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From: "aaron at aarongraham dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/108518] New: Format-overflow warning using `*.s` directive
 with null but zero-length string
Date: Tue, 24 Jan 2023 14:01:07 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 13.0
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X-Bugzilla-Who: aaron at aarongraham dot com
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108518

            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-overflo=
w=3D]
    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.=