#DPP #programming #pointers

Question

Comments

Q]Is the ans : A0 or A followed by garbage values ?

---

The program in the linked question is:

 

#include <stdio.h>

#include <stdlib.h>

 

int main() {

    int count = 0;

    char *p = (char *)malloc(sizeof(char));

 

    *p = 65;

    printf("%c", *p);

 

    p = realloc(p, 4 * sizeof(char));

 

    *p = 256;

    printf("%d", *(int *)p);

 

    return 0;

}

 

Step-by-step

 

1. malloc(sizeof(char))

 

p = malloc(1);

 

One byte is allocated.

 

2. Store 65

 

*p = 65;

printf("%c", *p);

 

65 corresponds to 'A' in the usual ASCII environment.

 

So the first output is:

 

A

 

3. realloc

 

p = realloc(p, 4);

 

The allocated block becomes 4 bytes. The old byte's value is preserved; the additional bytes do not have meaningful initialized values.

 

4. Critical statement

 

*p = 256;

 

p is a char *, so the value is converted to char. The exact resulting value depends on the implementation; it is not something you can safely treat as the integer 256 stored in 4 bytes.

 

5. Then:

 

printf("%d", *(int *)p);

 

This is the real problem.

 

You stored/accessed the allocated object through a char lvalue, and then attempt to access it through an int lvalue:

 

char object ---> accessed as int

 

That violates the C effective-type/strict-aliasing rules, producing undefined behavior. Also, the code assumes the 4-byte allocation is enough for an int, which is not guaranteed by the C standard.

 

Therefore

 

The answer is not

 

A0

 

and it is also not

 

A + garbage values

 

The only predictable part is:

 

A

 

After that, the program has undefined behavior, so the complete output is not defined. It could print A0, A followed by some other value, crash, or behave differently.

 

GATE takeaway

 

Whenever you see:

 

char *p = malloc(...);

...

printf("%d", *(int *)p);

 

don't automatically assume "read the bytes as an integer."

 

Check:

 

1. Effective type / strict aliasing

 

 

2. Alignment

 

 

3. Enough allocated bytes

 

 

4. Object representation / initialization

 

 

 

Here, undefined behavior is the decisive point.