C: Dynamic string using variable number of arguments

[negru]

I need to create dynamic string by given format(%d,%s,%f,%lf,%c) using variable number of arguments in function. This code gives me an error(main.exe has stopped working):

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

char *form(char *format,...);
char *form(char *format,...)
{
    char *res=(char *)calloc(1,1),val_int[12],*pos_int,*val_str,*pos_str,*val_float,*pos_float,*val_lf,*pos_lf,*val_char,*pos_char;
    va_list args;
    va_start(args,format);
    do
    {
      pos_int=strstr(format,"%d");pos_str=strstr(format,"%s");pos_float=strstr(format,"%f");pos_lf=strstr(format,"%lf");pos_char=strstr(format,"%c");
      if(pos_int && (!pos_str || pos_int < pos_str) && (!pos_float || pos_int < pos_float) &&
         (!pos_lf || pos_int < pos_lf) && (!pos_char || pos_int < pos_char))
      {
        itoa(va_arg(args,int),val_int,10);
        res=(char *)realloc(res,strlen(res)+(pos_int-format)+strlen(val_int)+1);
        strncat(res,format,pos_int-format);
        strcat(res,val_int);
        format=pos_int+2;
      }
      else if(pos_str && (!pos_int || pos_str < pos_int) && (!pos_float || pos_str < pos_float) &&
              (!pos_lf || pos_str < pos_lf) && (!pos_char || pos_str < pos_char))
      {
         val_str=va_arg(args,char *);
         res=(char *)realloc(res,strlen(res)+(pos_str-format)+strlen(val_str)+1);
         strncat(res,format,pos_str-format);
         strcat(res,val_str);
         format=pos_str+2;
      }

      else if(pos_float && (!pos_int || pos_float < pos_int) && (!pos_str || pos_float < pos_str) &&
              (!pos_lf || pos_float < pos_lf) && (!pos_char || pos_float < pos_char))
      {
         fcvt(va_arg(args,double),val_float,6,0);
         res=(char *)realloc(res,strlen(res)+(pos_float-format)+strlen(val_float)+1);
         strncat(res,format,pos_float-format);
         strcat(res,val_float);
         format=pos_float+2;
      }

      else if(pos_lf && (!pos_int || pos_lf < pos_int) && (!pos_str || pos_lf < pos_str) &&
              (!pos_float || pos_lf < pos_float) && (!pos_char || pos_lf < pos_char))
      {
          ecvt(va_arg(args,double),val_lf,6,0);
          res=(char *)realloc(res,strlen(res)+(pos_lf-format)+strlen(val_lf)+1);
          strncat(res,format,pos_lf-format);
          strcat(res,val_lf);
          format=pos_lf+2;
      }
      else if(pos_char && (!pos_int || pos_char < pos_int) && (!pos_str || pos_char < pos_str) &&
              (!pos_float || pos_char < pos_float) && (!pos_lf || pos_char < pos_lf))
      {
          val_char=va_arg(args,char *);
          res=(char *)realloc(res,strlen(res)+(pos_char-format)+strlen(val_char)+1);
          strncat(res,format,pos_char-format);
          strcat(res,val_char);
          format=pos_char+2;
      }
    }
    while(pos_int || pos_str || pos_float || pos_lf || pos_char);
    va_end(args);
    res=(char *)realloc(res,strlen(res)+strlen(format)+1);
    strcat(res,format);
    return res;
}

int main()
{
    char *s;
    s=form("John is %d years old and has an %c in %s. He is going to school for %lf years.",9,'A',"maths",1.5);
    printf("%s",s);
    free(s);
    return 0;
}

I assume the error is in functions(itoa,fcvt,ecvt). Thanks for replies.