What is proper way to read and delete message from SIM900? - gsm

Question

I would like to wait in loop for sms message arrival on my SIM900 and when message is detected , read that message and delete it from the system.
What's bothering me is what if another message comes while I am working on this one (Unsolicited message info).
What is proper algorithm to handle sms reading and deletion?

Answer 1

This is how I do similar task
At start delete all sms from sim memory
Enable new message alerts using AT+CNMI command
Use Interrupt based serial communication and make use of a ring buffer to store all in-coming data
Whenever you will get a new message the modem will send an alert to the controller.
parse the alert message to know the location of sms
After reading the SMS delete the sms
Below is another approach.
/*
* Description: This example shows hot to read a SMS from SIM memory.
* This example only shows the AT commands (and the answers of the module) used
* to read the SMS For more information about the AT commands, refer to the AT
* command manual.
*
* Copyright (C) 2013 Libelium Comunicaciones Distribuidas S.L.
* http://www.libelium.com
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* Version 0.2
* Author: Alejandro Gallego
*/
int8_t answer;
int x;
int onModulePin= 2;
char SMS[200];
void setup(){
pinMode(onModulePin, OUTPUT);
Serial.begin(115200);
Serial.println("Starting...");
power_on();
delay(3000);
// sets the PIN code
sendATcommand("AT+CPIN=****", "OK", 2000);
delay(3000);
Serial.println("Setting SMS mode...");
sendATcommand("AT+CMGF=1", "OK", 1000); // sets the SMS mode to text
sendATcommand("AT+CPMS=\"SM\",\"SM\",\"SM\"", "OK", 1000); // selects the memory
answer = sendATcommand("AT+CMGR=1", "+CMGR:", 2000); // reads the first SMS
if (answer == 1)
{
answer = 0;
while(Serial.available() == 0);
// this loop reads the data of the SMS
do{
// if there are data in the UART input buffer, reads it and checks for the asnwer
if(Serial.available() > 0){
SMS[x] = Serial.read();
x++;
// check if the desired answer (OK) is in the response of the module
if (strstr(SMS, "OK") != NULL)
{
answer = 1;
}
}
}while(answer == 0); // Waits for the asnwer with time out
SMS[x] = '\0';
Serial.print(SMS);
}
else
{
Serial.print("error ");
Serial.println(answer, DEC);
}
}
void loop(){
}
void power_on(){
uint8_t answer=0;
// checks if the module is started
answer = sendATcommand("AT", "OK", 2000);
if (answer == 0)
{
// power on pulse
digitalWrite(onModulePin,HIGH);
delay(3000);
digitalWrite(onModulePin,LOW);
// waits for an answer from the module
while(answer == 0){ // Send AT every two seconds and wait for the answer
answer = sendATcommand("AT", "OK", 2000);
}
}
}
int8_t sendATcommand(char* ATcommand, char* expected_answer, unsigned int timeout){
uint8_t x=0, answer=0;
char response[100];
unsigned long previous;
memset(response, '\0', 100); // Initialice the string
delay(100);
while( Serial.available() > 0) Serial.read(); // Clean the input buffer
Serial.println(ATcommand); // Send the AT command
x = 0;
previous = millis();
// this loop waits for the answer
do{
// if there are data in the UART input buffer, reads it and checks for the asnwer
if(Serial.available() != 0){
response[x] = Serial.read();
x++;
// check if the desired answer is in the response of the module
if (strstr(response, expected_answer) != NULL)
{
answer = 1;
}
}
// Waits for the asnwer with time out
}while((answer == 0) && ((millis() - previous) < timeout));
return answer;
}