libc - How to write my own printf() in C?

Question

Actually I am trying to write my own printf() in C by using varags. But I am not getting the correct solution for this. Can anyone help me out?

Answer 1

Before implementation of printf( ) function we have to deal with unusual problem which is variable arguments. As we know that printf can take many arguments besides string. So we have to use a standard library called stdarg.h to handle this variable argument problem. In this implementation context, we don’t need learn whole stdarg.h library because we use some macro functions of these library which is understandable directly by our C program.

Here is the code source which explain nice and fast

#include<stdio.h> 
#include<stdarg.h>                      

void Myprintf(char *,...);              //Our printf function
char* convert(unsigned int, int);       //Convert integer number into octal, hex, etc.


int main() 
{ 
    Myprintf(" WWW.FIRMCODES.COM 
 %d", 9); 

    return 0;
} 


void Myprintf(char* format,...) 
{ 
    char *traverse; 
    unsigned int i; 
    char *s; 

    //Module 1: Initializing Myprintf's arguments 
    va_list arg; 
    va_start(arg, format); 

    for(traverse = format; *traverse != ''; traverse++) 
    { 
        while( *traverse != '%' ) 
        { 
            putchar(*traverse);
            traverse++; 
        } 

        traverse++; 

        //Module 2: Fetching and executing arguments
        switch(*traverse) 
        { 
            case 'c' : i = va_arg(arg,int);     //Fetch char argument
                        putchar(i);
                        break; 

            case 'd' : i = va_arg(arg,int);         //Fetch Decimal/Integer argument
                        if(i<0) 
                        { 
                            i = -i;
                            putchar('-'); 
                        } 
                        puts(convert(i,10));
                        break; 

            case 'o': i = va_arg(arg,unsigned int); //Fetch Octal representation
                        puts(convert(i,8));
                        break; 

            case 's': s = va_arg(arg,char *);       //Fetch string
                        puts(s); 
                        break; 

            case 'x': i = va_arg(arg,unsigned int); //Fetch Hexadecimal representation
                        puts(convert(i,16));
                        break; 
        }   
    } 

    //Module 3: Closing argument list to necessary clean-up
    va_end(arg); 
} 

char *convert(unsigned int num, int base) 
{ 
    static char Representation[]= "0123456789ABCDEF";
    static char buffer[50]; 
    char *ptr; 

    ptr = &buffer[49]; 
    *ptr = ''; 

    do 
    { 
        *--ptr = Representation[num%base]; 
        num /= base; 
    }while(num != 0); 

    return(ptr); 
}