Convert serial read into a useable string using Arduino

Question

I m using two Arduinos to sent plain text strings to each other using newsoftserial and an RF transceiver   Each string is perhaps 20-30 characters in length  How do I convert Serial read   into a string so I can do if x     testing statements   etc

User · Answer

Here is a more robust implementation that handles abnormal input and race conditions.

It detects unusually long input values and safely discards them. For example, if the source had an error and generated input without the expected terminator; or was malicious.
It ensures the string value is always null terminated (even when buffer size is completely filled).
It waits until the complete value is captured. For example, transmission delays could cause Serial.available() to return zero before the rest of the value finishes arriving.
Does not skip values when multiple values arrive quicker than they can be processed (subject to the limitations of the serial input buffer).
Can handle values that are a prefix of another value (e.g. "abc" and "abcd" can both be read in).

It deliberately uses character arrays instead of the String type, to be more efficient and to avoid memory problems. It also avoids using the readStringUntil() function, to not timeout before the input arrives.

The original question did not say how the variable length strings are defined, but I'll assume they are terminated by a single newline character - which turns this into a line reading problem.

int read_line(char* buffer, int bufsize)
{
  for (int index = 0; index < bufsize; index++) {
    // Wait until characters are available
    while (Serial.available() == 0) {
    }

    char ch = Serial.read(); // read next character
    Serial.print(ch); // echo it back: useful with the serial monitor (optional)

    if (ch == '\n') {
      buffer[index] = 0; // end of line reached: null terminate string
      return index; // success: return length of string (zero if string is empty)
    }

    buffer[index] = ch; // Append character to buffer
  }

  // Reached end of buffer, but have not seen the end-of-line yet.
  // Discard the rest of the line (safer than returning a partial line).

  char ch;
  do {
    // Wait until characters are available
    while (Serial.available() == 0) {
    }
    ch = Serial.read(); // read next character (and discard it)
    Serial.print(ch); // echo it back
  } while (ch != '\n');

  buffer[0] = 0; // set buffer to empty string even though it should not be used
  return -1; // error: return negative one to indicate the input was too long
}

Here is an example of it being used to read commands from the serial monitor:

const int LED_PIN = 13;
const int LINE_BUFFER_SIZE = 80; // max line length is one less than this

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  Serial.print("> ");

  // Read command

  char line[LINE_BUFFER_SIZE];
  if (read_line(line, sizeof(line)) < 0) {
    Serial.println("Error: line too long");
    return; // skip command processing and try again on next iteration of loop
  }

  // Process command

  if (strcmp(line, "off") == 0) {
      digitalWrite(LED_PIN, LOW);
  } else if (strcmp(line, "on") == 0) {
      digitalWrite(LED_PIN, HIGH);
  } else if (strcmp(line, "") == 0) {
    // Empty line: no command
  } else {
    Serial.print("Error: unknown command: \"");
    Serial.print(line);
    Serial.println("\" (available commands: \"off\", \"on\")");
  }
}

User · Answer

This always works for me    String  SerialRead    quot  quot        void setup       Serial begin 9600          void loop       while  Serial available    gt  0           Only run when there is data available         SerialRead    char Serial read       Here every received char will be                                           added to  SerialRead     if   SerialRead indexOf  quot S quot    gt  0      Checks for the letter S              SerialRead    quot  quot                     Do something then clear the string

User · Answer

Use string append operator on the serial read    It works better than string concat   char r  string mystring    quot  quot    while serial available         r   serial read        mystring   mystring   r      After you are done saving the stream in a string mystring  in this case   use SubString functions to extract what you are looking for

User · Answer

Credit for this goes to magma  Great answer  but here it is using c   style strings instead of c style strings  Some users may find that easier  String string    quot  quot   char ch     Where to store the character read  void setup         Serial begin 9600       Serial write  quot Power On quot           boolean Comp String par        while  Serial available    gt  0     Don t read unless                                           there you know there is data               ch   Serial read       Read a character         string    ch     Add it                if  par    string           string    quot  quot          return true             else            dont reset string        return false                void loop         if  Comp  quot m1 on quot              Serial write  quot Motor 1 - gt  Online n quot              if  Comp  quot m1 off quot              Serial write  quot Motor 1 - gt  Offline n quot

