[chbot] snprintf

[Paul Davey]

I think there is some bogging down in worrying about whether the size
parameter to snprintf is the size of the destination buffer or a
restriction of what is printed.  However for what the warning is telling
you it being a restriction on what is printed is sufficient to provide the
truncation warning.

As far as snprintf is concerned the size parameter may as well be the size
of the output buffer as it is not allowed to write more than that.

The warning is telling you that you told snprintf to write at most 30
characters to buf but if your ssid array contains a long enough string then
snprintf will produce truncated output as it will stop as soon as it would
have to write 30 characters into buf.

I would also note that truncation can be an unintended consequence.  Use of
snprintf merely avoids unintended memory write related consequences.

On Wed, 31 Jul 2024, 09:37 Charles Manning, <cdhmanning at gmail.com> wrote:

> There is a solid reason to use snprintf rather than sprintf.
>
> snprintf will prevent writing off the end of the buffer.
>
> char str[5];
>
> sprintf(str,"%s", "This is a long string which will overwrite the end of
> the buffer and cause unintended consequences");
> snprints(str, sizeof(str),"%s", "This is a long string but it will be
> truncated to fit in the buffer so no unintended consequences");
>
> On Wed, Jul 31, 2024 at 7:16 AM Simeon Pilgrim <simeon.pilgrim at gmail.com>
> wrote:
>
>> Ok, good point, if we ignore the history of the language. Ignore the
>> warning and what itis pointing. Heck let ignore the fact you read a terse
>> man page.
>>
>> Why are you use snprintf, instead of sprintf, and what did you want
>> tohappen, because the all the non string saffety trick are valid formatting
>> rules. Aka use %*s and two variables
>>
>> On 30 Jul 2024, at 11:55 PM, Robin Gilks <gb7ipd at gmail.com> wrote:
>>
>> 
>> Absolutely correct
>> As I quoted previously from the man page
>>
>> *The functions snprintf() and vsnprintf() write at most size bytes
>> (including the terminating null byte ('\0')) to str.*Note there is no
>> qualification of what 'size' is referring to, only that it is a restriction
>> of what is written, nothing to indicate that it's the length of str.
>> That's the whole problem - sloppy specifications!
>>
>> I have a standard question I always ask of people who are specifying
>> anything: "what colour should it be?"
>> This avoids the exclamation from the developer one day before delivery
>> of: "colour, colour - nobody told us it had to have a colour!!"
>>
>>
>> On Tue, Jul 30, 2024 at 10:00 PM Bevin Brett <bevin_brett at hotmail.com>
>> wrote:
>>
>>> Getting rid of the struct and the indexing, what you have is
>>>
>>>     char ssid[30];
>>>     char buf[100]
>>>
>>>     snprintf (buf, sizeof (ssid), "SSID %s\n", ssid);
>>>
>>> which is a request to write 5 chars ("SSID ") followed by any number of
>>> chars ending in a NUL  (perhaps overrunning the end of ssid if there are no
>>> NUL in it) followed by \n and a trailing NUL into the first upto 30
>>> characters of buf.      The warning is assuming (incorrectly) that ssid
>>> will contain less than 30 non-NULL characters, hence decides may need upto
>>> 37 characters (should have been 36) (5 + 29 + 1 + 1) and you have only
>>> supplied 30
>>>
>>> /Bevin
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