NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/3384

Internet Message Format | 1996-08-05 | 3.0 KB

Path: SPOD2.dev.esoc.esa.de!328PT
From: [e-mail] (Phil Tregoning)
Newsgroups: comp.lang.c
Subject: Re: Please help ?!
Date: 28 Jan 1996 15:59:36 GMT
Organization: European Space Operations Center
Message-ID: <4eg6h8$[e-mail]>
References: <4dm889$[e-mail]> <4drnv1$[e-mail]> <4drq5i$[e-mail]> <4e6hse$[e-mail]> <[e-mail]>
Reply-To: [e-mail]
NNTP-Posting-Host: spod2.dev.esoc.esa.de

>> In referenced article, John R Buchan says...
>>
>> >>   cpy = (char *) malloc(MAXLEN);
>> >
>> >The cast is unnecessary and can hide errors.  You should remove it.
>
>[e-mail] (Ray Dunn) wrote:
>
>> [...] adding a cast to a pointer of the same
>> type to the malloc return is the *safest* thing you can do:
>>
>>   fred = malloc(n * sizeof(int));
>>
>> Oops - fred isn't an "int *" it's a "long *", but the compiler wont
>> issue any warnings, but in:
>>
>>   fred = (int *)malloc(n * sizeof(int));
>>
>> the compiler will issue an error.

[e-mail] (Mike Rubenstein) replied :

>Why will the compiler issue an error in that case?  It's completely
>legal code.

From K&R2 section 5.4 :

  "It is not legal to [stuff about illegal pointer arithmatic], or even,
   except for void *, to assign a pointer of one type to a pointer of
   another type without a cast."

If you assign a int * to a long * the compiler can certainly pick that up.

>The error that can be hidden by a cast is omitting the declaration of
>malloc() and not including stdlib.h.  Without the cast a diagnostic is
>required since an int cannot be automatically converted to a pointer.
>With the cast the compiler will convert the int it thinks is returned
>by malloc() to a pointer and few compilers will give any indication of
>the error (read: none that I'm aware of).

I've also frequently seen the advice not to cast the return from malloc()
as it supposedly can hide errors, and have followed this advice. However,
having read what Ray Dunn wrote I thought I would try it out on my compiler
(DEC C - with default warnings) to see which cases get picked up.

Missing out #include <stdlib.h>, and with a cast on malloc() gives a
warning and information message as follows :

        long_ptr = (int *)malloc(sizeof(int));
        ..................^
%CC-I-IMPLICITFUNC, In this statement, the identifier "malloc" is implicitly
 declared as a function.

        long_ptr = (int *)malloc(sizeof(int));
        ^
%CC-W-PTRMISMATCH, In this statement, the referenced type of the pointer value
 "(int ...)malloc(...)" is "int", which is not compatible with "long".

I would call that fair warning that something is amiss.

Including #include <stdlib.h>, and without a cast on malloc() gives no
messages at all on "long_ptr = malloc(sizeof(int));" because a void pointer
(returned from malloc()) is being converted to a long pointer, which is
OK as far as the compiler is concerned.

Given the results of this, I think a may start casting the return from
malloc().

Phil Tregoning