C99 Question

[Vijay Kumar R Zanvar]

Hi,

Which section of C99 says that return value
of malloc(3) should not be casted?

Thanks.

--
Vijay Kumar R Zanvar
My Home Page - http://www.geocities.com/vijoeyz/

[Martin Dickopp]

Re: C99 Question

"Vijay Kumar R Zanvar" <vijoeyz@hotpop .com> writes:
[color=blue]
> Which section of C99 says that return value
> of malloc(3) should not be casted?[/color]

None. The standard only defines the language, it does not advise on
good programming style.

Martin

[Richard Heathfield]

Re: C99 Question

Vijay Kumar R Zanvar wrote:
[color=blue]
> Hi,
>
> Which section of C99 says that return value
> of malloc(3) should not be casted?[/color]

The same section that says you should not microwave your cat.

Casting malloc is deprecated by the regular subscribers in this newsgroup
because:

(a) it does nothing good;
(b) it doesn't stop anything bad happening;
(c) it can (in certain circumstances in C90) actually /cause/ something bad
to happen.

Even if you're not persuaded by (a) and (b) together (which you probably
should be), (c) should be a showstopper.

The malloc function - Just Don't Cast.

--
Richard Heathfield : binary@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

[lallous]

Re: C99 Question

Hello,
"Richard Heathfield" <invalid@addres s.co.uk.invalid > wrote in message
news:3ff15532@n ews2.power.net. uk...[color=blue]
> Vijay Kumar R Zanvar wrote:
>
>
> The malloc function - Just Don't Cast.[/color]
This might be silly from me, but you mean don't cast as:
int *i = (int *) malloc(sizeof(i nt));
?
What other solutions are there then? just assign w/o casting?
[color=blue]
>
> --
> Richard Heathfield : binary@eton.pow ernet.co.uk
> "Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
> C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
> K&R answers, C books, etc: http://users.powernet.co.uk/eton[/color]

[Richard Bos]

Re: C99 Question

"Vijay Kumar R Zanvar" <vijoeyz@hotpop .com> wrote:
[color=blue]
> Which section of C99 says that return value
> of malloc(3) should not be casted?[/color]

There is no section of C99 that says that; if there were, you wouldn't
be able to do it. It's a very bad idea all the same.
In fact, it's similar to there not being any law forbidding you to shoot
yourself in the foot. You _are_ allowed to do that; but expect it to
hurt, and do not expect your insurance to pay for the medical costs.

Richard

[Richard Heathfield]

Re: C99 Question

lallous wrote:
[color=blue]
> Hello,
> "Richard Heathfield" <invalid@addres s.co.uk.invalid > wrote in message
> news:3ff15532@n ews2.power.net. uk...[color=green]
>> Vijay Kumar R Zanvar wrote:
>>
>>
>> The malloc function - Just Don't Cast.[/color]
> This might be silly from me, but you mean don't cast as:
> int *i = (int *) malloc(sizeof(i nt));
> ?[/color]

Yes, I mean that this cast is pointless and can be harmful.
[color=blue]
> What other solutions are there then? just assign w/o casting?[/color]

T *p = malloc(n * sizeof *p);

works for any object type T. You can omit n if it's 1, of course.

Your example would be better written as: int *i = malloc(sizeof *i);

--
Richard Heathfield : binary@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

[pete]

Re: C99 Question

Vijay Kumar R Zanvar wrote:[color=blue]
>
> Hi,
>
> Which section of C99 says that return value
> of malloc(3) should not be casted?[/color]

How about an errata from K&R2 instead ?

