Question

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

const int * func()
{
    int * i = malloc(sizeof(int));
    (*i) = 5;    // initialize the value of the memory area
    return i;
}

int main()
{
    int * p = func();
    printf("%d\n", (*p));
    (*p) = 3;       // attempt to change the memory area - compiles fine
    printf("%d\n", (*p));
    free(p);
    return 0;
}

Why does the compiler allow me to change (*p) even if func() returns a const pointer?

I’m using gcc, it shows only a warning on the int * p = func(); line : “warning: initialization discards qualifiers from pointer target type”.

Thanks.

Answer 1

Your program is not valid. C forbids implicitly removing a const like that, and in conformance to the spec GCC should give you at least a warning for that code. You would need a cast to remove the const.

Having consumed a warning for that, you can however rely on the program to work (although not anymore from a Standards point of view), because the pointer is pointing to a malloc’ed memory area. And you are allowed to write to that area. A const T* pointing to some memory doesn’t mean that the memory is thereafter marked immutable.

Note that the Standard doesn’t require a compiler to reject any program. The Standard merely requires compilers to sometimes emit a message to the user. Whether that’s an error message or warning and how the message is emitted and whatever happens after that emission, isn’t specified by the Standard at all.