Re: [PATCH] dlfcn: Avoid one-element flexible array in Dl_serinfo

[Paul Eggert <eggert at cs dot ucla dot edu>]

On 5/24/19 1:42 AM, Florian Weimer wrote:
> This changes the size of the type and is not source-code-compatible.  I
> have not investigated whether the change is still reasonably safe, but
> usually, wo do not make such changes.

OK, in that case I suggest adding a comment explaining the situation, 
since it is a bit of a sore thumb. Something like the following perhaps? 
Or if this problem is likely to occur elsewhere, we could package the 
situation up into a macro and just use the macro here.

    /* An array of dls_cnt elements, each of type Dl_serpath.  */
  #if 0
    /* With no backward-compatibility concerns we’d use the following
       C99 flexible array member.  However, as this data structure
       predates C99 it had to contain a one-element array here, and we
       don't want to change the struct's size now.  */
    Dl_serpath dls_serpath[];
  #elif defined __GNUC__
    /* Avoid an unwanted array subscript check by the compiler, while
       preserving the size of the type.  */
    __extension__ union
    {
      Dl_serpath dls_serpath[0]; /* Actually longer, dls_cnt elements.  */
      Dl_serpath __dls_serpath_pad[1];
    };
  #else
    Dl_serpath dls_serpath[1];
  #endif