142(§6.5, toward the end):
The remark about casting the return value of malloc
("the proper method is to declare ... then explicitly coerce")
needs to be rewritten. The example is correct and works,
but the advice is debatable in the context of the
1988-1989 ANSI/ISO standards. It's not necessary
(given that coercion of void * to ALMOSTANYTYPE * is automatic),
and possibly harmful if malloc,
or a proxy for it, fails to be declared as returning void *.
The explicit cast can cover up an unintended error.
On the other hand, pre-ANSI, the cast was necessary,
and it is in C++ also.

http://cm.bell-labs.com/cm/cs/cbook/2ediffs.html

--
pete

[pete]

Re: C99 Question

Richard Heathfield wrote:[color=blue]
>
> lallous wrote:
>[color=green]
> > Hello,
> > "Richard Heathfield" <invalid@addres s.co.uk.invalid >
> > wrote in message
> > news:3ff15532@n ews2.power.net. uk...[color=darkred]
> >> Vijay Kumar R Zanvar wrote:
> >>
> >>
> >> The malloc function - Just Don't Cast.[/color]
> > This might be silly from me, but you mean don't cast as:
> > int *i = (int *) malloc(sizeof(i nt));
> > ?[/color]
>
> Yes, I mean that this cast is pointless and can be harmful.
>[color=green]
> > What other solutions are there then? just assign w/o casting?[/color]
>
> T *p = malloc(n * sizeof *p);
>
> works for any object type T. You can omit n if it's 1, of course.
>
> Your example would be better written as: int *i = malloc(sizeof *i);[/color]

Neither of your posts in this thread, mention stdlib.h.
In order to write the lines that you wrote, there has to be

#include <stdlib.h>

in the code, somewhere before those lines,
otherwise the compiler will generate a warning which,
(though the warning signifies an absence of stdlib.h to us,)
may suggest a cast, and confuse the programmer into thinking
that the return value of malloc should be cast.
And who's the programmer going to believe, clc or his compiler ?

--
pete

[Richard Heathfield]

Re: C99 Question

pete wrote:
[color=blue]
> Richard Heathfield wrote:[color=green]
>>[/color][/color]

<snip>
[color=blue][color=green]
>> Your example would be better written as: int *i = malloc(sizeof *i);[/color]
>
> Neither of your posts in this thread, mention stdlib.h.[/color]

Thanks for mentioning <stdlib.h> which, of course, provides the essential
function prototype that is so critical to correct malloc usage.
[color=blue]
> And who's the programmer going to believe, clc or his compiler ?[/color]

If he is wise, he will believe clc (and then he will find out precisely
*why* his compiler is right).

--
Richard Heathfield : binary@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

[lallous]

Re: C99 Question

Hello,

"Richard Heathfield" <invalid@addres s.co.uk.invalid > wrote in message
news:3ff16c4c@n ews2.power.net. uk...[color=blue]
> pete wrote:
>[color=green]
> > Richard Heathfield wrote:[color=darkred]
> >>[/color][/color]
>
> <snip>
>[color=green][color=darkred]
> >> Your example would be better written as: int *i = malloc(sizeof *i);[/color]
> >
> > Neither of your posts in this thread, mention stdlib.h.[/color]
>
> Thanks for mentioning <stdlib.h> which, of course, provides the essential
> function prototype that is so critical to correct malloc usage.
>[color=green]
> > And who's the programmer going to believe, clc or his compiler ?[/color]
>
> If he is wise, he will believe clc (and then he will find out precisely
> *why* his compiler is right).[/color]


I use the cast to avoid warning or when using malloc() in a C++ program.

The stdlib.h would remove the warning? How, compiler switch (like vc's
#pragma warn(disable:n) )?

--
Elias[color=blue]
> --
> Richard Heathfield : binary@eton.pow ernet.co.uk
> "Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
> C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
> K&R answers, C books, etc: http://users.powernet.co.uk/eton[/color]

[Richard Bos]

Re: C99 Question

"lallous" <lallous@lgwm.o rg> wrote:
[color=blue]
> "Richard Heathfield" <invalid@addres s.co.uk.invalid > wrote in message
> news:3ff16c4c@n ews2.power.net. uk...[color=green]
> > Thanks for mentioning <stdlib.h> which, of course, provides the essential
> > function prototype that is so critical to correct malloc usage.[/color][/color]
[color=blue]
> I use the cast to avoid warning or when using malloc() in a C++ program.[/color]

- don't use malloc() in a C++ program, use new;
- don't confound C with C++, and least of all good C with good C++;
- therefore, don't apply a C++ hack-around to proper C code.
[color=blue]
> The stdlib.h would remove the warning? How, compiler switch (like vc's
> #pragma warn(disable:n) )?[/color]

Of course not - by the simple expedient of providing a proper
declaration for malloc() and friends. As all good C code should do for
all used functions.

