gcc.info: Zero Length

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Arrays of Length Zero

   Zero-length arrays are allowed in GNU C.  They are very useful as
the last element of a structure which is really a header for a
variable-length object:
     struct line {
       int length;
       char contents[0];
     };
     {
       struct line *thisline = (struct line *)
         malloc (sizeof (struct line) + this_length);
       thisline->length = this_length;
     }
   In standard C, you would have to give `contents' a length of 1, which
means either you waste space or complicate the argument to `malloc'.