warning: incompatible implicit declaration of built-in function 'malloc'??

[Paminu]

On a gentoo linux system i don't get any warnings when I comlpile this code:

#include <stdio.h>
typedef struct test
{
void *content;
struct test_ *bob;

} test_;

int main(void)
{

test_ *tt;
tt =(test_ *) malloc(sizeof(test_));
return 0;
}

But on my home Ubuntu box I get this warning:

mos@ubuntu:~/lab1$ gcc test2.c -o test2
test2.c: In function 'main':
test2.c:16: warning: incompatible implicit declaration of built-in function
'malloc'
mos@ubuntu:~/lab1$
What does this warning mean and why does it appear on one system while it
does not appear on another?

[Vladimir S. Oka]

Paminu wrote:

On a gentoo linux system i don't get any warnings when I comlpile this code:
Good example of why casting the result of malloc() is a bad idea. It
masks the problem you have below...
#include <stdio.h>
You need:

#include <stdlib.h>

as well, if you want to use malloc().
typedef struct test
{
void *content;
struct test_ *bob;

} test_;

int main(void)
{

test_ *tt;
tt =(test_ *) malloc(sizeof(test_));
return 0;
}

But on my home Ubuntu box I get this warning:

mos@ubuntu:~/lab1$ gcc test2.c -o test2
test2.c: In function 'main':
test2.c:16: warning: incompatible implicit declaration of built-in function
'malloc'
mos@ubuntu:~/lab1$

What does this warning mean and why does it appear on one system while it
does not appear on another?

It means that you did not include <stdlib.h> and the compiler does not
know about malloc(), and is therefore exercising its right to assume it
returns and `int`. You're then warned about it, which is very nice on
the part of the compiler, as it's not required to do so (since, by
casting, you told it: "I know what I'm doing here").

My guess is that you're using different gcc versions on your two
machines, or at least different set of default options.

--
BR, Vladimir

[Martin Ambuhl]

Paminu wrote (with snippage applied):

On a gentoo linux system i don't get any warnings when I comlpile this code: [Code including a call to malloc without including <stdlib.h> or
otherwise providing an explicit declaration]:
tt =(test_ *) malloc(sizeof(test_)); test2.c: In function 'main':
test2.c:16: warning: incompatible implicit declaration of built-in function
'malloc' What does this warning mean and why does it appear on one system while it
does not appear on another?

malloc returns a void *, but by not providing a declaration you have
implicitly declared it as returning an int. One of your compilers knows
that the standard library function malloc returns a void * and notices
that you have implicitly declared it otherwise; the other compiler
doesn't have this information. Further, the unnecessary and bogus cast
of an int to a pointer has masked your error of attempting to assign an
int to a pointer and encouraged the compiler to ignore your error.

[Emmanuel Delahaye]

Paminu a écrit :

What does this warning mean and why does it appear on one system while it
does not appear on another?

In both cases, <stdlib.h>, where malloc() is prototyped, is missing.

--
A+

Emmanuel Delahaye

[FlyingBird]

typedef struct test
{
void *content;
struct test_ *bob;

} test_;

I don't understand why the struct can be written like this. More
specifically, why doesn't the compiler report an error when it sees
struct test_ *bob. test_ is a typedef a struct test. Can we add the
keyword struct before test_? I read FAQ 1.14
http://c-faq.com/decl/selfrefstruct.html but there is no such an
example.

[Emmanuel Delahaye]

FlyingBird a écrit :

typedef struct test
{
void *content;
struct test_ *bob;

} test_;

I don't understand why the struct can be written like this. More
specifically, why doesn't the compiler report an error when it sees
struct test_ *bob. test_ is a typedef a struct test. Can we add the
keyword struct before test_? I read FAQ 1.14
http://c-faq.com/decl/selfrefstruct.html but there is no such an
example.

It is perfecty legal to define an incomplete structure name loke this :

struct name

Being incomplete, the compiler don'sn't know what is the size of such a
structure, hence an instanciation of such a type is illegal :

struct name object; /* compile error */

But, and this is the magic of C, it is possible to define a pointer to
such a type.

struct name *p_object;
struct name const *p_object;

This can be used to define :

* a single pointer (not very useful),
* an element of structure (useful for recursive structures like node of
liked lists, trees etc.),
* a function parameter (same behaviour that a void *, but typed, hence
the ability of a type control by the compiler)

This trick is used to implement ADT (Abstract Data Types) that have an
'opaque' type from the outside (interface, user) and a well defined type
in the inside (implementation)

/* xxx.h */
/* add guards... */

/* public incomplete type */
struct xxx;

/* public functions */
struct xxx *xxx_create (void);
void xxx_delete (struct xxx *p_context);

void xxx_function (struct xxx *p_context);

/* xxx.c */
#include "xxx.h"

/* private complete type */
struct xxx
{
int x;
char *y;
};

/* public function */
struct xxx *xxx_create (void)
{
struct xxx *this = malloc(sizeof *this);
if (this != NULL)
{
/* clear */
static const struct xxx z = {0};
*this = z;
}
return this;
}

void xxx_delete (struct xxx *this)
{
free (this);
}

void xxx_function (struct xxx *this)
{
this->x = 123;
}

/* main.c */
#include "xxx.h"

#include <assert.h>

int main (void)
{
struct xxx *p = xxx_create();

if (p != NULL)
{
xxx_function(p);

/* ... */

/* end */
xxx_delete(p), p = NULL;
}

assert (p == NULL);
return 0;
}

--
A+

Emmanuel Delahaye

[Mark McIntyre]

On Tue, 07 Feb 2006 15:23:37 +0100, in comp.lang.c , Paminu
<sd**@asd.com> wrote:

On a gentoo linux system i don't get any warnings when I comlpile this code:
You need to increase warninglevels in your gentoo compiler.
#include <stdio.h>
typedef struct test
{
void *content;
struct test_ *bob;
Where did you declare this type?
test_ *tt;
tt =(test_ *) malloc(sizeof(test_));
NEVER cast the return of malloc. It hides a serious error which in
this case is the cause of your problems.
test2.c:16: warning: incompatible implicit declaration of built-in function
this is the warning you should have got on gentoo.
What does this warning mean

it means you forgot to include stdlib.h, which is a bad mistake.
Mark McIntyre
--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

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[FlyingBird]

Thanks for your perfect describing :-) I also found some useful
discussions in another thread "initializing a pointer?".