尝试从.c文件中的标头使用结构时，为什么会出现类型错误？

Question

I really don't understand what is happening here. I'm trying to access members of a struct in a .c file, but it's giving an 'error-type' when I try to access the struct variable. Anybody have any idea what's going on here?

CPU.h Header file:

#ifndef _CPU_H
#define _CPU_H

#include <stdint.h>

typedef struct cpu_registers
{
    union
    {
        struct
        {
            uint8_t f;
            uint8_t a;
        };
        uint16_t af;
    };
    union
    {
        struct
        {
            uint8_t c;
            uint8_t b;
        };
        uint16_t bc;
    };
} cpu_registers;

#endif /* _CPU_H */

CPU.c file:

#include "CPU.h"

cpu_registers regs;
regs.af = 0xFFFF;

Here are the errors upon compilation with clang:

CPU.c:4:1: error: unknown type name 'regs'
regs.af = 0xFFFF;
^
CPU.c:4:5: error: expected identifier or '('
regs.af = 0xFFFF;
    ^
2 errors generated.

Answer 1

You can declare and initialize global variables outside of functions, but you cannot do anything else with them.

So, you could do this:

cpu_registers regs = { .af = 0xFFFF };

However, do note that this will not work:

int val = 0xFFFF;
cpu_registers regs = { .af = val };

And - maybe a bit surprisingly - not this either:

const int val = 0xFFFF;
cpu_registers regs = { .af = val };