Add Poll example

Author: mprse

APIs_Platform/Poll/README.md

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+## Poll example
+
+The example transfers bidirectional data between two serial ports, acting as a virtual link.
+
+1. Example `mbed_app.json` file (`K64F`):
+
+```
+{
+    "config": {
+        "UART1_TX": "D1",
+        "UART1_RX": "D0",
+        "UART2_TX": "USBTX",
+        "UART2_RX": "USBRX"
+    }
+}
+```
+
+2. Flash the board, and ensure the target's USB is plugged into the PC (UART2).
+3. Connect second serial port to PC via USB to serial adapter (UART1).
+4. Open two serial consoles (select associated COM ports, transmission speed: 9600 bps).
+5. Reset the target.
+6. Data send from one console should appear in the other one and vice versa.
+
+**Note:** You can check the associated serial port using `mbedls` command (UART2).  
+**Note:** Note: On Windows you can check the associated COM port in the Device Manager (UART1).  
+**Note:** Please adapt your pins for target different than `K64F`.

APIs_Platform/Poll/main.cpp

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+// Transfer bidirectional data between two ports, acting as a virtual link.
+// poll() is used to monitor both ports for input.
+#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).
+BufferedSerial device1(MBED_CONF_APP_UART1_TX, MBED_CONF_APP_UART1_RX);
+BufferedSerial device2(MBED_CONF_APP_UART2_TX, MBED_CONF_APP_UART2_RX);
+
+// 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");
+    }
+}
+
+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);
+        }
+    }
+}