Why can’t I print it?

Question

It’s a simple code (I’m practicing pointers) It is not printing the result, the program closes... because? ;-;

#include <stdio.h>
#include <math.h>
#define PI 3.1416

void calc_esfera(float R, float *area, float *volume);

int main(){
    float raio, *area, *volume;

    printf("Raio: ");
    fflush(stdin);
    scanf("%f", &raio);

    calc_esfera(raio, area, volume);

    printf("Area: %.2f\n", *area);

    printf("Volume: %.2f", *volume);

}

void calc_esfera(float R, float *area, float *volume){

    *area = PI * pow(R, 2.0);
    *volume = 4 * PI * pow(R, 3.0);
}

Answer 1

Turns out you didn’t initialize the pointers. As the name itself indicates, they are pointers, so have to point to some place in memory that is where the values are placed.

Just when it does:

*area = PI * pow(R, 2.0);

You’re saying, in the place of memory where the pointer area point, put this value. But which location is this ? area has not even been assigned, and therefore results in undefined and potentially Segmentation fault.

The compiler itself helps you in this regard. See what I get when I compile your code:

||=== Build: Debug in Test (compiler: GNU GCC Compiler) ===|
... main.c|14|warning: 'area' is used uninitialized in this function [-Wuninitialized]|
... main.c|14|warning: 'volume' is used uninitialized in this function [-Wuninitialized]|
||=== Build finished: 0 error(s), 2 warning(s) (0 minute(s), 0 second(s)) ===|

How to fix ? Allocates space to the pointer, which would be the most normal:

float *area = malloc(sizeof(float));
float *volume = malloc(sizeof(float));

Now the pointers already point to a valid memory location, even if they have no value assigned by you. This will already make the following assignments work properly. Don’t forget to include <stdlib.h> to use the malloc.

Another alternative would be to create variables for the area and volume as values and pass their addresses to the function:

float area, volume;
...
calc_esfera(raio, &area, &volume);
//                ^------^-- aqui passa o endereço das variáveis no main

This last solution would even be preferable because it avoids the allocation in the heap, which causes more memory fragmentation and is slower. It also prevents you from having to worry about releasing memory with free when you no longer need it.