https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70813
Bug ID: 70813
Summary: Wrong warning "'0' flag ignored with precision and
‘%d’ gnu_printf format"
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: ch3root at openwall dot com
Target Milestone: ---
When compiling this program:
#include <stdio.h>
int main()
{
printf("abc"
"%05.*d\n", -1, 1);
}
I get the following error message:
example.c: In function ‘main’:
example.c:5:10: warning: '0' flag ignored with precision and ‘%d’ gnu_printf
format [-Wformat=]
printf("abc"
^~~~~
The output from the program:
abc00001
There are two issues here.
1. Wrong position for the warning.
2. The warning itself is wrong in this corner case. Yes, '0' flag is ordinarily
ignored with precision and %d per C11, 7.21.6.1p6: "For d, i, o, u, x, and X
conversions, if a precision is specified, the 0 flag is ignored." But the
precision is negative here and C11, 7.21.6.1p5, reads: "A negative precision
argument is taken as if the precision were omitted."
The output from the program demonstrates that indeed '0' flag is not ignored.
Maybe it's better to limit this warning to cases where precision is known at
compile time (e.g., for simplicity, when it's not '*')?
This gcc bug was pointed out to me in
https://twitter.com/spun_off/status/723558401599524865 .
