Programming Exercise 1: Area of a Circle

The area of a circle is pi * r2. Write a C program that inputs a single floating point numberrepresenting the value of the radius r. Input comes from standard input. Your program shouldoutput the area of the circle with a radius r. Now, the only problem is what value should you usefor pi? You have already worked with M_PI in Lab 6. In fact, inside of math.h is the followingline:

#define M_PI 3.14159265358979323846 /* pi */

so you need to include math.h and then use M_PI in an expression to calculate the area. Do nottype in this #define in your program, just include math.h. However, when compiling, withthe -ansi option you will need to add another option, otherwise things won’t go too well.

Try to build this program using:

gcc -ansi -pedantic -Wall assignment06pe01.c -o assignment06pe01

You should see that trouble exists. It will not recognize M_PI even though it is inside math.h.To get past this, you need to build the program by either dropping the -ansi option or by addingthe -D_GNU_SOURCE option. I prefer you choose the latter:

gcc -ansi -pedantic -Wall -D_GNU_SOURCE assignment06pe01.c -o assignment06pe01

You do not need to build the program with the -lm option since you are not missing anydefinitions from libm.a. You need to understand the previous sentence. Make certain that you dounderstand it. Do you?