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

Question

If I use malloc in my code:

int *x = malloc(sizeof(int));

I get this warning from gcc:

new.c:7: warning: implicit declaration of function ‘malloc’  
new.c:7: warning: incompatible implicit declaration of built-in function ‘malloc’

Answer 1

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*).