Returning a pointer to a constant string

[=?iso-8859-1?q?Santiago_Urue=F1a?=]

Hi,

I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:

warning: assignment from incompatible pointer type

This is the code:


const char msg[] = "Test message";

const char *message(void) {
return msg;
}

int main(void){
const char * str;

str = (const char *)message;
str = (char *)message;
str = message; /* GCC warning! */

str = (const char *)msg;
str = (char *)msg;
str = msg;

return 0;
}

Oddly, GCC only gives the warning if no cast is used, but it doesn't
complain if the cast discards the const qualifier. Is this behavior
OK? I'm using GCC 4.1.2. Thanks!

Best regards,

Santi

[Walter Roberson]

Re: Returning a pointer to a constant string

In article <1190068695.241 389.292540@g4g2 000hsf.googlegr oups.com>,
=?iso-8859-1?q?Santiago_Ur ue=F1a?= <suruena@gmail. comwrote:

>I tried to return a pointer to a constant string, but the compiler
>gives the following warning if a cast is not used:

warning: assignment from incompatible pointer type

>This is the code:

>const char msg[] = "Test message";

>const char *message(void) {
return msg;
>}

Pay close attention to the placement of the const qualifiers.

const char msg[] says that msg[someindex] will be a const char

const char *message(void)

says that message will return a pointer to a char and that the
pointer is constant. (I think. I'm not -positive-. I haven't had
much occasion to use const.)
--
All is vanity. -- Ecclesiastes

[JimS]

Re: Returning a pointer to a constant string

On Mon, 17 Sep 2007 15:38:15 -0700, Santiago Urueña
<suruena@gmail. comwrote:

>Hi,
>
>I tried to return a pointer to a constant string, but the compiler
>gives the following warning if a cast is not used:
>
warning: assignment from incompatible pointer type
>
>This is the code:
>
>
>const char msg[] = "Test message";
>
>const char *message(void) {
return msg;
>}
>
>int main(void){
const char * str;
>
str = (const char *)message;
str = (char *)message;
str = message; /* GCC warning! */

message is a pointer to a function returning const char *. The
previous two casts are covering up a gratuitous mistake!

Jim

[=?iso-8859-1?q?Santiago_Urue=F1a?=]

Re: Returning a pointer to a constant string

str = (const char *)message;

str = (char *)message;
str = message; /* GCC warning! */

>
message is a pointer to a function returning const char *. The
previous two casts are covering up a gratuitous mistake!
>

You are right! Silly me.

Anyway, the compiler doesn't give any warning even if the qualifier is
discarded by the cast:

str = (const char *)message();
str = (char *)message(); /* No warning! */
str = message();

Is this OK?

Thanks again.

Santi

[=?iso-8859-1?q?Santiago_Urue=F1a?=]

Re: Returning a pointer to a constant string

Anyway, the compiler doesn't give any warning even if the qualifier is

discarded by the cast:
>
str = (const char *)message();
str = (char *)message(); /* No warning! */
str = message();
>
Is this OK?
>

After thinking a little more about this, I think I have the answer:
the compiler shouldn't give a warning because a pointer to a non-const
object is being assigned to a pointer to a const object, and this is
totally OK because (anyway you cannot modify the object via the 'str'
pointer).

Thanks

[Keith Thompson]

Re: Returning a pointer to a constant string

Santiago Urueña <suruena@gmail. comwrites:

str = (const char *)message;
str = (char *)message;
str = message; /* GCC warning! */

>>
>message is a pointer to a function returning const char *. The
>previous two casts are covering up a gratuitous mistake!
>>

You are right! Silly me.
>
Anyway, the compiler doesn't give any warning even if the qualifier is
discarded by the cast:
>
str = (const char *)message();
str = (char *)message(); /* No warning! */
str = message();
>
Is this OK?

Please leave attribution lines in place for quoted text (i.e., lines
like "So-and-so <foo@bar.comwri tes:"). They make it easier to
follow the conversation, and it's just polite to credit people for
their words.

A cast specifies a type conversion, but what it *really* does is tell
the compiler "I know exactly what I'm doing, don't bother me with
warnings". Because of that property, almost all casts should be
viewed with suspicion. Adding a cast for the sole purpose of
silencing a compiler warning is almost always a mistake; the correct
solution is usually to fix the code so the cast isn't required, either
by arranging for things to be of the desired type in the first place
or by using types that are converted implicitly.

