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From: "modchipv12 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/89990] request warning: Use of out of scope compound literals
Date: Tue, 31 Mar 2020 02:34:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D89990

Andrew D'Addesio <modchipv12 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |modchipv12 at gmail dot com

--- Comment #5 from Andrew D'Addesio <modchipv12 at gmail dot com> ---
GCC already warns about this at compile time, but the warning sometimes doe=
sn't
appear (due to PR88058, as Andrew Pinski just mentioned). Plus, the warning=
 is
a bit confusing and could be reworded.

For example, create the following files:

foo.c:

    int foo(const unsigned char *buf)
    {
        (void)buf; /* unused parameter */
        return 1;
    }

test.c:

    int foo(const unsigned char *buf);

    struct mytype {
        char c;
    };

    static struct mytype d =3D { 42 };

    int test(int x)
    {
        const unsigned char buf[32];
        const struct mytype *ptr =3D &d;

        if (x !=3D 0)
            ptr =3D &(const struct mytype){ 43 };

        foo(buf);
    #ifdef CALL_FOO_TWICE
        foo(buf);
    #endif

        return ptr->c;
    }

    int main()
    {
        return test(1); /* returns 43 on GCC8, 0 on GCC9+ */
    }

Compiling with one foo() call gives us a warning:

    $ gcc -std=3Dc99 -Wall -Wextra -pedantic -O1 -o test test.c foo.c
    test.c: In function =E2=80=98main=E2=80=99:
    test.c:26:15: warning: =E2=80=98<U2cf0>.c=E2=80=99 is used uninitialize=
d in this function
[-Wuninitialized]
       26 |     return ptr->c;
          |            ~~~^~~
    $ ./test
    $ echo $?
    0

However, compiling with two foo() calls makes the warning disappear, for so=
me
reason:

    $ gcc -DCALL_FOO_TWICE -std=3Dc99 -Wall -Wextra -pedantic -O1 -o test t=
est.c
foo.c
    $ ./test
    $ echo $?
    0

My GCC version is 9.3.1 20200317 (Red Hat 9.3.1-1) on Fedora 31 x86-64.=