Re: linux strlen man page accepts NULL?

[Michael Kerrisk <mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>]

Hello Jon,

On Mon, Aug 10, 2009 at 2:06 PM, Jon Grant <jg-hus3n9K41k0@public.gmane.org> wrote:
> Hello
>
> 2009/8/7 Michael Kerrisk <mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>:
>> Jon,
>>
>> On Fri, Aug 7, 2009 at 1:55 PM, Jon Grant<jg-hus3n9K41k0@public.gmane.org> wrote:
>>> Looking at this man page:
>>>
>>> http://linux.die.net/man/3/strlen
>>>
>>> Should it not mention that a NULL address is a valid param? Or is it
>>> not a valid param?
>>
>> What makes you think it is a valid parameter?
>
> NULL points to 0x0, which could be mapped to something.

D'oh! Sorry -- now I'm with you. Yes, it could.

> On my embedded
> platform it is the beginning of the boot ROM. However typically 0x0 is
> an invalid address, in which case strlen should check for NULL, and
> return 0

I don't think it should check for this. If the addres is invalid, it
should be treated like any other invalid address -- usually a SIGSEGV
results.

> e.g.:
>
> size_t strlen(const char *str)
> {
>        const char *s;
>
> if(str == NULL)
> {
>    return 0;
> }
>
>        for (s = str; *s; ++s)
>                ;
>        return (s - str);
> }
>
>>> On most systems NULL is a special error pointer.
>>
>> I don't understand what you mean with this last sentence. Please explain.
>
> Well, the purpose of strlen is to count chars (excluding terminating
> '\0') but on most systems address 0x0 [which is what NULL is, (void*)0
> ].
>
> So if something is mapped by the hardware at 0x0 address then strlen
> could be used to count the number of characters at that location.
> However, on most systems his 0x0 address (NULL) indicates an invalid
> address.
>
> I am not on this mailing list, so please keep my email address in any replies.

As far as I can see, there's nothing in the documentation that needs
to be fixed. I'm doubtful that glibc's strlen() should be fixed
either. I agree that mappings at 0x0 are a strange corner case that
can produce unusual results in cases like these.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Watch my Linux system programming book progress to publication!
http://blog.man7.org/
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