keywords: [C]printf函数源码实现

printf函数源码实现
#include <stdio.h>  
#include <stdarg.h>  
  
  
//va_start(arg,format),初始化参数指针arg，将函数参数format右边第一个参数地址赋值给arg  
//format必须是一个参数的指针，所以，此种类型函数至少要有一个普通的参数,   
//从而提供给va_start ,这样va_start才能找到可变参数在栈上的位置。   
//va_arg(arg,char),获得arg指向参数的值，同时使arg指向下一个参数,char用来指名当前参数型  
//va_end 在有些实现中可能会把arg改成无效值，这里，是把arg指针指向了 NULL,避免出现野指针   
  
  
void print(const char *format, ...)  
{  
    va_list arg;  
    va_start(arg, format);  
  
    while (*format)  
    {  
        char ret = *format;  
        if (ret == '%')  
        {  
            switch (*++format)  
            {  
            case 'c':  
            {  
                        char ch = va_arg(arg, char);  
                        putchar(ch);  
                        break;  
            }  
            case 's':  
            {  
                        char *pc = va_arg(arg, char *);  
                        while (*pc)  
                        {  
                            putchar(*pc);  
                            pc++;  
                        }  
                        break;  
            }  
            default:  
                break;  
            }  
        }  
        else  
        {  
            putchar(*format);  
        }  
        format++;  
    }  
    va_end(arg);  
}  

int main()  
{  
    print("%s %s %c%c%c%c%c!\n", "welcome", "to", 'C', 'h', 'i', 'n', 'a');  
    system("pause");  
    return 0;  
}

Origin:
https://blog.csdn.net/iynu17/article/details/51588199

Format

Example:

/* printf example */
#include <stdio.h>

int main()
{
   printf ("Characters: %c %c \n", 'a', 65);
   printf ("Decimals: %d %ld\n", 1977, 650000L);
   printf ("Preceding with blanks: %10d \n", 1977);
   printf ("Preceding with zeros: %010d \n", 1977);
   printf ("Some different radices: %d %x %o %#x %#o \n", 100, 100, 100, 100, 100);
   printf ("floats: %4.2f %+.0e %E \n", 3.1416, 3.1416, 3.1416);
   printf ("Width trick: %*d \n", 5, 10);
   printf ("%s \n", "A string");
   return 0;
}

Output:

Characters: a A
Decimals: 1977 650000
Preceding with blanks:       1977
Preceding with zeros: 0000001977
Some different radices: 100 64 144 0x64 0144
floats: 3.14 +3e+000 3.141600E+000
Width trick:    10
A string

Reference:
https://www.cplusplus.com/reference/cstdio/printf/

察见渊鱼者不祥，智料隐匿者有殃。----《列子》