Hello I wrote this C program but it is not compiling with gcc -Wall .. it is giving the error """"

due_date.c: In function main:

due_date.c:14: warning: implicit declaration of function exit

due_date.c:14: warning: incompatible implicit declaration of built-in function exit

"""

The reason I am using exit is because I have to abort the program if a condition is not met "If the entered day for today is not in the range of 0 to 6, display an error message and abort the program. "

this is the program

#include

int main ()

{

/*defining local variables*/

int todays_date;

int number_of_days;

int D;

/*Taking today's date and the number of days till due date from the user*/

printf("enter today's number 0 for sunday 1 for monday ect: ");

scanf("%d",&todays_date);

/*Checking for errors*/

if(todays_date<0 || todays_date>6){

printf("You entered an invalid number.. try again: ");

exit(1);

}

printf("enter number of days: ");

scanf("%d",&number_of_days);

/*Calculating days in total*/

D = (todays_date + number_of_days)%7;

switch(D)

{

case 0:printf("number of days allowed: %d ",number_of_days+1);

printf("the work is due on Monday ");

break;

case 1:printf("number of days allowed: %d ",number_of_days);

printf("the work is due on Monday ");

break;

case 2: printf("number of days allowed: %d ",number_of_days);

printf("the work is due on Tuesday ");

break;

case 3: printf("number of days allowed: %d ",number_of_days);

printf("the work is due on Wednesday ");

break;

case 4: printf("number of days allowed: %d ",number_of_days);

printf("the work is due on Thursday ");

break;

case 5: printf("number of days allowed: %d ",number_of_days+3);

printf("the work is due on Monday ");

break;

case 6: printf("%d ",number_of_days+2);

printf("the work is due on Monday ");

break;

}

return 0;

}