c - Why does select return when USB cable is unplugged? -

i'm having problem linux c code reads ttyusb ports using select, fd_isset, read, etc. modem uses ftdi serial usb cable input. problem select unblocks when usb cable unplugged. there way prevent doing that?

count = 0; while ( g_running ) {    fd_zero(&readfdset);    maxfd = 0;    numtransports = 0;    logger( debug, "begin g_running loop - %d", count );    ( = 0; < max_config_ports; i++ ) {       if ( configports[i].commtype == 1 && configports[i].pttyhost != null ) {          fd_set( configports[i].pttyhost->fd, &readfdset );          logger( debug, "fd_set - fd=%d, index=%d", configports[i].pttyhost->fd, );          if ( configports[i].pttyhost->fd >= maxfd ) {             maxfd = configports[i].pttyhost->fd;          }          numtransports++;       }    }    maxfd++; // add 1 because select check range n-1 file descriptors    if ( maxfd != 0 ) { // indicates no ports available       logger( debug, "calling select() %d ports , maxfd of %d", numtransports, maxfd );       logger( info, "waiting input ..." );       select( maxfd, &readfdset, null, null, null ); // blocking until 1 available       if( result == -1 ){          logger( info, "select() error. errno: %d", errno );       } else if ( result > 0 ){          ( = 0; < max_config_ports; i++ ) {             if ( fd_isset( configports[i].pttyhost->fd, &readfdset ) ) { // input available                logger( info, "input on port %s", configports[i].pttyhost->serialpath );                result = serialportread( buffer, configports[i].pttyhost->fd );                if ( result <= 0 ) {                   // there error due file descriptor.                   // indicates tty ports no longer available                }             }          }       } else {          logger ( info, "select() returns 0" );       }    }    count++; } 

serialportread:

int serialportread( char *buf, int serialhandle ) {    //char ch;    char *ptr;    int res = 0;    int bytesread = 0;    int i;     logger( trace, "trace: serialportread() fd = %d", serialhandle );     ptr = buf;     // try 3 times    ( = 0; < 3; i++ ) {       while ( (res = read( serialhandle, ptr, 1 )) > 0 ) { // read 1 byte @ time          if ( *ptr == 0x0d ) { //there 0x0d terminate byte ecr             break;          }          ptr += res;       }       if ( res < 0 && errno == eagain ) {          continue;       } else {          break;       }    }     *ptr = 0x00; // set 0x00 terminate byte    //   pthread_mutex_unlock(&g_serial_trans_mutex);    if ( res < 0 ) {       // if res -1, there error       bytesread = -1;       logger( debug, "serialportread error. errno = %d", errno );    } else {       bytesread = (int) (ptr - buf);       logger( debug, "serialportread %d bytes", bytesread );    }     return bytesread; } 

when usb cable unplugged, select() unblocks, implying input available, fd_isset returns true. read(), in serialportread, return 0 bytes having been read. loops select() unblocks again saying input available, , on. thus, infinite loop of select(), fd_isset returning true, fd never gets cleared, read returns 0 , on. how can fix this? behavior expect select not falsely unblock when there not read?

note: when select unblocks returns positive number

select() returning because there information read - in case, fact file descriptor has reached "end of file". indicated read() returning 0.

when read() returns 0, should closing corresponding file descriptor.