实现自己的变参函数printf

在c/c++标准库中，变参函数很特别。printf, fprintf, sprintf等都属于变参函数。如果自己要写类似的参数可变的函数，通常会用到下面三个函数：

#include <stdarg.h>        void va_start(va_list ap, last);        type va_arg(va_list ap, type);        void va_end(va_list ap);

一般的用法是这样：
va_list args; //声明变量
va_start(args, fmt); //开始解析。args指向fmt后面的参数
TYPE var = va_arg(args, TYPE); //取下一个参数并返回。args指向下一个参数
va_end(args); //结束解析

下面就给出具体的例子：

#include <stdarg.h> #include <stdio.h> /*  * Conver int to string based on radix (usually 2, 8, 10, and 16)  */ char *itoa(int num, char *str, int radix) {     char string[] = "0123456789abcdefghijklmnopqrstuvwxyz";     char* ptr = str;     int i;     int 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; } /*  * A simple printf function. Only support the following format:  * Code Format  * %c character  * %d signed integers  * %i signed integers  * %s a string of characters  * %o octal  * %x unsigned hexadecimal  */ int my_printf( const char* format, ...) {     va_list arg;     int done = 0;     va_start (arg, format);     //done = vfprintf (stdout, format, arg);     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);             }             // Skip this two characters.             format += 2;         } else {             putc(*format++, stdout);         }     }     va_end (arg);     return done; } int main(int argc, char* argv[]) {     int n = 255;     char str[] = "hello, world!";     printf("n = %d\n", n);     printf("n = %i\n", n);     printf("n = %o\n", n);     printf("n = %x\n", n);     printf("first char = %c\n", str[0]);     printf("str = %s\n", str);     printf("%s\tn = %d\n", str, n);     // Test my_printf function     printf("---------------my_printf--------------\n");          my_printf("n = %d\n", n);     my_printf("n = %i\n", n);     my_printf("n = %o\n", n);     my_printf("n = %x\n", n);     my_printf("first char = %c\n", str[0]);     my_printf("str = %s\n", str);     my_printf("%s\tn = %d\n", str, n);     return 0; }

编译运行，结果如下：

[ray@localhost c++]$ gcc -o printf printf.c
[ray@localhost c++]$ ./printf
n = 255
n = 255
n = 377
n = ff
first char = h
str = hello,
hello, n = 255
---------------my_printf--------------
n = 255
n = 255
n = 377
n = ff
first char = h
str = hello,
hello, n = 255

转自:http://www.cublog.cn/u2/68938/showart_726427.html