One of the few cases where a cast is necessary is for some arguments
to variadic functions like printf(). For non-variadic functions, the
compiler knows the required type and is able to generate an implicit
conversion if necessary. For a variadic function, the compiler
doesn't necessarily have this information, so you have to give it some
help.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[CBFalconer]

Re: Returning a pointer to a constant string

Santiago Urueña wrote:

>
I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:
>
warning: assignment from incompatible pointer type
>
This is the code:
>
const char msg[] = "Test message";
>
const char *message(void) {
return msg;
}

msg is an array. &msg is a pointer to msg.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>


--
Posted via a free Usenet account from http://www.teranews.com

[Martin Ambuhl]

Re: Returning a pointer to a constant string

Santiago Urueña wrote:

Hi,
>
I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:
>
warning: assignment from incompatible pointer type
>
This is the code:
>
>
const char msg[] = "Test message";
>
const char *message(void) {
return msg;
}
>
int main(void){
const char * str;
>
str = (const char *)message;
str = (char *)message;
str = message; /* GCC warning! */
>
str = (const char *)msg;
str = (char *)msg;
str = msg;
>
return 0;
}

const char msg[] = "Test message";
/* msg (above) is a character array, but message is a function */
const char *message(void)
{
return msg;
}

int main(void)
{
const char *str;
str = message(); /* notice parenthesis; message is not a
pointer-to-char but a function
returning a (const) pointer-to-char */
str = msg;
return 0;
}

[anders]

Re: Returning a pointer to a constant string

Hmm as i see it
one type is a char [] and one is char *
that way
str = (char *)message();
works.
// Anders


On 18 Sep, 01:49, CBFalconer <cbfalco...@yah oo.comwrote:

Santiago Urueña wrote:
>

I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:

>

warning: assignment from incompatible pointer type

>

This is the code:

>

const char msg[] = "Test message";

>

const char *message(void) {
return msg;
}

>
msg is an array. &msg is a pointer to msg.
>
--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>
>
--
Posted via a free Usenet account fromhttp://www.teranews.co m

[Charlie Gordon]

Re: Returning a pointer to a constant string

"Walter Roberson" <roberson@ibd.n rc-cnrc.gc.caa écrit dans le message de
news: fcn02v$2j9$1@ca nopus.cc.umanit oba.ca...

In article <1190068695.241 389.292540@g4g2 000hsf.googlegr oups.com>,
=?iso-8859-1?q?Santiago_Ur ue=F1a?= <suruena@gmail. comwrote:
>

>>I tried to return a pointer to a constant string, but the compiler
>>gives the following warning if a cast is not used:

>

> warning: assignment from incompatible pointer type

>

>>This is the code:

>

>>const char msg[] = "Test message";

>

>>const char *message(void) {
> return msg;
>>}

>
Pay close attention to the placement of the const qualifiers.
>
const char msg[] says that msg[someindex] will be a const char

correct. It could also be written char const msg[].

const char *message(void)
>
says that message will return a pointer to a char and that the
pointer is constant.

No, such a pointer would be defined as char * const p; and it makes no sense
as a return value for a function.

(I think. I'm not -positive-. I haven't had much occasion to use const.)

It shows!
Please don't post misleading answers on subjects you know you don't master.

--
Chqrlie.

[Charlie Gordon]

Re: Returning a pointer to a constant string

"Santiago Urueña" <suruena@gmail. coma écrit dans le message de news:
1190068695.2413 89.292540@g4g20 00...legro ups.com...

Hi,
>
I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:
>
warning: assignment from incompatible pointer type
>
This is the code:
>
>
const char msg[] = "Test message";
>
const char *message(void) {
return msg;
}
>
int main(void){
const char * str;
>
str = (const char *)message;
str = (char *)message;
str = message; /* GCC warning! */
>
str = (const char *)msg;
str = (char *)msg;
str = msg;
>
return 0;
}
>
Oddly, GCC only gives the warning if no cast is used, but it doesn't
complain if the cast discards the const qualifier. Is this behavior
OK? I'm using GCC 4.1.2. Thanks!

Well well, gcc gives you a warning and you don't read it.
"assignment from incompatible pointer type" is not about the const
qualifier being discarded... after all str is a const char *, no const
qualifier is discard by these assignments, as the rest of the code shows.

The problem is much worse, and a good example of why casts must be avoided.
You are attempting to store a pointer to the function 'message', not the
result of an invocation because you missed the ().

str = message; is correctly diagnosed as an assignment from incompatible
pointer types.

By casting message to (char*) or (const char*), you are effectively telling
the compiler to shut up because you presumably know what you are doing (for
instance, you are trying to disassemble the function and you know the
respective representations of function pointers and char pointers are
compatible on your target). gcc respects that and does not complain.

But you are wrong. Your code invokes undefined behaviour.

Consequently: do not use casts. Turn extra gcc warnings on so it complains
about suspicious casts and a flew of other things.

--
Chqrlie.

[Charlie Gordon]

Re: Returning a pointer to a constant string

"CBFalconer " <cbfalconer@yah oo.coma écrit dans le message de news:
46EF127E.E35C44 72@yahoo.com...

