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

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’

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*).

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

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.

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.

Beside the other very good answers, I would like to do a little nitpick and cover something what is not discussed yet in the other answers.

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When you are at Linux, To use malloc() in your code,

You don´t actually have to #include <stdlib.h>.

(Although the use of stdlib.h is very common and probably every non-toy-program should include it either way because it provides a wide range of useful C standard library functions and macros)

You could also #include <malloc.h> instead.

But please note that the use of malloc.h is deprecated and it makes your code non-portable. If you want to use malloc() you should always and ever (except for explicit reasons to do otherwise) #include <stdlib.h>.

The reasons why, are best explained in the answers to this question:

https://stackoverflow.com/questions/12973311/difference-between-stdlib-h-and-malloc-h