Thanks Jonathan,
That certainly clarifies my confusion.
Much appreciated.
Pico
On Thu, Jun 22, 2017 at 4:31 PM, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
> On 22 June 2017 at 15:06, Pico Geyer wrote:
>> Hi all.
>>
>> I'm using g++ version 7.1.1 20170528 on Arch linux.
>> I've been rather surprised by a warning (turned into error by -Werror)
>> that only occurs when compiling with -O2
>> I've extracted this sample which seems to exhibit the same behaviour:
>> ---------------------------------------------
>> #include "stdio.h"
>> int get_amt()
>> {
>> int i;
>> int r = scanf("%d", &i);
>> if( r != 1) {
>> //Some error
>> }
>> return i;
>> }
>>
>> void standalone()
>> {
>> char str[10];
>> int amt = 0;
>> amt = get_amt();
>> /*XXX If I comment out this if block then no compile errors ?? */
>> if ( amt == 0 )
>> return;
>> snprintf( str, 10, "%d", (amt - 1) );
>> }
>> ---------------------------------------------
>> g++ -c -O2 -Werror -Wall test.cpp
>> test.cpp: In function ‘void standalone()’:
>> test.cpp:12:6: error: ‘%d’ directive output may be truncated writing
>> between 1 and 11 bytes into a region of size 10
>> [-Werror=format-truncation=]
>> void standalone()
>> ^~~~~~~~~~
>> test.cpp:12:6: note: directive argument in the range [-2147483648, 2147483646]
>> test.cpp:20:13: note: ‘snprintf’ output between 2 and 12 bytes into a
>> destination of size 10
>> snprintf( str, 10, "%d", (amt - 1) );
>> ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> ---------------------------------------------
>> However if I compile with -O1:
>> g++ -c -O1 -Werror -Wall test.cpp
>> #No errors.
>>
>> Could this be a g++ bug? (I didn't want to risk filing a bug if it isn't one)
>
> No.
>
>> If it is a legitimate warning, why is it only issued with -O2 ?
>
> Because the analysis that is needed to detect the bug only happens
> when certain optimisations are enabled.
>
>> I don't get this warning/error with older versions of g++ (Tested
>> 6.3.0 and 5.4).
>
> Because the -Wformat-overflow warnings are new in GCC 7, i.e. GCC got
> better. See https://gcc.gnu.org/gcc-7/changes.html#c-family
