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The poll functions perform timed waits on one or more file handles.

Mbed OS provides poll() in two forms:

  • int poll(struct pollfd fds[], nfds_t nfds, int timeout) the standard POSIX form taking an array of integer file descriptors.
  • int mbed::poll(pollfh fhs[], unsigned nfhs, int timeout), the variant taking an array of FileHandle *.

See the POSIX specification of poll for full details of the API. Mbed OS provides the POLLIN, POLLOUT, POLLERR and POLLHUP event flags, but implementation depends on the device.

Note that the poll is not affected by the blocking or nonblocking setting of the individual file handles. The timeout argument always determines the wait behavior: 0 for immediate (nonblocking) return, -1 for wait forever or positive for a limited wait.

As per the POSIX specification, poll always indicates that ordinary files (as opposed to devices) are readable and writable.

Poll reference

Poll example

 * Copyright (c) 2006-2020 Arm Limited and affiliates.
 * SPDX-License-Identifier: Apache-2.0
#include "mbed.h"

// Pins for each port are specified using mbed_app.json. Assume no flow control.
// (If there were flow control, being blocked by it could break the implementation,
// as you would stop reading input in the opposite direction).

// Precondition: "in" is readable
static void copy_some(FileHandle *out, FileHandle *in)
    // To ensure performance, try to read multiple bytes at once,
    // but don't expect to read many in practice.
    char buffer[32];

    // Despite the FileHandle being in its default blocking mode,
    // read() must return immediately with between 1-32 bytes, as
    // you've already checked that `in` is ready with poll()
    ssize_t read = in->read(buffer, sizeof buffer);
    if (read <= 0) {
        error("Input error");

    // Write everything out. Assuming output port is same speed as input,
    // and no flow control, this may block briefly but not significantly.
    ssize_t written = out->write(buffer, read);
    if (written < read) {
        error("Output error");

// Transfer bidirectional data between two ports, acting as a virtual link.
// poll() is used to monitor both ports for input.
int main()
    pollfh fds[2];

    fds[0].fh = &device1;
    fds[0].events = POLLIN;
    fds[1].fh = &device2;
    fds[1].events = POLLIN;

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

        // Transfer some data in either or both directions
        if (fds[0].revents) {
            copy_some(fds[1].fh, fds[0].fh);
        if (fds[1].revents) {
            copy_some(fds[0].fh, fds[1].fh);

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