Question

I am learning C and I did this (code below and I cannot understand why I am getting program return value of something like -1073740940 when it should be 0

#include <stdio.h>
#include <stdlib.h> //For using malloc;


struct Vertex{
    int x;
    int y;
    int z;
};

int main(){
    int ret = 0;
    struct Vertex myVertex;
    struct Vertex *myVertexPtr = malloc(sizeof(*myVertexPtr));


    myVertexPtr = &myVertex;

    myVertex.x = 1;
    myVertex.y = 2;
    myVertex.z = 3;

    printf("%d\n", myVertexPtr->x);
    printf("%d\n", myVertexPtr->y);
    printf("%d\n", myVertexPtr->z);

    getchar();

    free(myVertexPtr); //When this line is included I get the strange program return    value (And, "This program has stopped working properly windows error")
                   //When this line is not included it returns fine, but I'm under the impression it is good practice to free pointers

    return 0;
}

I am using MinGW GCC to compile

Answer 1

This line:

myVertexPtr = &myVertex;

overwrites the pointer returned by malloc(), so that you pass the wrong value to free() which causes the error. It also causes a memory leak, which would be a problem in a more long-running program. Don’t do this!

It should simply be removed, if you want to work with a vertex on the heap, do that:

myVertexPtr = malloc(sizeof *myVertexPtr);
myVertexPtr->x = 1;
myVertexPtr->y = 2;
myVertexPtr->z = 3;

If you want to have a separate pointer to a vertex on the stack, drop the malloc():

myVertexPtr = &myVertex;