File content as of revision 0:0f1c0f6579ab:

#include "mbed.h"

UARTSerial pc(USBTX, USBRX);
UARTSerial device(MBED_CONF_APP_UART1_TX, MBED_CONF_APP_UART1_RX);

static void copy_some(FileHandle *out, FileHandle *in) {
    // To ensure performance, allow to read multiple bytes at once, although
    // we don't expect to read many in practice.

    // read() will return immediately, as we've already
    // checked that `in` is ready with poll()
    char buffer[32];
    ssize_t read = in->read(buffer, sizeof buffer);
    if (read <= 0) {
        error("Input error");
    }

    // Then write them all out. Assuming output port is similar speed to input,
    // this may block briefly, but not significantly.
    ssize_t written = 0;
    while (written < read) {
        ssize_t w = out->write(buffer + written, read - written);
        if (w <= 0) {
            error("Output error");
        }
        written += w;
    }
}

int main() {
    char buffer[32];
    pollfh fds[2];

    fds[0].fh = &pc;
    fds[0].events = POLLIN;
    fds[1].fh = &device;
    fds[1].events = POLLIN;

    while (1) {
        // Block until either of the 2 ports is readable (or has an error)
        poll(fds, 2, -1);

        if (fds[0].revents) {
            copy_some(fds[1].fh, fds[0].fh);
        }
        if (fds[1].revents) {
            copy_some(fds[0].fh, fds[1].fh);
        }
    }
}