Just don't try to have both a zero sized array and a flexible array in the same struct. g++ 5 or earlier will compile that, but g++ 6 will not: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69550

Also for external declarations:

   /* file scope */
   int a[];

   /* later in same translation unit, or in another one: */

   int a[42];

In this case, we get an "incomplete array type" after the first declaration. The storage size isn't known and so sizeof a cannot be computed.

This is also a de facto kind of "flexible array", but not formally named such.

For instance we can do this:

   /* In proprietary kernel source code */
   /* Size deliberately unspecified */
   extern struct driver driver_table[]; 

The proprietary kernel code is compiled into .o files and linked into a .a archive library.

To complete the build, a driver table module must be added and some drivers. The module which defines driver_table can be open-source. It defines the size of driver_table by how many entries there actually are; users can configure as many or few drivers and re-build the kernel, without having to recompile the proprietary part in the .a archive.

Unixes were shipped like this once upon a time. The "device major number" is just the position in the device table.

Formal arguments of array type are transformed into pointers.