Why do I get " stray '#' in program " error?

Question

I am making an Arduino program with SIM800L module and would like to check credit status of the SIM card. When I manually enter the command: AT+CUSD=1,"*123#" into Serial Monitor, it works fine.

But in the Arduino (1.8.7) I am receiving an error stray "#" in program

when compiling this line of sketch: SIM800.print("AT+CUSD=1,"*123#"\r");

I am not experienced enough to fix this problem. Could somebody help me, please?

Thank you. Marko

• Jot: I think something else is stray. Why are there three double quotes in "*123#"\r". A backslash is a normal part of a string, why did you put it outside the string?
• Marko Šinkovc: There are four quotes in the command ("AT+CUSD=1,"*123#"\r"). When I manually enter this command into Serial Monitor, I type AT+CUSD=1,"*123#" .
• Jot: Sorry, there are four indeed. A double quote " in a string closes the string. You need the escape backslash and use \" in the same way as the escape is used for \r. en.cppreference.com/w/cpp/language/escape
• Marko Šinkovc: I understand that *123# is a string for provider to perform this function (return a info of my SIM credit). When I make a phone call from my mobitel, I just enter *123# and receive an answer with this info.
• Marko Šinkovc: It is interesting that a similar command: SIM800.print("ATD*123#\r"); works fine ( return OK) and without any compiler comment.

Answer 1

Use the escape backslash to get a double quote in a text. See: https://en.cppreference.com/w/cpp/language/escape

Is the *123$ between double quotes?

Serial.println("Hello");
Serial.println("I said: Hello to you");
Serial.println("I said: \"Hello\" to you");
Serial.println("AT+CUSD=1,\"*123#\"\r");
Serial.println("AT+CUSD=1,\"*123#\"");

I think the carriage return \r at the end is not needed when the println is used. The println adds a linefeed \n by the way.

• Marko Šinkovc: The last suggested command SIM800.println("AT+CUSD=1,\"*123#\""); is accepted by compiler but returns only "OK" - the same answer as basic command SIM800.println("AT+CUSD=1"); . It looks like as the parameter \"*123#\ is not forwarded to SIM800?
• Marko Šinkovc: .....or the provider does not recognise this parameter \"*123#\ ?
• PimV: You could try to send the ASCII code of these characters with Serial.write(). In that way you don't have to bother about escaping and the compiler doesn't see the # anymore.
• Jot: The command \"*123#\" is translated by the compiler into "*123#" including the two double quotes. I don't know if that is recognized, I see examples that have a comma and a number after it.
• Marko Šinkovc: Thank you all for the good will and your efforts to help me. Jot, can you please clarify what is meant by "... have a comma and a number after it."