This is right - a flexible array member is denoted with []. If you update the OPs example to try and put two of them in, you get this error from GCC: x.c:7:

This is right - a flexible array member is denoted with []. If you update the OPs example to try and put two of them in, you get this error from GCC: x.c:7:10: error: flexible array member not at end of struct