User · Answer

If you want to read messages from the serial port and you need to deal with every single message separately I suggest separating messages into parts using a separator like this  String getMessage       String msg  quot  quot     the message starts empty   byte ch     the character that you use to construct the Message    byte d        the separating symbol     if Serial available      checks if there is a new message            while Serial available    amp  amp  Serial peek    d    while the message did not finish                   ch Serial read      get the character           msg   char ch   add the character to the message           delay 1    wait for the next character              ch Serial read      pop the     from the buffer      if ch  d     id finished          return msg       else          return  quot NA quot         else       return  quot NA quot      return  quot NA quot  if no message     This way you will get a single message every time you use the function

User · Answer

This would be way easier  char data  21   int number of bytes received   if Serial available    gt  0        number of bytes received   Serial readBytesUntil  13 data 20      read bytes  max  20  from buffer  untill  lt CR gt   13   store bytes in data  count the bytes recieved      data number of bytes received    0     add a 0 terminator to the char array     bool result   strcmp  data   quot whatever quot       strcmp returns 0  if inputs match     http   en cppreference com w c string byte strcmp   if  result    0       Serial println  quot data matches whatever quot       else       Serial println  quot data does not match whatever quot

User · Answer

I could get away with this   void setup       Serial begin 9600      void loop       String message         while  Serial available        message concat  char  Serial read       if  message            Serial println message

User · Answer

String content    quot  quot   char character   if Serial available    gt 0         reset this variable      content    quot  quot          make string from chars     while Serial available   gt 0            character   Serial read            content concat character           send back        Serial print  quot   quot        Serial print content       Serial print  quot   quot        Serial flush

User · Answer

You can use Serial readString   and Serial readStringUntil   to parse strings from Serial on the Arduino  You can also use Serial parseInt   to read integer values from serial  int x  String str       void loop           if Serial available    gt  0                 str   Serial readStringUntil   n            x   Serial parseInt              The value to send over serial would be my string n5 and the result would be str    quot my string quot  and x   5

User · Answer

If you re using concatenate method then don t forget to trim the string if you re working with if else method

User · Answer

I was asking the same question myself and after some research I found something like that  It works like a charm for me  I use it to remote control my Arduino     Buffer to store incoming commands from serial port String inData       void setup         Serial begin 9600       Serial println  quot Serial conection started  waiting for instructions    quot           void loop         while  Serial available    gt  0                char recieved   Serial read            inData    recieved                   Process message when new line character is recieved         if  recieved      n                         Serial print  quot Arduino Received   quot                Serial print inData                                   You can put some if and else here to process the message juste like that               if inData     quot     n quot       DON T forget to add  quot  n quot  at the end of the string                Serial println  quot OK  Press h for help  quot                                       inData    quot  quot      Clear recieved buffer

