That already exists, as a Microsoft extension, and if the struct is declared within the union, in standard C: https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Unnam

That already exists, as a Microsoft extension, and if the struct is declared within the union, in standard C: https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Unnamed-Fields.html https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Unnamed-Fields.... (However, in your example, c1.a is ambiguous, so it won't compile.)