[Bug c/108518] New: Format-overflow warning using `*.s` directive with null but zero-length string

[Bug c/108518] New: Format-overflow warning using `*.s` directive with null but zero-length string

[aaron at aarongraham dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108518

            Bug ID: 108518
           Summary: Format-overflow warning using `*.s` directive with
                    null but zero-length string
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aaron at aarongraham dot com
  Target Milestone: ---

https://godbolt.org/z/YGra91Woa

#include <stdio.h>

int main() {
    // This causes a format-overflow warning, but it
    // should not warn if size() is 0
    printf("%.*s\n", 0, (char*)0);
}

The warning is:

<source>: In function 'int main()':
<source>:6:13: warning: '%.*s' directive argument is null [-Wformat-overflow=]
    6 |     printf("%.*s\n", 0, (char*)0);
      |             ^~~~

I see this commonly when using std::string_view with printf. In cases where it
knows that you're passing a default-constructed string_view it produces this
warning.

It should not produce this warning if the length being printed is 0.

[Bug c/108518] Format-overflow warning using `*.s` directive with null but zero-length string

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108518

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think it is still undefined if the argument is null even if the length of 0
is supplied from my reading of the standard:
If no l length modifier is present, the argument shall be a pointer to the
initial element of an array of character type.


If the precision is specified, no more than that many bytes are written.

[Bug c/108518] Format-overflow warning using `*.s` directive with null but zero-length string

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108518

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
In similar fashion memcpy/memset with a size argument of 0 passed, they have to
be a non-null pointer.