Re: linux strlen man page accepts NULL?

[Jon Grant <jg-hus3n9K41k0@public.gmane.org>]

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. 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

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.

Best regards, Jon
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