Conversion from string to char array

I know this is Arduino Forum but I think this question is related to programming, I am using ESP8266 for the project. I need to convert the string incoming on the serial through UART lines and receiving those using the below code. Also I am converting the received string to char array as i need to insert some more characters in between hence I need the conversion. I have used 3 different commands to convert the above string into char array as you can see below

#include <SoftwareSerial.h>
#include <Arduino.h>

#include <stdio.h>

#include <string.h>



SoftwareSerial s; //RX TX
using namespace std;
String str= "";
String str_tx;

const char* c;
double buff[24];
unsigned long int str_len = 0;
double val;
char* token; 
//char* rest = char_array;
int i=0;
void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  s.begin(115200, SWSERIAL_8N1, 13, 15, false, 256); //115200, SWSERIAL_8N1, 13, 15, false, 512
  pinMode(13, INPUT);
  pinMode(15, OUTPUT);

  Serial.println("in setup 1");
  Serial.println("in setup 2");
  Serial.println("in setup 3");
  
}

void loop() {
  // put your main code here, to run repeatedly:2
 while(s.available() >0)
 {

  char ch=s.read();
  str.concat(ch);
  if( ch =='\n' )
  { str.trim();
    Serial.println(str);
    str_tx = str;
    Serial.println(str_tx);
    
    str_len = str_tx.length()+1;
    Serial.println(str_len);
    //char char_array[sizeof(str_tx)];
    char char_array[str_len];
    //strcpy(char_array, str_tx.c_str());
    //str_tx.toCharArray(char_array, str_len);
    std:copy(str_tx.begin(),str_tx.end(),char_array);
    Serial.println(char_array);
    char_array[sizeof(str_tx)] = '\0';
    //char_array[65] = 0;
    str="";
    str_tx = "";
    Serial.println("------------------------------------");
  }
  /*
  str = "";
  str = s.readStringUntil('\n');
  str.trim();
  Serial.println(str);
  str_tx = str;
  Serial.println(str_tx);
  
  str_tx = str;
  str_tx.replace("\n","");
   
  str_len = str_tx.length();
  Serial.println(str_len);
  Serial.println(str_tx.c_str());*/

}

 
}

The output of the above code comes as below, you can see the first time conversion occurs but the next time only empty string shows up, in all the 3 cases its showing the same hence I need to find out what exactly this issue is caused by, would appreciate if anyone could thro some light on it. Thankyou