will be assigned to 21 and 22 respectively by default */

The above enum functionality can also be implemented by #define preprocessor directive as given below. Above enum example is same as given below.

#define Jan 20;
#define Feb 21;
#define Mar 22;

Example enum program in C

#include <unistd.h>

void ft_putchar(char c)
{
  write(1, &c, 1);
}

void ft_putstr(char *str)
{
  while (*str != '\0')
    write(1, str++, 1);
}

int main() {
  enum MONTH { Jan = 0, Feb, Mar };
  enum MONTH month = Mar;
  if (month == 0)
    ft_putstr("Value of Jan");
  else if (month == 1)
    ft_putstr("Month is Feb");
  if (month == 2)
    ft_putstr("Month is Mar");
  return (0);
}

Output:

Month is Mar

Test it out here: http://www.pythontutor.com/c.html#mode=edit