C语言中结构体封装全局变量用法说明

typedef	struct	global_data {
	bd_t		*bd;
	unsigned long	flags;
	unsigned long	baudrate;
	unsigned long	have_console;	
	unsigned long	reloc_off;	
	unsigned long	env_addr;	
	unsigned long	env_valid;	
	unsigned long	fb_base;	
#ifdef CONFIG_VFD
	unsigned char	vfd_type;	
#endif
#if 0
	unsigned long	cpu_clk;	
	unsigned long	bus_clk;
	phys_size_t	ram_size;	
	unsigned long	reset_status;	
#endif
	void		**jt;		
} gd_t;


#define DECLARE_GLOBAL_DATA_PTR     register volatile gd_t *gd asm ("r8") 
//定义了一个要放在寄存器r8中的全局变量，名字叫gd，类型是一个指向gd_t类型变量的指针
//这个gd在程序中经常被访问，因此放在register当中提升效率

	ulong gd_base;
	gd_base = CFG_UBOOT_BASE + CFG_UBOOT_SIZE - CFG_MALLOC_LEN - CFG_STACK_SIZE - sizeof(gd_t)
	gd = (gd_t*)gd_base; //强制类型转换，分配基地址

#ifdef GLOBAL_VAR
#define GLOBAL_EXTERN

#else
#define GLOBAL_EXTERN extern
#endif

GLOBAL_EXTERN int g_var1;
GLOBAL_EXTERN int g_var2;

#define GLOBAL_VAR
#include "global.h"
#undef GLOBAL_VAR