Here is code I have written:

#include <stdio.h>
#include <ESP8266WebServer.h>

#define WIFI_RETRY_DELAY 500

const char* wifi_ssid = "ssid";
const char* wifi_passwd = "password";

ESP8266WebServer http_rest_server(80);

const int numParams = 20;
String params [numParams];

String toString(String my_array[], int size)
  String string;
  for (int i = 0; i < size; i++) {
    string = string + ", " + my_array[i];
  return string;

int init_wifi() {
  int retries = 0;
  Serial.println("Connecting to WiFi AP..........");
  WiFi.begin(wifi_ssid, wifi_passwd);
  // check the status of WiFi connection to be WL_CONNECTED
  while ((WiFi.status() != WL_CONNECTED) && (retries < MAX_WIFI_INIT_RETRY)) {
  return WiFi.status(); // return the WiFi connection status

void readParams(String (& params) [numParams])
  int mn = WiFi.scanNetworks();
  for (int i = 0; i < mn; i++)
    //    Serial.println(WiFi.SSID(i).c_str()); // prints name of all hotspots in serial monitor.
    params [i] = WiFi.SSID(i).c_str();

void listHotspots() {
  String results = toString(params, sizeof(params));
  http_rest_server.send(200, "text/plain", results);

void config_rest_server_routing() {
  http_rest_server.on("/", HTTP_GET, []() {
    http_rest_server.send(200, "text/html",
                          "Welcome to the ESP8266 REST Web Server");
  http_rest_server.on("/hotspots", HTTP_GET, listHotspots);

void setup(void) {


  readParams (params);

  if (init_wifi() == WL_CONNECTED) {
    Serial.print("Connetted to ");
    Serial.print("--- IP: ");
  else {
    Serial.print("Error connecting to: ");


  Serial.println("HTTP REST Server Started");

void loop(void) {

when I access the root url: i.e. It opens the root page fine, but when I try to open the It reboots the ESP8266 until the page stops to make a new attempts and I get nothing in result.

In the serial monitor I am seeing this exception when I try to visit the ip/hotspot page .

Exception (9):
epc1=0x4020148a epc2=0x00000000 epc3=0x00000000 excvaddr=0x4d2d5456 depc=0x00000000


ctx: cont
sp: 3ffffde0 end: 3fffffc0 offset: 01a0
3fffff80:  3fffdad0 3ffee553 3ffee5a4 40201347  
3fffff90:  402086b8 6505a8c0 feefeffe feefeffe  
3fffffa0:  3fffdad0 00000000 3ffee5e0 40206614  
3fffffb0:  feefeffe feefeffe 3ffe8510 40100479  

 ets Jan  8 2013,rst cause:4, boot mode:(3,6)

wdt reset
load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
Connecting to WiFi AP..........
######Connetted to Hometelecom1N5E--- IP:
HTTP REST Server Started

is it something wrong with the toString function ?

  • cause 4 is a watchdog reset
    – Juraj
    Commented May 25, 2020 at 18:46
  • so what might be causing watchdog reset ? Commented May 25, 2020 at 18:47
  • not calling a function which 'feeds the dog' long enough. decode the stack trace
    – Juraj
    Commented May 25, 2020 at 18:48
  • nevermind, I figured out solved my problem Commented May 25, 2020 at 19:36

1 Answer 1


When a program crashes, you can debug the crash based on the Fatal exception number.

 Exception (9)  Unaligned read/write operation addresses
     1. Unaligned read/write Cache addresses
     2. Wild pointers

This leads than as Juraj points out to a WD reset:

 rst:4     Hardware WDT reset

To get the exact part of the code paste the >>>stack>>> into the window of the espExeption decoder in the ArduinoIDE. Details how to install and use.

My educated guess is that the heavy use of String class in combination with lot of String operations with the getHotspot function leads to heap corruption and crashes the Esp8266. Try defining fixed char arrays and get rid of the String class to get a stable running system. Post the espExeption decoder result as edit to your question if you thing something else might be fishy


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