How to count number of digits of printf argument?

So, I know the question sounds strange but I don't really know how to phrase it better. I'll try to explain: I'm writing my own version of printf, this is the code I have so far:

Code:

---

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


void print(char *, ...);
char *convert(unsigned int, int);
char *to_lower(unsigned int, int);
unsigned int count_digits(unsigned int);


int main()
{
    char str[12]="World";
    char c='A';
        int n = 5, x= 1011, y = 12, z = 5, t = 10;


    print("Hello s c |%s| |%c| 5 |%d| 5 some random %X %x %b %o words %4d\n", str, c, n, x, y, z, t, z);
}


void print(char *format, ...)
{
        int num, precision = 0;


    va_list lst;
    va_start(lst, format);


    while(*format != '\0')
    {
        if(*format != '%')
        {
            putchar(*format);
            format++;
            continue;
        }


                format++;


                if(isdigit(*format))
                {
                        precision = atoi(format);
                        format++;
                        continue;
                }


                //format++;


        switch(*format)
        {
                        case 'c':
                putchar(va_arg(lst, int));
                break;
                        case 'd':
                num = (va_arg(lst, int));


                if(num > 0)
                {
                    fputs(convert(num,10), stdout);
                }
                else
                {
                    num = -num;
                    putchar('-');
                    fputs(convert(num,10), stdout);
                }
                break;
            case 's':
                                fputs(va_arg(lst, char *), stdout);
                                break;
                        case 'X':
                                num = va_arg(lst, int);
                                fputs(convert(num,16), stdout);
                                break;
                        case 'x':
                                num = va_arg(lst, int);
                                fputs(to_lower(num,16), stdout);
                                break;
                        case 'b':
                                num = va_arg(lst, int);
                                fputs(convert(num,2), stdout);
                                break;
                        case 'o':
                                num = va_arg(lst, int);
                                fputs(convert(num,8), stdout);
                                break;
        }


        format++;
    }


        va_end(lst);
}


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);
}


char *to_lower(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);
}


unsigned int count_digits(unsigned int n)
{
        unsigned int counter = 0, lastDigit;


        while(n != 0)
        {
                lastDigit = n%10;
                n = n/10;
                counter++;
        }


        return counter;
}

---

Output:
Hello s c |World| |A| 5 |5| 5 some random 3F3 c 101 12 words d

(As you can see it's only printing 'd' from '%4d' because I've extracted the 4). The program is working fine so far but I have a problem. I'm trying to implement the functionality where if we write:
%nd, where n is a natural number, an integer of n characters will be printed (and if the number to be printed has less digits then spaces shall be used for padding up to 'n' character)(e.g: 12 with %5d is <space><space><space>12). Right now I can extract the number between % and d but I don't know how to find the number of digits of the argument to be used, since we only specify that in main. If I knew the number of digits, I would write the following code inside case 'd':

Code:

---

for(i = 0; i < (precision - noDigits); i++)
{
    putchar(' ');
}

---

I hope you understand what I'm trying to do, if not I'll try to explain more clearly. Thank you!!

Code:

---

        if(isdigit(*format))
        {
            precision = atoi(format);
            format++;
            continue;
        }

---

I think this should be

Code:

---

if(isdigit(*format)) {
    precision = strtol(format,&format,10);
}

---

This will store (in your case, 4) in the precision, and advance format to point to the 'd' of the conversion specifier.

> an integer of n characters will be printed (and if the number to be printed has less digits then spaces shall be used for padding up to 'n' character)
Well rather than doing this
> fputs(convert(num,10), stdout);

Maybe something like

Code:

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char *out = convert(num,10);
noDigits = strlen(out);
// your padding code
fputs(out,stdout);

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