Why do I get a warning everytime 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’

I'm new to C. Am I doing something wrong?

Answer 1

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

Answer 2

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

#include <stdlib.h>

Answer 3

You need to #include <stdlib.h> which includes malloc's declaration. If you don't do that, the compiler thinks you want to define your own function named malloc (and it warns you because a) you don't explicitly declare it and b) there already is a built-in function by that name).

Answer 4

malloc has type (void*) and you want (int*) so it's better to add typecast in addition to include

#include <stdlib.h>
int *x = (int*)malloc(sizeof(int));

Answer 5

make a habit of looking your functions up in help.

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

man malloc

gives pretty useful results.

googling man malloc will show you what I mean.

of course in unix you also get apropos for things that are related.