Capturing serial input in string/char array

Question

I have a software serial link between an Arduino Uno and a TC35 GSM Module to send and receive SMS messages. Sending SMS'/calls is not a problem as it is a matter of sending the appropriate AT command to the GSM module. However I wish to use the AT+CMGR=1 command (which checks the first SMS stored on the SIM card) to check if there is any messages and store the message as a char array so that I can then check if the SMS contains the word 'on' or 'off' to activate a LED.

The AT+CMGR=1 command should return the following:

AT+CMGR=1
+CMGR: "REC READ","+3538xxxxxxxx",,"13/03/23,14:29:37+00"
Set

OK

But in the method below when I print 'data' it just returns:

Message contains:

AT

Any pointers would be much appreciated.

void checkMessage() {

    gsmSerial.println("AT+CMGR=1");    //Reads the first SMS

    for (x=0;x < 255;x++){            
        data[x]='\0';                        
    } 
    x=0;
    do{
        while(gsmSerial.available()==0);
        data[x]=gsmSerial.read();  
        x++;           
        if(data[x-1]==0x0D&&data[x-2]=='"'){
            x=0;
        }
    }while(!(data[x-1]=='K'&&data[x-2]=='O'));

    data[x-3]='\0';        //finish the string before the OK

    Serial.println("\r\nMessage contains: \r");
    Serial.println(data);    //shows the message

    delay(1000);
}

data is the char array; char data[256]. gsmSerial is a software serial connection between Arduino and GSM Module — BLL27
For simplicity of code, I would recommend you have to have ONLY the word ON (all caps) so you don't have to search the whole message. This would also prevent accidental "Is this thing on?" to turn on the LED. — Anonymous Penguin

Carey Gregory Carey Gregory · Accepted Answer · 2013-03-25T20:26:16

I don't understand your intent of doing this:

if(data[x-1]==0x0D&&data[x-2]=='"'){ 
    x=0; 
}

It appears you're discarding your data and reading new data whenever you hit a line containing a trailing quote and newline. So the response to the command of interest is being discarded. I haven't tested it, but I think it would work if you deleted those three lines.

I should also mention that [x-1] and [x-2] are referencing memory prior to the data buffer. That's a very bad practice that can and will cause undefined behavior. You should only check the index minus some value when the result of that computation will be a positive value. A negative array index will access memory prior to the start of the array.

Capturing serial input in string/char array

1 Answers