Anonymous union within struct not in c99?

Question

here is very simplified code of problem I have:

 enum node_type {     t_int, t_double };  struct int_node {     int value; };  struct double_node {     double value; };  struct node {     enum node_type type;     union {         struct int_node int_n;         struct double_node double_n;     }; };  int main(void) {     struct int_node i;     i.value = 10;     struct node n;     n.type = t_int;     n.int_n = i;     return 0; } 

And what I don't undestand is this:

 $ cc us.c  $ cc -std=c99 us.c  us.c:18:4: warning: declaration does not declare anything us.c: In function ‘main’: us.c:26:4: error: ‘struct node’ has no member named ‘int_n’ 

Using GCC without -std option compiles code above without any problems (and the similar code is working pretty well), but it seems that c99 does not permit this technique. Why is it so and is it possible to make is c99 (or c89, c90) compatible? Thanks.

Answer 1

Anonymous unions are a GNU extension, not part of any standard version of the C language. You can use -std=gnu99 or something like that for c99+GNU extensions, but it's best to write proper C and not rely on extensions which provide nothing but syntactic sugar...

Edit: Anonymous unions were added in C11, so they are now a standard part of the language. Presumably GCC's -std=c11 lets you use them.

Answer 2

I'm finding this question about a year and a half after everybody else did, so I can give a different answer: anonymous structs are not in the C99 standard, but they are in the C11 standard. GCC and clang already support this (the C11 standard seems to have lifted the feature from Microsoft, and GCC has provided support for some MSFT extensions for some time).