Re: strlen

[Jonny Grant <jg@jguk.org>]

On 06/07/2021 23:11, Florian Weimer wrote:
> * Jonny Grant:
> 
>> The reason it does not crash appears to be because of this warning
>> which removes the call to strlen due to the return not being
>> checked?
> 
> GCC uses the information that the argument to strlen cannot be null on
> that particular path.

It's a shame GCC doesn't give a warning

It may be GCC is using '__builtin_strlen'

<string.h> marks the param as nonnull. However, I am surprised this does not trigger the GCC warning -Werror=nonnull

/* Return the length of S.  */
extern size_t strlen (const char *__s)
     __THROW __attribute_pure__ __nonnull ((1));

Perhaps that is just a macro that is not actually used......

If I add another function -Werror=nonnull does give a warning
void test(const char * const p) __attribute__((nonnull));

https://godbolt.org/z/x37sbfWaG

<source>:15:9: error: argument 1 null where non-null expected [-Werror=nonnull]
   15 |     test(NULL);

> 
>> strlen.c:11:3: warning: statement with no effect [-Wunused-value]
>>    11 |   strlen (str);
>>       |   ^~~~~~~~~~~~
>>
>> https://godbolt.org/z/caoes5nGa
> 
> That site probably uses different library headers.
> 
> As posted, with Debian's GCC 8.3, I get this for the main function:
> 
> main:
> 	xorl	%eax, %eax
> 	ret
> 


In that case maybe https://man7.org/linux/man-pages/man3/strlen.3.html should have a "NOTES" section stating something similar to your wording ?

NOTES

The behavior of strlen(NULL) is depends on different things. It may cause a SEGV exception, or the compiler may optimize and remove the code path handling NULL.


Cheers, Jonny