User · Answer

Many great answers  here is my 2 cents with exact functionality as requested in the question    Plus it should be a bit easier to read and debug   Code is tested up to 128 chars of input    Tested on Arduino uno r3  Arduino IDE 1 6 8   Functionality    Turns Arduino onboard led  pin 13  on or off using serial command input    Commands    LED ON    LED OFF   Note  Remember to change baud rate based on your board speed      Turns Arduino onboard led  pin 13  on or off using serial command input      Pin 13  a LED connected on most Arduino boards  int const LED   13      Serial Input Variables int intLoopCounter   0  String strSerialInput           the setup routine runs once when you press reset  void setup           initialize the digital pin as an output    pinMode LED  OUTPUT         initialize serial port   Serial begin 250000      CHANGE BAUD RATE based on the board speed        initialized   Serial println  Initialized           the loop routine runs over and over again forever  void loop           Slow down a bit        Note  This may have to be increased for longer strings or increase the iteration in GetPossibleSerialData   function    delay 1     CheckAndExecuteSerialCommand         void CheckAndExecuteSerialCommand         Get Data from Serial   String serialData   GetPossibleSerialData      bool commandAccepted   false     if  serialData startsWith  LED ON            commandAccepted   true      digitalWrite LED  HIGH        turn the LED on  HIGH is the voltage level        else if   serialData startsWith  LED OFF            commandAccepted   true      digitalWrite LED  LOW         turn the LED off by making the voltage LOW       else if  serialData                Serial println        Serial println      Command Failed            Serial println   t    serialData       Serial println        Serial println        Serial println      Invalid Command            Serial println        Serial println  Try         Serial println   tLED ON        Serial println   tLED OFF        Serial println           if  commandAccepted          Serial println        Serial println      Command Executed            Serial println   t    serialData       Serial println           String GetPossibleSerialData       String retVal    int iteration   10     10 times the time it takes to do the main loop   if  strSerialInput length    gt  0             Print the retreived string after looping 10 iteration  ex times     if  intLoopCounter  gt  strSerialInput length     iteration                retVal   strSerialInput          strSerialInput               intLoopCounter   0             intLoopCounter           return retVal     void serialEvent         while  Serial available                strSerialInput concat  char  Serial read

User · Answer

From Help with Serial Read   getting string  char inData 20      Allocate some space for the string char inChar -1     Where to store the character read byte index   0     Index into array  where to store the character  void setup         Serial begin 9600       Serial write  quot Power On quot       char Comp char  This        while  Serial available    gt  0     Don t read unless                                           there you know there is data              if index  lt  19     One less than the size of the array                     inChar   Serial read       Read a character            inData index    inChar     Store it            index       Increment where to write next            inData index      0      Null terminate the string                     if  strcmp inData This      0           for  int i 0 i lt 19 i                  inData i  0                  index 0         return 0             else          return 1            void loop         if  Comp  quot m1 on quot    0            Serial write  quot Motor 1 - gt  Online n quot              if  Comp  quot m1 off quot    0           Serial write  quot Motor 1 - gt  Offline n quot

User · Answer

Unlimited string readed  String content    quot  quot   char character       while Serial available           character   Serial read         content concat character            if  content     quot  quot          Serial println content

User · Answer

The best and most intuitive way is to use serialEvent   callback Arduino defines along with loop   and setup    I ve built a small library a while back that handles message reception  but never had time to opensource it  This library receives  n terminated lines that represent a command and arbitrary payload  space-separated  You can tweak it to use your own protocol easily  First of all  a library  SerialReciever h   ifndef   SERIAL RECEIVER H    define   SERIAL RECEIVER H        class IncomingCommand     private      static boolean hasPayload    public      static String command      static String payload      static boolean isReady      static void reset           isReady   false        hasPayload   false        command    quot  quot         payload    quot  quot             static boolean append char c          if  c      n             isReady   true          return true                if  c         amp  amp   hasPayload            hasPayload   true          return false                if  hasPayload          payload    c        else         command    c        return false                boolean IncomingCommand  isReady   false  boolean IncomingCommand  hasPayload   false  String IncomingCommand  command   false  String IncomingCommand  payload   false        endif     ifndef   SERIAL RECEIVER H    To use it  in your project do this   include  lt SerialReceiver h gt       void setup       Serial begin 115200     IncomingCommand  reset           void serialEvent       while  Serial available          char inChar    char Serial read        if  IncomingCommand  append inChar         return         To use the received commands  void loop       if   IncomingCommand  isReady        delay 10       return       executeCommand IncomingCommand  command  IncomingCommand  payload      I use registry pattern to handle commands  but you are free to do whatever suits your project better       IncomingCommand  reset

[arduino] Convert serial.read() into a useable string using Arduino?

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