不安全地使用void *指针吗？

Question

For example in this code:

#include <stdio.h>

int main()
{
  char i = 0;
  char j = 0;
  char *p = &i;
  void *q = p;
  int *pp = q;
  printf("%d %d\n", i, j);
  *pp = -1;
  printf("%d %d\n", i, j);

  return 0;
}

if I compile with gcc version 8.1.0 x64 (Ubuntu 8.1.0-5ubuntu1~16.04) the output is:
0 0
-1 -1
now, I use a cast to int *

#include <stdio.h>

int main()
{
  char i = 0;
  char j = 0;
  char *p = &i;
  void *q = p;
  int *pp = (int *)q;
  printf("%d %d\n", i, j);
  *pp = -1;
  printf("%d %d\n", i, j);

  return 0;
}

the result is the same as the previous.
With clang-6.0 x64 the output is:
0 0
-1 0
Is the code apparently a buffer overflow?
I hope I've explained that clearly.

Answer 1

You actually have two problems:

The first is that you break strict aliasing and have undefined behavior .

The second problem is that on most modern platforms the size of an int is four bytes, and the pointer pp is pointing only to a single byte. The assignment you make to *pp will therefore write out of bounds and also lead to undefined behavior.

Answer 2

Cast or no cast, you are essentially try to use incompatible types, leading to violating strict aliasing rules.

According to C11 , chapter §6.5,

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:88)

— a type compatible with the effective type of the object,

— a qualified version of a type compatible with the effective type of the object,

— a type that is the signed or unsigned type corresponding to the effective type of the object,

— a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,

— an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or

— a character type.

But, you try to access the memory allocated for a char via an int type. This violates strict aliasing.

That said, the standard guarantees that size of a char is 1 byte, size of an int is >= char . So, the access would be out of bound and lead to undefined behavior.