SIM800L string concatenation - string

Question

I have this code from a website that I'm using as a guide to send SMS message from a SIM800L connected to my Arduino Mega.
#include <Sim800l.h>
#include <SoftwareSerial.h>
Sim800l Sim800l; //declare the library
char* text;
char* number;
bool error;
void setup(){
Sim800l.begin();
text="Testing Sms";
number="[phone]";
error=Sim800l.sendSms(number,text);
// OR
//error=Sim800l.sendSms("[phone]","the text go here");
}
void loop(){
//do nothing
}
I added some bits of code in the middle so that it will receive a string input from a user in my Python GUI via serial connection.
#include <Sim800l.h>
#include <SoftwareSerial.h>
Sim800l Sim800l; //declare the library
char* text;
char* number;
bool error;
String data;
void setup(){
Serial.begin(9600);
}
void loop(){
if (Serial.available() > 0)
{
data = Serial.readString();
Serial.print(data);
sendmess();
}
}
void sendmess()
{
Sim800l.begin();
text="Power Outage occured in area of account #: ";
number="[phone]";
error=Sim800l.sendSms(number,text);
// OR
//error=Sim800l.sendSms("[phone]","the text go here");
}
I am trying to concatenate the data from my serial.readString() to the end of the text. Conventional methods like the + and %s don't work.
In Arduino IDE I'm getting this error:
error: cannot convert ‘StringSumHelper’ to ‘char*’ in assignment
If I'm correct char* is a pointer that points to an address. Is there anyway to add the string from the serial monitor to the text?

Answer 1

You have to convert the Arduino String object to a standard C string. You can do this by using the c_str() method of the String class. It will return a char* pointer.
Now you can concatenate the two string with the strncat function from C library, string.h and also using strncpy as well.
#include <string.h>
char message[160]; // max size of an SMS
char* text = "Power Outage occured in area of account #: ";
String data;
/*
* populate <String data> with data from serial port
*/
/* Copy <text> to message buffer */
strncpy(message, text, strlen(text));
/* Calculating remaining space in the message buffer */
int num = sizeof(message) - strlen(message) - 1;
/* Concatenate the data from serial port */
strncat(message, data.c_str(), num);
/* ... */
error=Sim800l.sendSms(number, message);
Note it will simply chop off the remaining data if there is not enough space in the buffer.