Write your own printf() function in c

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I was wondering where I can find the C code that’s used so that when I write printf(“Hello World!”) in my C program to know that it has to print that string to standard output(i.e. monitor screen). I looked in <stdio.h>, but there I could only find its prototype int printf(const char *format, …), but not how it looks like internally.

Then i start googling for that but i hardly find a solution for which i am looking for. So after long time spend on net, i found this nice formal C Program.

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, like

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.

Code to implement printf( )

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

Pseudocode

Module 1: Initializing Myprintf’s arguments

In this section, we initialize the arguments of Myprintf( ) function by using standard argument library.

va_list arg;

This line declares a variable, arg, which we use to manipulating the argument list containing variable arguments of Myprintf( ). The data type of the variable is va_list, a special type defined by <stdarg.h>.

va_start(arg, format);

This line initializes arg variable with function’s last fixed argument i.e. format. va_start() uses this to figure out where the variable arguments begin.

Module 2: Fetching and executing arguments

i = va_arg(arg,int);

va_arg() fetches the next argument from the argument list. The second parameter to va_arg() is the data type of the argument we expect.

Note: va_arg( ) function will never receive arguments of type char, short int, or float. va_arg( ) function only accept arguments of type char *, unsigned int, int or double.

Module 3: Closing argument list to necessary clean-up

	va_end(arg);

Finally, when we’re finished processing the all arguments, we call va_end(), which performs any necessary cleanup.

Original standard printf( )

Here’s the GNU version of printf… you can see it passing in stdout to vfprintf:

__printf (const char *format, ...)
{
   va_list arg;
   int done;

   va_start (arg, format);
   done = vfprintf (stdout, format, arg);
   va_end (arg);

   return done;
}

Here’s a link to vfprintf… all the formatting ‘magic’ happens here.

This is the same thing as we use varargs(stdarg.h library) to get parameters in a variable argument list. Other than that, they’re just traditional C.

Limitation of standard variable argument functions is described in another post named “How can I write a function that takes a variable number of arguments ? ” which you can see below in suggested reading.

Source

http://www.eskimo.com/
http://sourceware.org/
http://stackoverflow.com/
https://www.google.com/

FIRMWARE DEVELOPER

Write your own printf() function in c

Code to implement printf( )

Pseudocode

Original standard printf( )

Source

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