Richard

[Servé Lau]

Re: C99 Question

"lallous" <lallous@lgwm.o rg> wrote in message
news:bsrr2u$o7i i$1@ID-161723.news.uni-berlin.de...[color=blue]
> I use the cast to avoid warning or when using malloc() in a C++ program.[/color]

First, you should use new in a C++ program, and you don't have to cast new.

Second, the whole point is that if you forget to include stdlib.h and you
try to use malloc, the compiler lets malloc return int instead of void *. If
you don't cast the return value, the compiler should give an error that
there is no automatic conversion from int to pointer type. If you do cast
the return value, the compiler thinks you are saying "shut up, I know what
I'm doing" and will happily generate some code that converts an int to a
pointer. This causes big problemos on platforms where sizeof(int) != sizeof
(void *) and 64 bit platforms are coming pretty soon.
[color=blue]
> The stdlib.h would remove the warning? How, compiler switch (like vc's
> #pragma warn(disable:n) )?[/color]

Do not remove the warning, write correct code.

[pete]

Re: C99 Question

lallous wrote:[color=blue]
>
> Hello,
>
> "Richard Heathfield" <invalid@addres s.co.uk.invalid > wrote in message
> news:3ff16c4c@n ews2.power.net. uk...[color=green]
> > pete wrote:
> >[color=darkred]
> > > Richard Heathfield wrote:
> > >>[/color]
> >
> > <snip>
> >[color=darkred]
> > >> Your example would be better written as: int *i = malloc(sizeof *i);
> > >
> > > Neither of your posts in this thread, mention stdlib.h.[/color]
> >
> > Thanks for mentioning <stdlib.h> which, of course, provides the essential
> > function prototype that is so critical to correct malloc usage.
> >[color=darkred]
> > > And who's the programmer going to believe, clc or his compiler ?[/color]
> >
> > If he is wise, he will believe clc
> > (and then he will find out precisely *why* his compiler is right).[/color]
>
> I use the cast to avoid warning or when using
> malloc() in a C++ program.[/color]

The rumors are, that well written C++ programs
which look like C++ programs, don't use malloc.
[color=blue]
> The stdlib.h would remove the warning? How, compiler switch (like vc's
> #pragma warn(disable:n) )?[/color]

You need to understand the reason for the warning.

malloc returns type void*. The prototype for malloc is in stdlib.h.
If a function, like malloc, doesn't have a prototype in scope,
then the compiler will assume that the function returns type int.
If you assign an int to a pointer, you get a warning or an error.

If stdlib.h is #included, then the compiler will know that
malloc returns type void*. When you assign a void* value
to any type of pointer to an object, there is no problem, no warning.

--
pete

[lallous]

