Initializing array of structs strange behavior

Question

I am using an array of structs and then set up the elements like the following:

#include <stdio.h>
#include <stdlib.h>

typedef struct _point {
    char s;
    unsigned int x;
    unsigned int y;
} point;

point* vehicle;

int main(int argc, char *argv[]) {
    /* 26 Vehicles */
    vehicle = malloc(26*sizeof(point*));
    for (int i = 0; i < 26; i++) {
        vehicle[i].s = ' ';
    }

    /* Print already existing vehicles */
    for (int i = 0; i < 26; i++) {
        if (vehicle[i].s != ' ') {
            printf("%c: x=%d y=%d\n", vehicle[i].s, vehicle[i].x, vehicle[i].y);
        }
    }

    return 0;
}

NOTE : this is not the actual code (which is too big to post) but the set up of the array and structs is the same.

As you can see, I set every vehicle[i].s to the space character, but the loop prints the following (not it this example code but in my actual code):

 : x=32215344 y=0 P: x=0 y=33 : x=2105376 y=0 

Question : how can it be that after the first loop, some elements are modified in the "background" without assigning them in the code? Or can some other malloc operations overwrite/reallocate the memory?

Answer 1

The problem, as I see it is in

 vehicle = malloc(26*sizeof(point*));

you're allocating memory for the pointer-to-the-data-type type, whereas you should be allocating for the data type itself.

To elaborate, you want to allocate memory for 26 elements of type point (ie, struct _point ), not 26 point * .

Change

vehicle = malloc(26*sizeof(point));

or, for better,

vehicle = malloc(26*sizeof * vehicle);

Otherwise, you're running short of allocated memory when you try to dererference the pointer to access n s of instances. So, you end up accessing out-of-bound memory which causes undefined behavior .

That said, just an advice, if you know the size to be allocated beforehand, ( 26 , for example), don't use dynamic memory, there's no need for it. Use an array.