Why pointer memory gets successfully able to save and retrive data from void memory allocation

Question

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

int main()
{
    char *p = malloc(sizeof(void));
    p[0] = 'a';
    p[1] = 'b';
    p[2] = 'c';
    p[3] = 'd';
    p[4] = 'e';
    printf("%c\n",p[0]);
    printf("%c\n",p[1]);
    printf("%c\n",p[2]);
    printf("%c\n",p[3]);
    printf("%c\n",p[4]);
    return 0;
}

// In above code only one byte memory should be allocated as sizeof(void) will be only '1', but why I am able to successfully print all the characters?

similarly for below program also it is printing successfully

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

int main()
{
    char *p = malloc(0);
    p[0] = 'a';
    p[1] = 'b';
    p[2] = 'c';
    p[3] = 'd';
    p[4] = 'e';
    printf("%c\n",p[0]);
    printf("%c\n",p[1]);
    printf("%c\n",p[2]);
    printf("%c\n",p[3]);
    printf("%c\n",p[4]);
    return 0;
}

Please clarify why both of this cases are successfully printing the msg even though memory is not allocated for this?

I parked my car illegally. Explain why it was not towed away. — Weather Vane, Jun 28 '17 at 09:04
please check both the dupes. Combined together they answer your question. — Sourav Ghosh, Jun 28 '17 at 09:04
Yes, may be succeeded, or may be fail. Because it's undefined behavior. And you write bytes in non-allocated memory block. So the program may be crash. — mattn, Jun 28 '17 at 09:04
Add this toward the end of main() : `free(p); p = malloc(sizeof(void));` and see what the effect of buffer overflow is to your app. Buffer overflows can be difficult to track, because the error may pop up much later in the instruction flow. — Michaël Roy, Jun 28 '17 at 09:23

score 0 · Answer 1 · answered Jun 28 '17 at 08:58

0

You're exploiting undefined behaviour.

On other occasions the compiler might eat your cat.

(What might be happening under the hood is that the C runtime library is giving you back more memory than you've actually asked for as some kind of optimisation.)

answered Jun 28 '17 at 08:58

Bathsheba

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Why pointer memory gets successfully able to save and retrive data from void memory allocation

1 Answers1