Implementation of printf() function

Question

I'm making a hobbyist kernel and I'm trying to implement a printf() function.

Here's my implementation:

void kprint(uint8_t *format, ...) {
  va_list ap;
  va_start(ap, format);

  uint8_t *ptr;

  for (ptr = format; *ptr != '\0'; ptr++) {
    if (*ptr == '%') {
      ptr++;
      switch (*ptr) {
        case 's':
          puts(va_arg(ap, uint8_t *), 0x0F, xPos, yPos);
          break;
      }
    } else {
      puts(ptr, 0x0F, xPos, yPos);
      ptr++;
    }
  }

  va_end(ap);
}

When I want to print "Hello World!" using this function it returns me that:

"Hello %sllo %so %sWorld"

Here's function call:

kprint("Hello %s", "World");

Answer 1

the main issue is that you're using puts to print the rest of the string instead of current char.

 } else {
   puts(ptr, 0x0F, xPos, yPos);
   ptr++;
}

also you're incrementing ptr when it's already done in the loop. Useful to consume format argument, but not in that case.

A working implementation (not using the strange puts prototype) that works on a standard system (tested on windows gcc):

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

void zprintf(uint8_t *format, ...)
  {
      va_list ap;
      va_start(ap, format);

      uint8_t *ptr;

      for (ptr = format; *ptr != '\0'; ptr++) {
          if (*ptr == '%') {
              ptr++;
              switch (*ptr) {
                  case 's':
                      fputs(va_arg(ap, uint8_t *),stdout);
                      break;
                 case '%':
                      putchar('%');
                      break;
              }
             } else {
               putchar(*ptr);

            }
           }

           va_end(ap);
}

int main(){

    zprintf("%% Hello %s\n","World");    
    return 0;
}

prints:

% Hello World

(as a bonus, this implementation handles the escaped % char)

Using only your special implementation of puts you could change

 } else {
   puts(ptr, 0x0F, xPos, yPos);
   ptr++;
}

to a working:

 } else {
   char c[2];
   c[0] = *ptr; c[1] = '\0';
   puts(c, 0x0F, xPos, yPos);
 }

lame but would work :)

Answer 2

void kprint(char *format, ...) 
{
    va_list ap;
    va_start(ap, format);

    char *ptr = format;

  while(*ptr)
  {
    if (*ptr == '%') 
    {
      ptr++;
      switch (*ptr++) 
      {
        case 's':
          printf("%s", va_arg(ap, char *));
          break;
      }
    } 
    else 
    {
      putchar(*ptr++);
    }
  }

  va_end(ap);
}

int main()
{
    kprint("Hello %s World%s", "AAAAA", " BBBBBB");

    return 0;
}