Re: C99 Question


"pete" <pfiland@mindsp ring.com> wrote in message
news:3FF17BE3.5 44@mindspring.c om...[color=blue]
> lallous wrote:[color=green]
> >
> > Hello,
> >
> > "Richard Heathfield" <invalid@addres s.co.uk.invalid > wrote in message
> > news:3ff16c4c@n ews2.power.net. uk...[color=darkred]
> > > pete wrote:
> > >
> > > > Richard Heathfield wrote:
> > > >>
> > >
> > > <snip>
> > >
> > > >> Your example would be better written as: int *i = malloc(sizeof[/color][/color][/color]
*i);[color=blue][color=green][color=darkred]
> > > >
> > > > Neither of your posts in this thread, mention stdlib.h.
> > >
> > > Thanks for mentioning <stdlib.h> which, of course, provides the[/color][/color][/color]
essential[color=blue][color=green][color=darkred]
> > > function prototype that is so critical to correct malloc usage.
> > >
> > > > And who's the programmer going to believe, clc or his compiler ?
> > >
> > > If he is wise, he will believe clc
> > > (and then he will find out precisely *why* his compiler is right).[/color]
> >
> > I use the cast to avoid warning or when using
> > malloc() in a C++ program.[/color]
>
> The rumors are, that well written C++ programs
> which look like C++ programs, don't use malloc.
>[color=green]
> > The stdlib.h would remove the warning? How, compiler switch (like vc's
> > #pragma warn(disable:n) )?[/color]
>
> You need to understand the reason for the warning.
>
> malloc returns type void*. The prototype for malloc is in stdlib.h.
> If a function, like malloc, doesn't have a prototype in scope,
> then the compiler will assume that the function returns type int.[/color]
Yes, I know that.
[color=blue]
> If you assign an int to a pointer, you get a warning or an error.
>
> If stdlib.h is #included, then the compiler will know that
> malloc returns type void*. When you assign a void* value
> to any type of pointer to an object, there is no problem, no warning.[/color]
I know that C++ uses new, however C++ enforces type casting and that is why
I cast return value of malloc() to the desired type. C would access to
convert void* to any other pointer.

Anyway, thank you all for the info.
Elias[color=blue]
>
> --
> pete[/color]

[pete]

Re: C99 Question

lallous wrote:[color=blue]
>
> "pete" <pfiland@mindsp ring.com> wrote in message
> news:3FF17BE3.5 44@mindspring.c om...[color=green]
> > lallous wrote:[color=darkred]
> > >
> > > Hello,
> > >
> > > "Richard Heathfield" <invalid@addres s.co.uk.invalid > wrote in message
> > > news:3ff16c4c@n ews2.power.net. uk...
> > > > pete wrote:
> > > >
> > > > > Richard Heathfield wrote:
> > > > >>
> > > >
> > > > <snip>
> > > >
> > > > >> Your example would be better written as: int *i = malloc(sizeof[/color][/color]
> *i);[color=green][color=darkred]
> > > > >
> > > > > Neither of your posts in this thread, mention stdlib.h.
> > > >
> > > > Thanks for mentioning <stdlib.h> which, of course, provides the[/color][/color]
> essential[color=green][color=darkred]
> > > > function prototype that is so critical to correct malloc usage.
> > > >
> > > > > And who's the programmer going to believe, clc or his compiler ?
> > > >
> > > > If he is wise, he will believe clc
> > > > (and then he will find out precisely *why* his compiler is right).
> > >
> > > I use the cast to avoid warning or when using
> > > malloc() in a C++ program.[/color]
> >
> > The rumors are, that well written C++ programs
> > which look like C++ programs, don't use malloc.
> >[color=darkred]
> > > The stdlib.h would remove the warning? How, compiler switch (like vc's
> > > #pragma warn(disable:n) )?[/color]
> >
> > You need to understand the reason for the warning.
> >
> > malloc returns type void*. The prototype for malloc is in stdlib.h.
> > If a function, like malloc, doesn't have a prototype in scope,
> > then the compiler will assume that the function returns type int.[/color]
> Yes, I know that.
>[color=green]
> > If you assign an int to a pointer, you get a warning or an error.
> >
> > If stdlib.h is #included, then the compiler will know that
> > malloc returns type void*. When you assign a void* value
> > to any type of pointer to an object, there is no problem,
> > no warning.[/color]
> I know that C++ uses new, however C++ enforces type
> casting and that is why
> I cast return value of malloc() to the desired type. C would access to
> convert void* to any other pointer.
>
> Anyway, thank you all for the info.[/color]

And so, to sum up, the big advantage of not casting,
is that the warning will alert the programmer if the code
has changed in some way such that the prototype for malloc
is no longer in scope,
which is something that really does happen in real programs.

Not having the prototype in scope, gives your code undefined behavior,
which means that it might work right in your tests
and still have problems in the field.

--
pete