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From: "eb at emlix dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/106503] New: "const char []" in local scope never initialized
Date: Tue, 02 Aug 2022 07:31:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106503

            Bug ID: 106503
           Summary: "const char []" in local scope never initialized
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eb at emlix dot com
  Target Milestone: ---

Given the following test program:

------------
#include <sys/uio.h>
#include <string.h>

#define WRITEL(str) \
                do { \
                        wdata[wpos].iov_base =3D (void*)(str); \
                        wdata[wpos].iov_len =3D strlen(str); \
                        wlen +=3D wdata[wpos].iov_len; \
                        wpos++; \
                } while (0)

int main(int argc, char **argv)
{
        struct iovec wdata[20];
        unsigned int wpos =3D 0;
        ssize_t wlen =3D 0;
        int i =3D (argc > 1) ? 1 : 0;

        WRITEL("foo");
        if (argc) {
                const char junk[] =3D "abc";
                WRITEL(junk + i);
        } else {
                const char *junk =3D "def";
                WRITEL(junk + i);
        }
        WRITEL("baz\n");

        return writev(1, wdata, wpos) > 0 ? 0 : 1;
}
------------

For gcc 10 and before, and gcc 11, 12, or 13
(b06a282921c71bbc5cab69bc515804bd80f55e92) when used with -O0, this outputs:

$ ./Ch
fooabcbaz

>>From gcc 11 on when using -O1 or more it does not seem to initialize the "j=
unk"
buffer, so it may output random things:

$ ./Ch=20
foocbaz
$ ./Ch=20
foo=EF=BF=BDbaz
$ ./Ch=20
foo+baz
$ ./Ch=20
foo baz
$ ./Ch=20
foo[baz

I have seen the same behavior on both amd64 and sparc32, with distro compil=
ers
(openSUSE, Gentoo) as well as an unpatched gcc13 built with Gentoo ebuilds.=