> To find an invalid FD when select() fails with EBAD, we can use something like:
> http://ufwi.org/projects/nufw/repository/revisions/b4f66edc5d4dc837f75857f8bffe9015454fdebc/entry/src/nuauth/tls_nufw.c#L408

Oh, the link is dead. Copy/paste of the code:
---
/* errno == EBADF */
int i;
/* A client disconnects between FD_SET and select.
 * Will try to find it */
for (i=0; i<context->mx; ++i){
    struct stat s;
    if (FD_ISSET(i, &context->tls_rx_set)){
        if (fstat(i, &s)<0) {
            log_message(CRITICAL, DEBUG_AREA_USER,
                    "Warning: %d is a bad file descriptor.", i);
            FD_CLR(i, &context->tls_rx_set);
        }
    }
}
continue;   /* retry select */
---

In short: call fstat() if check if the FD is valid or not. O(n) complexity.