Linux C编程连载(7) - 变参数实现printf

 【代码清单】

#include <stdio.h>#define va_list void*#define va_end(arg)#define va_start(arg, start) arg = (va_list)(((char *)&(start)) + sizeof(start))#define va_arg(arg, type) *(type*)arg; arg = (char *)arg +sizeof(type)char *itoa(int num, char *str, int radix){char string[] = "0123456789abcdefghijklmnopqrstuvwxyz";char *ptr = str;int i,j;while(num){*ptr++ = string[ num%radix ];num /= radix;if(num < radix){*ptr++ = string[num];*ptr = '\0';break;}}j = ptr - str -1;for(i=0; i<(ptr-str)/2; i++){int temp = str[i];str[i] = str[j];str[j--] = temp;}return str;}int mprintf(const char *format, ...){va_list arg;int done = 0;va_start(arg, format);while(*format != '\0'){if(*format == '%'){if(*(format+1) == 'c'){char c = (char)va_arg(arg,int);putc(c,stdout);} else if(*(format+1) == 'd'||*(format+1) == 'i'){char store[20];int i = va_arg(arg,int);char *str = store;itoa(i,store,10);while(*str != '\0') putc(*str++,stdout);} else if(*(format+1) == 'o'){char store[20];int i = va_arg(arg,int);char *str = store;itoa(i,store,8);while(*str != '\0') putc(*str++,stdout);} else if(*(format+1) == 'x'){char store[20];int i = va_arg(arg,int);char *str = store;itoa(i,store,16);while(*str != '\0') putc(*str++,stdout);} else if( *(format+1) == 's' ){char* str = va_arg(arg, char*);while( *str != '\0') putc(*str++, stdout);}format += 2;} else {putc(*format++, stdout);}}    va_end (arg);    return done;}int main(int argc, char* argv[]){     int n = 255;    char str[] = "hello, world!";    // Test vprintf function    mprintf("n = %d\n", n);    mprintf("n = %i\n", n);     mprintf("n = %o\n", n);    mprintf("n = %x\n", n);    mprintf("first char = %c\n", str[0]);     mprintf("str = %s\n", str);    mprintf("%s\tn = %d\n", str, n);    return 0;}

【解析】

(1)

#define va_start(arg, start) arg = (va_list)(((char *)&(start)) + sizeof(start))

让arg指向start(即format)之后的地址，即可变参数的首地址。

(2)

#define va_arg(arg, type) *(type*)arg; arg = (char *)arg +sizeof(type)

定义了一个va_arg函数，该函数有两个参数，arg和type。并指明了arg是type*类型的指针。该函数的作用是将指针指向下一个可变参数。

【链接】函数定义的两种形式

(1) int func(int a, int b);

(2) int func(a,b) int a, int b;

 

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