SMS in modem's memory I am working on SMS communication with my phone and Arduino. My intension is simple. When somebody send an SMS, the Arduino should read a text file located on my server and forward the content to the same number.

The problem here is, when I send an SMS to my Arduino, which is waiting for my message, it does not read my message. But when I separately run this example code, I see my SMS is received. It means my SMS is already present in the modem memory. But why does this happen? How to solve this? Here is my code:

// libraries
#include &lt;GSM.h&gt;

// PIN Number
#define PINNUMBER ""

// APN data
#define GPRS_APN       "GPRS_APN" // replace your GPRS APN
#define GPRS_LOGIN     "login"    // replace with your GPRS login
#define GPRS_PASSWORD  "password" // replace with your GPRS password

// initialize the library instance
GSMClient client;
GPRS gprs;
GSM gsmAccess; 
GSM_SMS sms;

// URL, path &amp; port (for example: arduino.cc)
char server[] = "yourdomain.com";
char path[] = "/current.txt";
int port = 80; // port 80 is the default for HTTP

char k[30]; 
char z = '*';
int s=0;
String place;
char remoteNum[20];  // telephone number to send sms

void setup() {
    // initialize serial communications and wait for port to open:
    Serial.begin(9600);
    while (!Serial) {
        ; // wait for serial port to connect. Needed for Leonardo only
    }
    Serial.println("SMS connected");
    // connection state
    boolean notConnected = true;
    while(notConnected) {
        if((gsmAccess.begin(PINNUMBER)==GSM_READY) &amp;
           (gprs.attachGPRS(GPRS_APN, GPRS_LOGIN, GPRS_PASSWORD)==GPRS_READY))
            notConnected = false;
        else {
            Serial.println("Not connected");
            delay(1000);
        }
    }
    Serial.println("GPRS connected...");
    Serial.println("Server Connecting.");
    // if you get a connection, report back via serial:
    serverr();
}

void loop() {
    if (sms.available()) {
        Serial.println("Message received from:");
        sms.remoteNumber(remoteNum, 20);
        Serial.println(remoteNum);
        sms.flush();
        Serial.println("MESSAGE DELETED");
    }
}

void smss() {
  // I just want phone number
    if (client.available()) {
        char c = client.read();
        //***************************************************
        //       Filtering a specific message
        //**************************************************
        if (z==c) {
            s=1;
        }
        if(s == 1) {
            int i=0;
            k[i] = c;
            i=i+1;
            // Serial.print(k);
        }
        place +=k;
        //Serial.print(place);
    }
    //***************************************************  

    if (!client.available() &amp;&amp; !client.connected()) {
        sms.beginSMS(remoteNum);
        Serial.print("Place =");
        Serial.print(place);
        sms.print(place);
        sms.endSMS(); 
        sms.flush();
        Serial.println("MESSAGE DELETED");
        Serial.println();
        Serial.println("disconnecting.");
        client.stop();
        // do nothing forevermore:
        for(;;)
            ;
    }
}

// if you didn't get a connection to the server:
// Serial.print(server);

void serverr() {
    if (client.connect(server, port)) {
        client.print("GET /current.txt");
        Serial.print("GET /current.txt");
        client.println(" HTTP/1.1");
        Serial.println(" HTTP/1.1");
        client.println("Host: www.yourdomain.com");
        Serial.println("Host: www.yourdomain.com");
        client.println("User-Agent: Arduino");
        Serial.println("User-Agent: Arduino");
        client.println("Accept: text/html");
        Serial.println("Accept: text/html");
        client.println("Connection: close");
        Serial.println("Connection: close");
        client.println();
        Serial.println();

        Serial.println("\nCOMPLETE!\n");
        //client.stop();
    } else {
        Serial.println("connection failed");
        Serial.println("\n FAILED!\n");
    }
}
I see one problem with the global structure of your program; everytime loop() is run, it: 


checks if an SMS is present, and gets the originating phone number if present, otherwise it just goes on, ie smss() function returns without modifying remoteNumber
send an SMS to remoteNumber


One problem here is that your loop() will repeatedly try to send an SMS to the same remoteNumber even if no SMS was received; actually, to solve this issue, you should modify smss() to:


either wait (forever) for an SMS to be received and return only in this situation
or return a value (eg bool) that indicates whether an SMS was received or not; then an SMS should be sent if and only if an SMS was just received.


Here is an example:

void setup()
{
    GSM and GPRS initialization
    Connection to server
} 

void loop()
{
    if (smss())
    {
        Reading the txt file content in server

        sms.beginSMS(remoteNum);
        sms.print(txtMsg);
        sms.endSMS(); 
    }
}

bool smss() // I just want phone number
{
    if (sms.available())
    {
        Serial.println("Message received from:");
        sms.remoteNumber(remoteNum, 20);
        Serial.println(remoteNum);
        sms.flush();
        Serial.println("MESSAGE DELETED");
        return true;
    }
    return false;
}


Note that this code does not integrate error support as it should; indeed sms.remoteNumber() may return error codes (any value &gt;1), in which case you should consider no SMS was received at all.