#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define INDEX(x) "",(x)  

char *f;        

char *g(int a, char *ext){ char *F;
  F = (char *) malloc(strlen(ext)+5*sizeof(char));
  sprintf(F,"%s%i.%s",INDEX(a),ext); return(F);
}

int main(){ f=g(1000,"pnm"); return 0; }