ESP8266- Buffer storage capacity from the UART - c

Question

I have an issue while transferring the values of string from UART to buffer. I am using ESP8266 to receive strings on serial from STM32 device. I have total 600 string and i have CSV file that is being transmitted from the STM32 device. I have used proper filtering for every row on the NodeMCU side... I am having no clue why after 300 string the value transmitted to the buffer gets changed as well as the string read on UART also changes exactly after 300/305 string being transmitted. please let me know if there's any mistake in the code.
int i,j,k,l=0;
int httpCode=0;
String fields[24] = {"","month","year","hours","minutes","seconds","rimin","riavg","rimax","yimin","yiavg","yimax","bimin","biavg","bimax","nimin","niavg","nimax","eimin","eiavg","eimax","simin","siavg","simax"};
String element = "API_key=123&mac=0fa&day=";
String postApi1[300];
String postApi2[300];
String str= "";
String str_tx = "";
char char_array[128];
char* token;
char* rest = char_array;
WiFiClient client;
HTTPClient http;
void setup()
{
Serial.begin(230400);
s.begin(9600);
pinMode(13, INPUT);
pinMode(15, OUTPUT);
}
void loop()
{
if (s.available() >0)
{
for(j=0;j<300;j++)
{
i=0;
str = s.readStringUntil('\n');
Serial.println(str);
Serial.println("j");
postApi1[j]= str;
Serial.println(postApi1[j]);
str = "";
}
for(k=0;k<300;k++)
{
i=0;
str = s.readStringUntil('\n');
Serial.println(str);
Serial.println("k");
postApi2[k]= str;
Serial.println(postApi2[k]);
str = "";
}
}
}
Here is the code i am using and following is the Serial output of the code.

Answer 1

I think that the readStringUntil function waits for the terminator char. Is the '\n' sent also after 300th character? Also, try to avoid Serial.println in the read function, since the print function is quite slow, you might miss some characters. I would also avoid two for loops, instead, declare a variable, count number of received bytes and according to this save the "str" into "postApi1" or "postApi2".
Also, what is the purpose of "i" variable?

Answer 2

Hey #Andrej I tried your suggestion but it worked as follows
#include <Arduino.h>
#include <ESP8266WiFi.h>
#include <ESP8266WiFiMulti.h>
#include <SoftwareSerial.h>
#include <ESP8266HTTPClient.h>
#include <WiFiClient.h>
#include <stdio.h>
#include <string.h>
using namespace std;
ESP8266WiFiMulti WiFiMulti;
SoftwareSerial s(13,15); //RX TX
double buff[24]; //Initialized variable to store recieved data
double val;
char c;
int i,j,k,l=0;
int httpCode=0;
String fields[24] = {"","month","year","hours","minutes","seconds","rimin","riavg","rimax","yimin","yiavg","yimax","bimin","biavg","bimax","nimin","niavg","nimax","eimin","eiavg","eimax","simin","siavg","simax"};
String element = "API_key=123&mac=0fa&day=";
String postApi[616];
String str= "";
String str_tx = "";
char char_array[128];
char* token;
char* rest = char_array;
WiFiClient client;
HTTPClient http;
void setup()
{
Serial.begin(230400);
s.begin(9600);
pinMode(13, INPUT);
pinMode(15, OUTPUT);
//Serial.setDebugOutput(true);
//ESP.wdtDisable();
//Serial.begin(115200);
// Serial.setDebugOutput(true);
Serial.println();
Serial.println();
Serial.println();
for (uint8_t t = 4; t > 0; t--) {
Serial.printf("[SETUP] WAIT %d...\n", t);
Serial.flush();
delay(1000);
}
WiFi.mode(WIFI_STA);
WiFiMulti.addAP("ECLP", "p##$w0rD");
if ((WiFiMulti.run() == WL_CONNECTED))
{
Serial.print("WIFI connected");
}
}
void loop()
{
if (s.available() >0)
{
for(j=0;j<616;j++)
{
i++;
str = s.readStringUntil('\n');
//Serial.println(str);
//Serial.println(j);
postApi[j]= str;
//Serial.println(postApi[j]);
str = "";
}
}
if(i == 616)
{
i=0;
for(k=0;k<616;k++)
{
Serial.println(k);
Serial.print(" : ");
Serial.print(postApi[k]);
}
}
}
And receive on the output the following thing
Just for the ref i printed "k" value with output to know at which string the data loss occurs in buffer