In C, can a const variable be modified via a pointer?

by

const actually doesn’t mean “constant”. Something that’s “constant” in C has a value that’s determined at compile time; a literal 42 is an example. The const keyword really means read-only. Consider, for example:

const int r = rand();

The value of r is not determined until program execution time, but the const keyword means that you’re not permitted to modify r after it’s been initialized.

In your code:

const int x=1;
int *ptr;
ptr = &x;
*ptr = 2;

the assignment ptr = &x; is a constraint violation, meaning that a conforming compiler is required to complain about it; you can’t legally assign a const int* (pointer to const int) value to a non-const int* object. If the compiler generates an executable (which it needn’t do; it could just reject it), then the behavior is not defined by the C standard.

For example, the generated code might actually store the value 2 in x — but then a later reference to x might yield the value 1, because the compiler knows that x can’t have been modified after its initialization. And it knows that because you told it so, by defining x as const. If you lie to the compiler, the consequences can be arbitrarily bad.

Actually, the worst thing that can happen is that the program behaves as you expect it to; that means you have a bug that’s very difficult to detect. (But the diagnostic you should have gotten will have been a large clue.)

More Related Contents:

  1. What is the difference between char * const and const char *?
  2. Why can’t I convert ‘char**’ to a ‘const char* const*’ in C?
  3. constant pointer vs pointer on a constant value [duplicate]
  4. Constant pointer vs Pointer to constant [duplicate]
  5. Why can I change a local const variable through pointer casts but not a global one in C?
  6. Difference between char* and const char*?
  7. const usage with pointers in C
  8. Confusion regarding modification of const variable using pointers
  9. Why can I change the value of a const char* variable?
  10. constness and pointers to pointers
  11. C pointer to array/array of pointers disambiguation
  12. How do pointer-to-pointers work in C? (and when might you use them?)
  13. Pointers in C: when to use the ampersand and the asterisk?
  14. Is NULL always zero in C?
  15. What does the ‘array name’ mean in case of array of char pointers?
  16. What exactly is meant by “de-referencing a NULL pointer”?
  17. What does malloc(0) return? [duplicate]
  18. Variably modified array at file scope
  19. Is NULL always false?
  20. Does the size of pointers vary in C? [duplicate]
  21. What is the meaning of “wild pointer” in C?
  22. Pass a two dimensional array to a function of constant parameter
  23. Explain this implementation of malloc from the K&R book
  24. using printf to print out floating values
  25. How to check if a pointer is valid? [duplicate]
  26. Is it possible to allocate array inside function and return it using reference?
  27. Deep Analysis of Const Qualifier in C
  28. Dereference void pointer
  29. How to check if a pointer is freed already in C?
  30. Casting void pointers

Leave a Comment