modbus_get_socket always return -1

[audi0615]

Hi.

I'd like to inquire the amount of data available at a serial port using "ioctl". something like this.

int nread; ioctl(Serial, FIONREAD, &nread);

this is for implementing a non-blocking RTU slave but modbus_get_socket(ctx) always return -1. how can i get socket (file descriptor) of a serial port??