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From: "quentin at armitage dot org.uk" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101451] Incorrect -Wstringop-truncation warning
Date: Thu, 12 Aug 2021 21:05:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101451

--- Comment #5 from Quentin Armitage <quentin at armitage dot org.uk> ---
In the code of my original example (which I have simplified)

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
#include <string.h>

static char dest[16];
static char src[16] =3D "012345678901234";

int main(__attribute__(void)
{
  do { dest[0] =3D '\0'; strncat(dest, src, sizeof(dest) - 1); } while (0);
}
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D

there is no truncation since 15 bytes are copied to dest, with a NUL byte
added, and the maximum length of a string in src is 15 bytes plus terminati=
ng
NUL byte.

Also, the warning is not generated if -O2 is not specified, but it is gener=
ated
if -O2 is specified (-O1 does not generate the warning). To me this doesn't
seem right.=