#include <errno.h>
#include <string.h>
#include <sys/select.h>

int whichisready(int fd1, int fd2) {
   int maxfd;
   int nfds;
   fd_set readset;

   if ((fd1 < 0) || (fd1 >= FD_SETSIZE) ||
       (fd2 < 0) || (fd2 >= FD_SETSIZE)) {
      errno = EINVAL;
      return -1;
   }
   maxfd = (fd1 > fd2) ? fd1 : fd2;
   FD_ZERO(&readset);
   FD_SET(fd1, &readset);
   FD_SET(fd2, &readset);
   nfds = select(maxfd+1, &readset, NULL, NULL, NULL);
   if (nfds == -1)
      return -1;
   if (FD_ISSET(fd1, &readset)) 
      return fd1;
   if (FD_ISSET(fd2, &readset)) 
      return fd2;
   errno = EINVAL; 
   return -1;
}