Bad call with const signature in C

Question

Possible duplicate:
Why is it not legal to convert (pointer to pointer to non-constant) to (pointer to pointer to constant)

Why am I getting a warning (gcc 42.2) with the following call to foo?

void foo(const char **str) { (*str)++; } (...) char **str; foo(str); (...) 

I understand why we cannot call a function, except char ** with const char ** , but it seems to me the other way around, so why the following warning?

 warning: passing argument 1 of 'foo' from incompatible pointer type 

Answer 1

It is not right. There is no real opportunity to argue with the compiler, since it is supported by the specification. Here is an example that explains why this is wrong:

 void func(const char **p) { *p = "abc"; } void func2(void) { char *a; func(&a); *a = 'x'; } 

If the compiler did not spit out the error, your program will probably crash because you will rewrite the string literal (which is often marked as read-only in memory).

Thus, you cannot implicitly cast char ** to const char ** , because it will allow you to remove the const qualifier from any value - basically, it will allow you to ignore const as desired without an explicit cast.

The main reason the compiler gives a warning instead of an error is because a lot of old code does not use the const qualifier, wherever it would be if the code was written today.