Is the compiler allowed to recycle freed pointer variables?

by

Upon an object reaching the end of its lifetime, all pointers to it become indeterminate. This applies to block-scope variables and to malloced memory just the same. The applicable clause is, in C11, 6.2.4:2.

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime.

Using indeterminate memory for anything, including apparently harmless comparison or arithmetic, is undefined behavior (in C90; later standards complicate the matter terribly but compilers continue to treat usage of indeterminate memory as undefined behavior).

As an example, how about the following program printing that p and q are both different and the same? The results of execution with various compilers are shown here.

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

int main(int argc, char *argv[]) {
  char *p, *q;
  uintptr_t pv, qv;
  {
    char a = 3;
    p = &a;
    pv = (uintptr_t)p;
  }
  {
    char b = 4;
    q = &b;
    qv = (uintptr_t)q;
  }
  printf("Roses are red,\nViolets are blue,\n");
  if (p == q)
    printf ("This poem is lame,\nIt doesn't even rhyme.\n");
  else {
    printf("%p is different from %p\n", (void*)p, (void*)q);
    printf("%"PRIxPTR" is not the same as %"PRIxPTR"\n", pv, qv);
  }
}

More Related Contents:

  1. How does free know how much to free?
  2. Does free(ptr) where ptr is NULL corrupt memory?
  3. Should one really set pointers to `NULL` after freeing them?
  4. C – Accessing data AFTER memory has been free()ed?
  5. How are we able to access the pointer after deallocating the memory?
  6. How to check if a pointer is freed already in C?
  7. what is meant by (void *) variable and (void *) & variable in C [closed]
  8. How to find the ‘sizeof’ (a pointer pointing to an array)?
  9. Crash or “segmentation fault” when data is copied/scanned/read to an uninitialized pointer
  10. How come an array’s address is equal to its value in C?
  11. What is the difference between char array and char pointer in C?
  12. How do I understand complicated function declarations?
  13. How can I allocate memory and return it (via a pointer-parameter) to the calling function?
  14. C pointers : pointing to an array of fixed size
  15. Why does this code segfault on 64-bit architecture but work fine on 32-bit?
  16. C: Which character should be used for ptrdiff_t in printf?
  17. Why does std::cout convert volatile pointers to bool?
  18. Pointer vs array in C, non-trivial difference
  19. Passing arrays and matrices to functions as pointers and pointers to pointers in C
  20. Is it good practice to free a NULL pointer in C? [duplicate]
  21. How to optimize these loops (with compiler optimization disabled)?
  22. C: pointer to array of pointers to structures (allocation/deallocation issues)
  23. How to explain C pointers (declaration vs. unary operators) to a beginner?
  24. Why does NSError need double indirection? (pointer to a pointer)
  25. What is the real difference between Pointers and References?
  26. Does C have an equivalent of std::less from C++?
  27. How to avoid long chain of free’s (or deletes) after every error check in C?
  28. What happens to memory after free()?
  29. How to assign pointer address manually in C programming language?
  30. What does this C statement mean?

Leave a Comment