SIM800L 串接

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="+542926556644";
    error=Sim800l.sendSms(number,text);
    // OR 
    //error=Sim800l.sendSms("+540111111111","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="+639164384650";
  error=Sim800l.sendSms(number,text);
  // OR 
  //error=Sim800l.sendSms("+540111111111","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.