Re: strlen

[Florian Weimer <fw@deneb.enyo.de>]

* Jonny Grant:

> On 04/09/2020 20:21, Florian Weimer wrote:
>> * Jonny Grant:
>> 
>>> Hello
>>>
>>> https://man7.org/linux/man-pages/man3/strlen.3.html
>>>
>>> Is it possible to clarify :-
>>>
>>> * glibc will SIGSEGV if 's' is NULL
>>> * there are no ERROR returns
>> 
>> That would be misleading.  Whether strlen (NULL) is undefined also
>> depends on the compiler.  They know that the argument cannot be zero
>> and optimize accordingly.
>> 
>
> Hi,
>
> Do you know a compiler that has a different behaviour? I only tested
> gcc and clang. How would a compiler optimise?

Here's an example:

#include <stddef.h>
#include <stdio.h>
#include <string.h>

void
f (const char *str)
{
  strlen (str);
  if (str == NULL)
    puts ("str is NULL");
}

int
main (void)
{
  f (NULL);
  return 0;
}

When built with -O2, GCC 8 prints nothing, and there is no crash.

My point it's not just the C *library* that makes strlen (NULL)
undefined.  It's undefined according to the language, and even if we
changed the glibc implementation, things would still go wrong.