C: Why I am given similar things over multiple mallocs? - c

Question

I wrote a demo on dynamic 2D arrays:
# include <stdio.h>
# include <stdlib.h>
# define getarray2d(obj, dst) int (* dst)[obj->w] = (int (*)[obj->w])&(obj->data)
typedef struct _myarray {
int *data;
size_t h;
size_t w;
} myarray;
myarray * mallocarray2d(size_t h, size_t w) {
myarray *arr = (myarray *)malloc(sizeof(myarray));
int *data = (int *)malloc(h * w * sizeof(int));
arr->data = data;
arr->h = h; arr->w = w;
}
void freearray2d(myarray *arr) {
free(arr->data); free(arr);
}
void printarray2d(myarray *arr) {
getarray2d(arr, array2d);
size_t h = arr->h; size_t w = arr->w; size_t i, j;
for (i = 0; i < h; i++) {
for (j = 0; j < w; j++) {
printf("%12d ", array2d[i][j]);
}
printf("\n");
}
}
int main() {
myarray *arr = mallocarray2d(3, 4);
printarray2d(arr);
freearray2d(arr);
}
As expected, output is a "random" array:
F:\Users\23Xor\Desktop>a.exe
-695987216 555 3 0
4 0 -1651400550 268491609
-695968000 555 -695991984 555
But things became interesting after I ran it for multiple times:
F:\Users\23Xor\Desktop>a.exe
630939408 398 630915408 398
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
-1688379632 607 -1688403632 607
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
1305501456 532 1305477456 532
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
906452752 352 906428752 352
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
-1575002352 473 -1575026352 473
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
1594253072 518 1594229072 518
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
444751632 400 444727632 400
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
Elements at pos (0, 1) (2, 1) (2, 3) are always equal, and elements at pos (0, 2) (0, 3) (1, 0) (1, 1) always remain 3 0 4 0
Is there a specific reason for this?
EDIT: I corrected my faults:
# include <stdio.h>
# include <stdlib.h>
# define getarray2d(obj, dst) int (* dst)[obj->w] = (int (*)[obj->w])(obj->data)
typedef struct _myarray {
void *data;
size_t h;
size_t w;
} myarray;
myarray * mallocarray2d(size_t h, size_t w) {
myarray *arr = (myarray *)malloc(sizeof(myarray));
void *data = malloc(h * w * sizeof(int));
arr->data = data;
arr->h = h; arr->w = w;
return arr;
}
myarray * callocarray2d(size_t h, size_t w) {
myarray *arr = (myarray *)malloc(sizeof(myarray));
void *data = calloc(h * w, sizeof(int));
arr->data = data;
arr->h = h; arr->w = w;
return arr;
}
void freearray2d(myarray *arr) {
free(arr->data); free(arr);
}
void printarray2d(myarray *arr) {
getarray2d(arr, array2d);
size_t h = arr->h; size_t w = arr->w; size_t i, j;
for (i = 0; i < h; i++) {
for (j = 0; j < w; j++) {
printf("%12d ", array2d[i][j]);
}
printf("\n");
}
}
int main() {
myarray *arr = mallocarray2d(3, 4);
printarray2d(arr);
freearray2d(arr);
}
result:
F:\Users\23Xor\Desktop>a.exe
630939408 398 630915408 398
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
-1688379632 607 -1688403632 607
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
1305501456 532 1305477456 532
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
906452752 352 906428752 352
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
-1575002352 473 -1575026352 473
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
1594253072 518 1594229072 518
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
F:\Users\23Xor\Desktop>a.exe
444751632 400 444727632 400
2037603443 1835365491 1666986547 1697539181
1140876664 1702259058 1952531570 977485153
the last 8 elements even stops changing.

Answer 1

Since reading uninitialized data invokes undefined behavior, there is no satisfactory answer to your question. The official answer is: don't do that, it's not allowed by the rules of the C language, so the C language makes no guarantees about what will happen if you break the rules. You might end up reading all-zeroes, or all-42's, or 0xDEADBEEF, or your program might crash, or any other conceivable behavior might occur, because the resulting executable is not considered a valid C program.
At a practical level, you're printing out the values of whatever leftover garbage happened to be present in the area of RAM that malloc(h * w * sizeof(int)) returned to you when you called it. Why those values happened to be there (and not something different) is a function of how your C runtime's heap allocator works, combined with whatever the heap was being used for previously in your program's startup (e.g. before main() was called)... you could probably work out why they end up in that particular state, if you want to spend several days stepping through the source code of your C runtime with a debugger and a notepad. It probably wouldn't be worth the effort, though.