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How printf() works ??

Discussion in 'C' started by HARIT, Jul 25, 2011.

  1. HARIT

    HARIT New Member

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    anyone can explain me...How printf() function works.

    there any way to find out the coding of any library function in Turbo C??
     
  2. eriyer

    eriyer New Member

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    To get the code for Turbo C library functions you need to use tlib (included with turboc) - extract the function from the corresponding library (.lib file in directory lib) - disassemble to get an assembly language listing - I do not know if a better tool is available.

    As to how printf works I am appending the foll program - I am also enclosing as zip file
    Code:
    #include <stdio.h>
    #include <stdarg.h>
    #include <string.h>
    
    /*
    http://msdn.microsoft.com/en-us/library/kb57fad8%28v=vs.71%29.aspx 
    I used the above link for this example - to get the method of picking trailing args from the stack
    */
    void display_int(int d);
    void demo_printf(char *narration, ...); /* use ONLY 3 '.' i.e.  ... */
    void display_str(char *s);
    
    void main()
    {
    	demo_printf("the value of %s is between %d and %d\n***", "PI", 3, 4);
    }
    
    /* our toy implementation of printf */
    void demo_printf(char *narration, ...) /* use ONLY 3 '.' i.e.  ... to denote variable no of trailing arguments*/
    {
    	/* this illustrates use of only %s, %d and \n */
    	va_list argptr;
    	char *ptr;
    	int d, i;
    	
    	va_start(argptr, narration);
    	
    	for( i = 0; i < strlen(narration); i++ )
    	{
    		if( narration[ i ] != '%' && narration[ i ] != '\\' )
    			putc(narration[ i ], stdout);
    		else if( narration[ i ] == '%' )
    		{
    			if( narration[ i + 1 ] == 'd' )
    			{
    				d = va_arg(argptr, int);
    				display_int(d);
    				i++; /* skip next char in for loop */
    			}
    			else if( narration[ i + 1 ] == 's' )
    			{
    				ptr = va_arg(argptr, char *);
    				display_str(ptr);
    				i++; /* skip next char in for loop */
    			}
    		}
    		else if( narration[ i + 1 ] == 'n' )
    		{
    			putc('\n', stdout);
    			i++;
    		}
    	}
    	
    	va_end(argptr);
    	
    }
    
    void display_str(char *s)
    {
    	int i;
    	
    	for( i = 0; i < strlen(s); i++ )
    		putc(s[ i ], stdout);
    	
    }
    
    void display_int(int d)
    {
    	char intstr[ 15 ];
    	int i;
    	
    	i = sizeof(intstr) - 1;
    	intstr[ i-- ] = 0; /* null terminate*/
    	for( ; i >= 0 && d; i-- )
    	{
    		intstr[ i ] = '0' + d % 10;
    		d /= 10;
    	}
    	i++; /* to offset last decrement in for loop */
    	
    	display_str(intstr + i);
    }
    
    
     
  3. lionaneesh

    lionaneesh Active Member

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    We have a basic tutorial on How Format String Functions work , You can have a look.
     
  4. K SAICHARAN

    K SAICHARAN New Member

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    I helps to print what ever is typed between inverted comas.
     
  5. HARIT

    HARIT New Member

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    its not clear to me....please explain
     

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