Re: Bug in Newlib's snprintf implementation

[Jeffrey Walton <noloader at gmail dot com>]

On Fri, Feb 23, 2018 at 5:21 AM, Corinna Vinschen <vinschen@redhat.com> wrote:
> On Feb 23 05:12, Jeffrey Walton wrote:
>> On Fri, Feb 23, 2018 at 4:43 AM, Corinna Vinschen <vinschen@redhat.com> wrote:
>> > Slight correction: snprintf returns the size of the string it would
>> > have produced if it hadn't truncated the output.  So a size value
>> > of 4 ("test" would have been the desired result) is correct.
>>
>> I don't believe this is correct. The string of 'test' and the NULL
>> requires 5, not 4. 4 indicates truncation.
>
> 4 is correct:
>
>> According to the man page (https://linux.die.net/man/3/snprintf):
>>
>> <SNIP>
>> RETURN VALUE
>>
>> Upon successful return, these functions return the number of
>> characters printed (excluding the null byte used to end output to
>> strings).
>>
>> The functions snprintf() and vsnprintf() do not write more than size
>> bytes (including the terminating null byte ('\0')). If the output was
>> truncated due to this limit then the return value is the number of
>> characters (excluding the terminating null byte) which would have been
>               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>> written to the final string if enough space had been available. Thus,
>> a return value of size or more means that the output was truncated.
>> (See also below under NOTES.)
>> </SNIP>
>
> The desired result is a string with 4 chars, "test".  The return value
> reflects the desired result, *excluding* the NUL, if the truncation
> wouldn;t have been necessary.  That's 4.

Perhaps I am missing something obvious, so please forgive my ignorance.

The code is going to produce a string 'tes\0', not 'test\0'. The
string 'tes\0' is truncated, and the return value of 4 indicates that.
The return value should have been 5.

Again, my apologies if I am missing something obvious.

Jeff