Santiago Urueña wrote:

>>
>I tried to return a pointer to a constant string, but the compiler
>gives the following warning if a cast is not used:
>>
> warning: assignment from incompatible pointer type
>>
>This is the code:
>>
>const char msg[] = "Test message";
>>
>const char *message(void) {
> return msg;
>}

>
msg is an array. &msg is a pointer to msg.

But message returns a pointer to char, correctly constructed by 'return
msg;' or 'return &msg[0];'.

Returning &msg here would be abusing the rules.

--
Chqrlie.

[=?iso-8859-1?q?Santiago_Urue=F1a?=]

Re: Returning a pointer to a constant string

On 18 sep, 12:48, "Charlie Gordon" <n...@chqrlie.o rgwrote:

"Santiago Urueña" <suru...@gmail. coma écrit dans le message de news:
1190068695.2413 89.292...@g4g20 00hsf.googlegro ups.com...
>
>
>

Hi,

>

I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:

>

warning: assignment from incompatible pointer type

>

This is the code:

>

const char msg[] = "Test message";

>

const char *message(void) {
return msg;
}

>

int main(void){
const char * str;

>

str = (const char *)message;
str = (char *)message;
str = message; /* GCC warning! */

>

str = (const char *)msg;
str = (char *)msg;
str = msg;

>

return 0;
}

>

Oddly, GCC only gives the warning if no cast is used, but it doesn't
complain if the cast discards the const qualifier. Is this behavior
OK? I'm using GCC 4.1.2. Thanks!

>
Well well, gcc gives you a warning and you don't read it.
"assignment from incompatible pointer type" is not about the const
qualifier being discarded... after all str is a const char *, no const
qualifier is discard by these assignments, as the rest of the code shows.
>
The problem is much worse, and a good example of why casts must be avoided.
You are attempting to store a pointer to the function 'message', not the
result of an invocation because you missed the ().
>
str = message; is correctly diagnosed as an assignment from incompatible
pointer types.
>
By casting message to (char*) or (const char*), you are effectively telling
the compiler to shut up because you presumably know what you are doing (for
instance, you are trying to disassemble the function and you know the
respective representations of function pointers and char pointers are
compatible on your target). gcc respects that and does not complain.
>
But you are wrong. Your code invokes undefined behaviour.
>
Consequently: do not use casts. Turn extra gcc warnings on so it complains
about suspicious casts and a flew of other things.
>

Thank you very much for all your responses. Of course I see the
problem now, it seems I was too asleep for not seeing that silly
mistake.

I always use -Wall and -Wextra until all warnings are corrected. I'm
of those persons that believe that the compiler usually knows much
better, I don't really know why in this case the message seemed to me
a possible compiler bug.

Keith, Thanks for the suggestion about variadic functions, it's always
good to know tips like that. I don't usually use casts neither,
specially from now on... :-)

Best regards,

[=?iso-8859-1?q?Santiago_Urue=F1a?=]

Re: Returning a pointer to a constant string

On 18 sep, 01:49, CBFalconer <cbfalco...@yah oo.comwrote:

Santiago Urueña wrote:
>

I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:

>

warning: assignment from incompatible pointer type

>

This is the code:

>

const char msg[] = "Test message";

>

const char *message(void) {
return msg;
}

>
msg is an array. &msg is a pointer to msg.
>

I'm not a language lawyer, but the & operator returns the _address_ of
an object, not the pointer. But in this case 'msg' is directly the
address of the array.

I declared 'msg' as 'const char msg[] = "...";' and not as 'const char
*const msg = "...";' because in this case there is no need to reserve
memory for a pointer to the string, but just to the string itself.

Best regards,

Santi

[Keith Thompson]

Re: Returning a pointer to a constant string

Santiago Urueña <suruena@gmail. comwrites:

On 18 sep, 01:49, CBFalconer <cbfalco...@yah oo.comwrote:

>Santiago Urueña wrote:

I tried to return a pointer to a constant string, but the compiler
gives the following warning if a cast is not used:

>>

warning: assignment from incompatible pointer type

>>

This is the code:

>>

const char msg[] = "Test message";

>>

const char *message(void) {
return msg;
}

>>
>msg is an array. &msg is a pointer to msg.
>>

I'm not a language lawyer, but the & operator returns the _address_ of
an object, not the pointer. But in this case 'msg' is directly the
address of the array.

Given an object obj, the phrases "a pointer to obj" and "the address
of obj" are synonymous.

Since msg is declared as an array, the expression '&msg' is the
address *of the array*, whereas the expression 'msg' (in most
contexts) yields the address of the arrsy's first element, the same as
'&msg[0]'.

[...]

Section 6 of the comp.lang.c FAQ is an excellent resource.

There's a program called "cdecl" that you might also find useful (if
you can find and install it).

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"