Example of custom printf specifier

customprintf.c

#include <stdio.h>
#include <stdlib.h>
#include <printf.h>



static int print_coin (FILE *stream,
              const struct printf_info *info,
              const void *const *args)
{
  int headstails;
  char *buffer;
  int len;

  /* figure out our string */
  headstails = *((const int *) (args[0]));
  if (info->width!=1)
    {
      buffer=(headstails&1)?"HEADS":"TAILS";
    }
  else
    {
      buffer=(headstails&1)?"H":"T";
    }
  /* Pad to the minimum field width and print to the stream. */
  len = fprintf (stream, "%*s",
                 (info->left ? -info->width : info->width),
                 buffer);

  return len;
}


static int print_coin_arginfo (const struct printf_info *info, size_t n,
                      int argtypes[],int size[])
{
  /* We always take exactly one argument and this is the coin flip */
  if (n > 0)
    argtypes[0] = PA_INT;
  return 1;
}

// This is the only externally reachable function

void register_coin_printf(void)
{
  register_printf_specifier('H',print_coin,print_coin_arginfo);
}

#if 1   // remove for library use

int main (int argc, char *argv[])
{
  char output[64];
  /* Register the print function for Coin Flips */
  register_coin_printf(); 
  printf("%H %H %H %H\n",1,2,3,4);
  printf("%1H %1H\n",0,1);
  printf("%20H %20H\n",100,33);
  printf("%20H %-20H\n",100,33);
  sprintf(output,"%H",1);
  printf("->%s\n",output);
  return 0;
}

#endif

Use -Wno-format or add these pragrams:
#pragma GCC diagnostic ignored "-Wformat"
#pragma GCC diagnostic ignored "-Wformat-extra-args"