error in C - `system' undeclared (first use this function)

[gordhan P]

This is my first course in C, I do not know why this error. It did compile before but now this error. Please help..

#include <stdio.h>
#include <math.h>
 
 
//Functions
void calc_BMI(double weight, int height);
 
 
int main() {
int height, selection, updated_weight;
float weight;
 
 
     printf("Welcome to the Exercise UCF Program!\n");
 
     printf("What is your height in inches?\n");
     scanf("%d", &height);
 
     printf("What is your weight in pounds?\n");
     scanf("%f", &weight);
 
     printf("Please Select One of the Following:\n");
     printf("1 - Update Weight\n");
     printf("2 - Calculate BMI\n");
     printf("3 - Calorie Counter\n");
     printf("4 - Weight Calculation\n");
     printf("5 - Average Mile\n");
     printf("6 - Quit\n");
     scanf("%d", &selection);
 
     switch (selection) {
//enter weight
           case 1:
                printf("What is your current weight in pounds?\n");
                scanf("%d", updated_weight);
 
                break;
//Calculate BMI
           case 2:
                if (weight < 1000 && height < 100) 
                  calc_BMI(weight, height);
 
                //if(body_mass_index <= 18.5)
                //printf("Your body weight status is underweight.\n");
 
                //if(body_mass_index >= 18.5 && body_mass_index <= 24.9)
                //printf("Your body weight status is normal.\n");
 
                //if(body_mass_index >= 25 && body_mass_index <= 29.9)
                //printf("Your body weight status is overweight.\n");
 
                //if(body_mass_index > 30)
                //printf("Your body weight status is obese.\n");
                break; 
                }
                system ("pause");
     return 0;
 
     }
 
 
//BMI
void calc_BMI(double weight, int height) {
     double body_mass_index = 0;
 
     body_mass_index = weight*703/(height*height);
 
     if(body_mass_index <= 18.5)
       printf("Your body weight status is underweight.\n");
 
     if(body_mass_index >= 18.5 && body_mass_index <= 24.9)
       printf("Your body weight status is normal.\n");
 
     if(body_mass_index >= 25 && body_mass_index <= 29.9)
       printf("Your body weight status is overweight.\n");
 
     if(body_mass_index > 30)
       printf("Your body weight status is obese.\n");
 
 
}

[Dominik Przybys]

system is in stdlib.h
Just add line

#include <stdlib.h>

at the begining of the program.