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From: "vapier at gentoo dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug other/52944] New: [4.5/4.6 Regression] __builtin_object_size(..., 1) no longer returns (size_t)-1 for consecutive flexible/zero-length array members
Date: Thu, 12 Apr 2012 03:50:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52944

             Bug #: 52944
           Summary: [4.5/4.6 Regression] __builtin_object_size(..., 1) no
                    longer returns (size_t)-1 for consecutive
                    flexible/zero-length array members
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vapier@gentoo.org
            Target: x86_64-linux-gnu


consider the code:

struct stct {
    int i;
    union {
        short k;
        char buf[0];
    };
    char tail[];
};
char buf[100];
main()
{
    struct stct *foo = (void *)buf;
    printf("%i\n", __builtin_object_size(foo->buf, 1));
}

when compiled with gcc-4.4, we get -1.  but with gcc-4.5 and gcc-4.6, we get 0.
 granted, this code is a bit odd, but in some cases, it makes sense.  imo, the
trailing series of flexible/zero-length array members should get the same
treatment rather than just the last one.  gcc doesn't allow flexible array
members inside of unions which is unfortunate.

with tftp, the packet is described by:
struct tftphdr {
    short opcode;
    union {
        unsigned short tu_block;
        short tu_code;
        char tu_stuff[0];
    };
    char th_data[];
};

when opcode is 1, the rest of the packet is a C string.  i.e. the buffer:
    char x[] = { 1, 0, 'f', 'i', 'l', 'e', '\0', };
    opcode = 1, tu_stuff = "file"

when opcode is 3 though, the tu_block field will be a number, and the rest of
the data will be in th_data.  i.e. the buffer:
    char x[] = { 2, 0, 3, 0, <8KiB>, };
    opcode = 2, tu_block = 3, th_data = 8KiB