《C预处理》Linux内核中可变参数宏的用法

C语言预处理包括：宏定义、文件包含、条件编译等；预处理命令都以#开头：宏(#define)、文件包含(#include)、条件编译(#ifdef或#ifndef)。

本节主要分析宏定义的特殊用法。

一、#与##

1.#

#符号用作一个预处理运算符，它可以把语言符号转换成字符串。

eg：x是一个宏参数，那么#x可以把参数名转换成相应的字符串，这个过程叫字符串化。

test1.c

#include <stdio.h>#define PSQR(x) printf("the square of" #x "is %d.\n",(x)*(x))int main(void){    int y =4;    PSQR(y);    PSQR(2+4);    return 0;}/*result:the square ofyis 16.the square of2+4is 36.*/

gcc test1.c -o test1

./test1

the square ofyis 16.the square of2+4is 36.

2.##

该运算符把两个语言符号组合成单个语言符号。

test2.c

#include <stdio.h>#define XNAME(n) x##n#define PXN(n) printf("x"#n" = %d\n",x##n)int main(void){    int XNAME(1)=12;//int x1=12;    PXN(1);//printf("x1 = %d\n", x1);    return 0;}/*result:x1 = 12*/

gcc test2.c -o test2

./test2

x1 = 12

二、可变参数宏与__VA_ARGS__

__VA_ARGS__是一个可变参数宏，是新的C99规范中新增的，目前只有gcc支持（VC6.0的编译器不支持）。

宏定义中参数列表的最后一个参数为省略号（也就是三个点）。这样预定义宏_ _VA_ARGS_ _就可以被用在替换部分中，替换省略号所代表的字符串。

简单例子：

test3.c

#include <stdio.h>#define PR(...) printf(__VA_ARGS__)int main(){    int wt=1,sp=2;    PR("hello\n");    PR("weight = %d, shipping = %d\n",wt,sp);    return 0;}/*result:helloweight = 1, shipping = 2*/

gcc test3.c -o test3

./test3

helloweight = 1, shipping = 2

三、在Linux内核系统调用中的应用

公共接口：

common.h（相当与内核中的systemcalls.h）

#define __SC_DECL1(t1, a1)    t1 a1#define __SC_DECL2(t2, a2, ...) t2 a2, __SC_DECL1(__VA_ARGS__)#define __SC_DECL3(t3, a3, ...) t3 a3, __SC_DECL2(__VA_ARGS__)#define __SC_DECL4(t4, a4, ...) t4 a4, __SC_DECL3(__VA_ARGS__)#define __SC_DECL5(t5, a5, ...) t5 a5, __SC_DECL4(__VA_ARGS__)#define __SYSCALL_DEFINEx(x, name, ...)\ long sys##name(__SC_DECL##x(__VA_ARGS__))#define SYSCALL_DEFINEx(x, sname, ...)\__SYSCALL_DEFINEx(x, sname, __VA_ARGS__)#define SYSCALL_DEFINE0(name)   long sys_##name(void)#define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__)#define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)

1.无参数

test4.c

#include <stdio.h>#include "common.h"long sys_tank(void);long sys_tankai(void);SYSCALL_DEFINE0(tank){  printf("TK--------_>>>>>>tank call ok!!!\n");}SYSCALL_DEFINE0(tankai){  printf("TK--------_>>>>>>tankai call ok!!!\n");}int main(){  sys_tank();  sys_tankai();  return 0;}/*result:TK----->>>lianjie(a,b) is 20TK----->>>lianjie(4,5) is 45TK--------_>>>>>>tank call ok!!!TK--------_>>>>>>tankai call ok!!*/

gcc test4.c -o test4

./test4

TK----->>>lianjie(a,b) is 20TK----->>>lianjie(4,5) is 45TK--------_>>>>>>tank call ok!!!TK--------_>>>>>>tankai call ok!!

2.一个参数

test5.c

#include <stdio.h>#include "common.h"long sys_tank(int fd);SYSCALL_DEFINE1(tank, int, fd){  printf("TK--------_>>>>>>call ok!!!>>fd is %d\n",fd);  return 0;}int main(){  sys_tank(3);  return 0;}/*SYSCALL_DEFINE1(tank,int,fd) = SYSCALL_DEFINEx(1, _tank, int, fd)SYSCALL_DEFINEx(1, _tank, int, fd) = __SYSCALL_DEFINEx(1, _tank, int, fd)__SYSCALL_DEFINEx(1, _tank, int, fd) = long sys_tank(__SC_DECL1(int,fd))long sys_tank(__SC_DECL1(int,fd))//__SC_DECL1(int,fd) = int fd//#define __SC_DECL1(t1, a1)    t1 a1//#define __SC_DECL2(t2, a2, ...) t2 a2, __SC_DECL1(__VA_ARGS__)//#define __SC_DECL3(t3, a3, ...) t3 a3, __SC_DECL2(__VA_ARGS__)long sys_tank(__SC_DECL1(int,fd)) = long sys_tank(int fd);*//*result:TK--------_>>>>>>call ok!!!>>fd is 3*/

gcc test5.c -o test5

./test5

TK--------_>>>>>>call ok!!!>>fd is 3

3.五个参数情况，其他不再罗列

test6.c

#include <stdio.h>#include "common.h"long sys_mount(char *dev_name, char *dir_name, char *type, unsigned long flags, void *data);SYSCALL_DEFINE5(mount, char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data){  printf("TK--------_>>>>>>call ok!!!>>>dev_name is %s,dir_name is %s,flags is %d\n",dev_name,dir_name,flags);  return 0;}int main(){  sys_mount(".", "/", NULL, 0, NULL);  return 0;}/*SYSCALL_DEFINE5(mount,char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data) = SYSCALL_DEFINEx(5, _mount, char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data)SYSCALL_DEFINEx(5, _mount, char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data) = __SYSCALL_DEFINEx(5, _mount, char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data)__SYSCALL_DEFINEx(5, _mount, char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data) = long sys_tank(__SC_DECL5(char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data))long sys_tank(__SC_DECL1(int,fd))//__SC_DECL5(char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data) = char * dev_name, __SC_DECL4(char *, dir_name, char *, type, unsigned long, flags, void *, data)//......//__SC_DECL5(char *, dev_name, char *, dir_name, char *, type, unsigned long, flags, void *, data) = char *dev_name, char *dir_name, char *type, unsigned long flags, void *data//#define __SC_DECL1(t1, a1)    t1 a1//#define __SC_DECL2(t2, a2, ...) t2 a2, __SC_DECL1(__VA_ARGS__)//#define __SC_DECL3(t3, a3, ...) t3 a3, __SC_DECL2(__VA_ARGS__)//#define __SC_DECL4(t4, a4, ...) t4 a4, __SC_DECL3(__VA_ARGS__)//#define __SC_DECL5(t5, a5, ...) t5 a5, __SC_DECL4(__VA_ARGS__)long sys_mount(__SC_DECL5(int,fd)) = long sys_mount(char *dev_name, char *dir_name, char *type, unsigned long flags, void *data);*//*result://TK--------_>>>>>>call ok!!!>>>dev_name is .,dir_name is /,flags is 0*/

gcc test6.c -o test6

./test6

TK--------_>>>>>>call ok!!!>>>dev_name is .,dir_name is /,flags is 0