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From: "lavr at ncbi dot nlm.nih.gov" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/100403] New: Bogus "function may return address of local
 variable" warning
Date: Mon, 03 May 2021 18:52:00 +0000
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X-Bugzilla-Version: 10.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100403

            Bug ID: 100403
           Summary: Bogus "function may return address of local variable"
                    warning
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lavr at ncbi dot nlm.nih.gov
  Target Milestone: ---

$ gcc --version
gcc (GCC) 10.2.0


$ cat test.c
#include <stdio.h>
#include <string.h>

#define RECLEN  128


struct R {
    int  head;
    int  code;
    char text[1];
};


const char* fun(int n, const struct R* p)
{
    union {
        struct R r;
        char     rec[RECLEN];
    } x;
    const char* err =3D 0;

    memset(&x, 0, sizeof(x));
    if (p)
        memcpy(&x.r, p, sizeof(*p));
    else
        err =3D "Invalid argument";

    if (!err) {
        static const char magic[] =3D "MAGIC";
        const char* msg;
        if (memcmp(x.rec, magic, sizeof(magic)-1) =3D=3D 0)
            msg =3D x.rec;
        else if (x.r.text[0])
            msg =3D x.r.text;
        else
            msg =3D "Error w/code";
        if (msg)
            printf("%s\n", msg);
        if (x.rec <=3D msg  &&  msg < x.rec + sizeof(x))
            err =3D "Error detected";
        else
            err =3D msg;
    } else
        printf("%s\n", err);
    return err;
}


$ gcc -Wall -O2 -c test.c
test.c: In function =E2=80=98fun=E2=80=99:
test.c:45:12: warning: function may return address of local variable
[-Wreturn-local-addr]
   45 |     return err;
      |            ^~~
test.c:19:7: note: declared here
   19 |     } x;
      |       ^
test.c:19:7: note: declared here


Noted that does not matter whether "sizeof(x)" or "sizeof(x.rec)" is used at
the end of the "if()" statement on line 39.=