如何定义指向结构的指针？

Question

I would like to define to the struct

typedef struct
{
    unsigned long GPFSEL[6];
    unsigned long Reserved_1;
    unsigned long GPSET[2];
    unsigned long Reserved_2;

//Ignoring the reserved and test bytes
} GPIO_REGS_;

One solution would be this

volatile  GPIO_REGS_ * const GPIO_REGS  = ((volatile GPIO_REGS_ *) 0x20200000UL);

In this case I can reach the register as follow:

GPIO_REGS->GPSET[0];

But how should I define the GPIO_REGS variable that I can use as follow

GPIO_REGS.GPSET[0];

Why the following won't work?

#define GPIO_REGS  (*(( GPIO_REGS_ *) 0x20200000UL));

Answer 1

GPIO_REGS_ my_gpio_reg;

GPIO_REGS_ * my_gpio_reg_ptr;

my_gpio_reg_ptr = &my_gpio_reg;

Answer 2

The problem with the code in your question

#define GPIO_REGS  (*(( GPIO_REGS_ *) 0x20200000UL));

is the semicolon at the end. You should remove it

#define GPIO_REGS  (*(( GPIO_REGS_ *) 0x20200000UL))

(The real lesson here is to avoid macros at all costs. They are notoriously tricky. Every time you think you understand them, you just get a false sense of security and then more buggy code is written!)

Anyway, with your original use of the macro:

GPIO_REGS.GPSET[0];

it was expanded as:

(*(( GPIO_REGS_ *) 0x20200000UL));.GPSET[0];

See that the semicolon doesn't fit here?

Keep your macros small. Don't put ; on the end. And always put brackets around them. The brackets should be the outermost thing - do not put a semicolon on the end. (We could write other warnings about macros all day long.