C global anonymous struct / union

Question

I have a uint64 variable which often only requires high or low 32 bit access. I am using a 32-bit ARM Cortex M0, and to help with speed and I am trying to overlap the uint64 var

Answer 1

You define a union without an instance, which means that there is no union object containing the members. You could probably do something like this:

main.h:

typedef union {
  uint64_t ab;
  struct { uint32_t a, b; };
} u_t;

extern u_t u;

main.c:

u_t u = { .a = 1; };

And if you really wanted to (in main.h):

#define a u.a
#define b u.b
#define ab u.ab

If you do use #defines, be wary that they affect any declaration/use of the identifiers (a, b, ab), even those in a different scope. I recommend that instead you just access the values explictly via the u object (as u.a, u.b, u.ab).

I have removed volatile from the declarations because I highly doubt you really needed it. But you can certainly add it back if you wish.

(Note: the question originally had code split across two files, main.h and main.c. My answer correspondingly has code for two files. However, these could be combined into one easily).