Why do we not get a warning when passing a string where we should pass an int?

Question

If we try something like

char *str = "hello";
if (isalpha(str))
    printf("%s\n", str);

we'd get a segmentation fault, because isalpha() takes int whose value must be in the range of an unsigned char or EOF.

But why doesn't the compiler complain about passing a char * where an int is expected? If char * gets implicitly-casted down to int, we might lose data!

In fact, if we have something like

void doSomething(int i);

int main(void)
{
    char *str = "hello";
    doSomething(str);
}

void doSomething(int i)
{
    // some code
}

The compiler will give this warning:

warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

Assuming -Wpointer-to-int-cast is enabled by default, what's the difference here?

Answer 1

It turns out that isalpha is defined as a macro in the ctype.h header file, in my case, according to this answer (by @ouah on Stack Overflow):

Add -Wsystem-headers to get the warning

From gcc documentation (emphasize mine)

-Wsystem-headers

Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read.

Answer 2

If I understand your question correctly, then I would say that the warning is compiler specific. I'm not one with extensive knowledge on compilers but when I ran it on my mac using Coderunner it produced the following warning:

warning: incompatible pointer to integer conversion passing 'char *' to parameter of type 'int'

this result from passing the [-Wint-conversion] flag.