Is it Undefined Behaviour to cast away the constness of a function parameter?

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The attempt to write to *dst is UB if the caller passes you a pointer to a const object, or a pointer to a string literal.

But if the caller passes you a pointer to data that in fact is mutable, then behavior is defined. Creating a const char* that points to a modifiable char doesn’t make that char immutable.

So:

char c;
clearstring(&c);    // OK, sets c to 0
char *p = malloc(100);
if (p) {
    clearstring(p); // OK, p now points to an empty string
    free(p);
}
const char d = 0;
clearstring(&d);    // UB
clearstring("foo"); // UB

That is, your function is extremely ill-advised, because it is so easy for a caller to cause UB. But it is in fact possible to use it with defined behavior.

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