Re: C++ string::find functions and size_t

[Rik]

On 02/27/2015 03:37 PM, Andreas Weber wrote:
> Am 27.02.2015 um 18:47 schrieb rik:
>> 2/27/15
>>
>> Dan Sebald found a subtle bug in the use of the C++ string find functions
>> that was leading to segfaults.
>>
>> -- Code --
>> size_t pos = file.find_first_not_of ("|");
>> if (pos > 0)
>>   file = file.substr (pos);
>> else
>> -- End Code --
>
> I think the idea here was to check if the first char in file is "|" and
> omit the check (because we are building a pipe) for an existent
> directory in the else path.
>
>> The issue is that the size_t is an unsigned quantity and the find functions
>> do not return -1 on failure as do regular C functions.  Instead they return
>> std::string::npos (a very large number) when they fail to find the search term.
>>
>> This was easily corrected to
>>
>> -- Code --
>> size_t pos = file.find_first_not_of ("|");
>> if (pos != std::string::npos)
>>   file = file.substr (pos);
>> else
>> -- End Code --
>
> Please correct me if I'm wrong but shouldn't this then be
>
> -- Code --
> size_t pos = file.find_first_not_of ("|");
> if (pos != std::string::npos && pos > 0)
> -- End Code --

I didn't check the purpose of the code, but if that is the case, shouldn't it be simpler to just write

-- Code --
if (! file.empty () && file[0] == '|')
  file = file.substr (1);  // Strip leading pipe character
else
-- End Code --

This avoids playing around with the position entirely.

I made that change to both drawnow and in __osmesa_print__ and the latter now passes the built-in self tests.  See cset http://hg.savannah.gnu.org/hgweb/octave/rev/6b7aee95c54c

--Rik