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’

Solution 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*).

Solution 2

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

Solution 3

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.

Solution 4

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.

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Comments

[Mechanical snail]

Only if you already know that the line #include <stdlib.h> in the synopsis means you have to write that in your program.

[Jens]

Terminology nit: There's no such thing as a built-in function in C. malloc is simply a function from the Standard C Library.

[sepp2k]

@Jens I don't see a problem with referring to standard library functions as built-ins - and neither do the gcc people apparently since the error message used the word "built-in", too (which is why I did).

[Jens]

@sepp2k Well, I do see a problem. The usage is non-standard. Compilers can have built-in functions, and gcc may have malloc built-in. But as I wrote, in C, there are no built-in functions.

[MikeKulls]

If someone has read the other answers here then they will now know that.