Http GET in a loop - only works once

Projects Networking, Protocols, and Devices
1

I'm using the following libraries with the following two objects:

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>

WiFiClientSecure client;
HTTPClient sender;

Establishing a connection using the following code (works fine):

  // connect to wifi  
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  
  uint8_t i = 0; // temp variable for while loop
  
  while (WiFi.status() != WL_CONNECTED)
  {
      delay(200);
      i++;
      #ifdef DEBUG
      Serial.print(".");
      #endif
      if (i == numOfWIFIConRetries)
      {
        #ifdef DEBUG
        Serial.print(" Failed to connect to WIFI!");
        while(1); // stuck here
        #endif
      }
  }

The function I'm interesting in running several times, gets a char array (as pointer) and send it as a query displays the returned value. This works fine for the first time only. Why is that?

float queryWhatIsTheRate(char* coin)
{
  String payload;
  
  if (sender.begin(client, strcat(coin, "USDT"))))
  {
    if (sender.GET() == HTTP_CODE_OK) 
    {
      payload = sender.getString();
      #ifdef DEBUG
      Serial.println(payload);
      #endif
    }
    else
    {
      #ifdef DEBUG
      //Serial.printf("HTTP-Error: ", sender.errorToString(httpCode).c_str());
      Serial.printf("HTTP-error");
      #endif
    }
  }
  // temp return val
  return 0;
}

So the first time would be a response from the website's API and the rest of the following calls would result in 'HTTP-Error' being printed.

2

Have you tried calling sender.end() after a HTTP transaction? Maybe that helps.

3

Since strcat adds USDT to coin, I suspect that the next time you call it, the end of the string will be USDTUSDT, which the API you're calling does not like.

4

Oops. Good point.

5

I've tried calling the function 3 times (so not in a loop):

  queryWhatIsTheRate("BTC");
  queryWhatIsTheRate("ETH");  
  queryWhatIsTheRate("DOGE");

And I've added sender.end() before the return. This way the string won't get longer each time. first response is OK. 2nd & 3rd HTTP-error, code 400.

6

When you call strcat it will add USDT to the string passed. A string like "BTC" occupies four characters. There is no space reserved for the extra letters, but C++ doesn't care. You just wrote on memory that you shouldn't have. Who knows what you overwrote - small wonder that things aren't working as you hoped.

7

So you suggest taking in the char* as a parameter and concat it with the other char array inside the function into a new char* that will be large enough to host the entire string?

8

I'm not sure I quite got the specifics there, but yes, you need a buffer with enough space for both strings to be added.

9

This topic was automatically closed 120 days after the last reply. New replies are no longer allowed.