Why do I get a warning every time I use malloc?

You haven't done #include <stdlib.h>.

You need to add:

#include <stdlib.h>

This file includes the declaration for the built-in function malloc. If you don't do that, the compiler thinks you want to define your own function named malloc and it warns you because:

1. You don't explicitly declare it and
2. There already is a built-in function by that name which has a different signature than the one that was implicitly declared (when a function is declared implicitly, its return and argument types are assumed to be int, which isn't compatible with the built-in malloc, which takes a size_t and returns a void*).

Make a habit of looking your functions up in help.

Most help for C is modelled on the unix manual pages.

Using :

man malloc

gives pretty useful results.

Googling man malloc will show you what I mean.

In unix you also get apropos for things that are related.

You need to include the header file that declares the function, for example:

#include <stdlib.h>

If you don't include this header file, the function is not known to the compiler. So it sees it as undeclared.