C语言中利用#define和#undef定义初始化数组，实现不同的初始化效果

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本文通过Linux 2.6.22中u盘代码的例子，展示了如何利用C语言预处理器#define和#undef定义不同类型的数组进行初始化。通过切换#define的值，可以实现整型变量a1和字符型变量a2分别用同一个宏AAA初始化，从而达到灵活赋值的效果。

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Linux 2.6.22的u盘代码实现中有如下代码

第一种宏定义

#ifndef CONFIG_USB_LIBUSUAL

#define UNUSUAL_DEV(id_vendor, id_product, bcdDeviceMin, bcdDeviceMax, \
		    vendorName, productName,useProtocol, useTransport, \
		    initFunction, flags) \
{ USB_DEVICE_VER(id_vendor, id_product, bcdDeviceMin,bcdDeviceMax), \
  .driver_info = (flags)|(USB_US_TYPE_STOR<<24) }

#define USUAL_DEV(useProto, useTrans, useType) \
{ USB_INTERFACE_INFO(USB_CLASS_MASS_STORAGE, useProto, useTrans), \
  .driver_info = (USB_US_TYPE_STOR<<24) }

static struct usb_device_id storage_usb_ids [] = {

#	include "unusual_devs.h"
#undef UNUSUAL_DEV
#undef USUAL_DEV
	/* Terminating entry */
	{ }
};

MODULE_DEVICE_TABLE (usb, storage_usb_ids);
#endif /* CONFIG_USB_LIBUSUAL */

第二种宏定义

#define UNUSUAL_DEV(idVendor, idProduct, bcdDeviceMin, bcdDeviceMax, \
		    vendor_name, product_name, use_protocol, use_transport, \
		    init_function, Flags) \
{ \
	.vendorName = vendor_name,	\
	.productName = product_name,	\
	.useProtocol = use_protocol,	\
	.useTransport = use_transport,	\
	.initFunction = init_function,	\
}

#define USUAL_DEV(use_protocol, use_transport, use_type) \
{ \
	.useProtocol = use_protocol,	\
	.useTransport = use_transport,	\
}

static struct us_unusual_dev us_unusual_dev_list[] = {
#	include "unusual_devs.h" 
#	undef UNUSUAL_DEV
#	undef USUAL_DEV

	/* Terminating entry */
	{ NULL }
};

unusual_devs.h中内容大致如下

UNUSUAL_DEV( 0x07cf, 0x1001, 0x1000, 0x9999,
		"Casio",
		"QV DigitalCamera",
		US_SC_8070, US_PR_CB, NULL,
		US_FL_NEED_OVERRIDE | US_FL_FIX_INQUIRY ),

/* Submitted by Hartmut Wahl <hwahl@hwahl.de>*/
UNUSUAL_DEV( 0x0839, 0x000a, 0x0001, 0x0001,
		"Samsung",
		"Digimax 410",
		US_SC_DEVICE, US_PR_DEVICE, NULL,
		US_FL_FIX_INQUIRY),

UNUSUAL_DEV(  0x084d, 0x0011, 0x0110, 0x0110,
		"Grandtech",
		"DC2MEGA",
		US_SC_DEVICE, US_PR_DEVICE, NULL,
		US_FL_BULK32),

/* Submitted by Jan De Luyck <lkml@kcore.org> */
UNUSUAL_DEV(  0x08bd, 0x1100, 0x0000, 0x0000,
		"CITIZEN",
		"X1DE-USB",
		US_SC_DEVICE, US_PR_DEVICE, NULL,
		US_FL_SINGLE_LUN),

/* Control/Bulk transport for all SubClass values */
USUAL_DEV(US_SC_RBC, US_PR_CB, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_8020, US_PR_CB, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_QIC, US_PR_CB, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_UFI, US_PR_CB, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_8070, US_PR_CB, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_SCSI, US_PR_CB, USB_US_TYPE_STOR),

/* Control/Bulk/Interrupt transport for all SubClass values */
USUAL_DEV(US_SC_RBC, US_PR_CBI, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_8020, US_PR_CBI, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_QIC, US_PR_CBI, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_UFI, US_PR_CBI, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_8070, US_PR_CBI, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_SCSI, US_PR_CBI, USB_US_TYPE_STOR),

/* Bulk-only transport for all SubClass values */
USUAL_DEV(US_SC_RBC, US_PR_BULK, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_8020, US_PR_BULK, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_QIC, US_PR_BULK, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_UFI, US_PR_BULK, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_8070, US_PR_BULK, USB_US_TYPE_STOR),
USUAL_DEV(US_SC_SCSI, US_PR_BULK, 0),

如上就实现了用同一个文件实现了us_unusual_dev_list和storage_usb_ids不同结构体类型数组的初始化

使用方法

#define AAA2

int a1 = AAA;

#undef AAA

#define AAA'2'

char a2= AAA;

这样a1和a2的不同类型，却都可以使用AAA来初始化