Clearing string buffer with memset after a serial read

Question

In using the SerialTransfer.h / pySerialTransfer libraries to send commands between my laptop and an Arduino Mega, I am sending multiple sequential strings between the devices. However, I'm having trouble "clearing" the receiving buffer on the Arduino side - it never shrinks, despite a few different attempts to clear the string (memset; a for loop).

I'm sending a string to the Arduino of the format , where there are anywhere from 1 to 5 asterisks. Then the Arduino is simply sending the exact same string back, without any processing (as of yet). And in the python terminal, I'm printing it all out; this is what I get:

SENT (17 bytes): <rand_len_str=**>
RECD (17 bytes): <rand_len_str=**>

SENT (18 bytes): <rand_len_str=***>
RECD (18 bytes): <rand_len_str=***>

SENT (19 bytes): <rand_len_str=****>
RECD (19 bytes): <rand_len_str=****>

SENT (17 bytes): <rand_len_str=**>
RECD (19 bytes): <rand_len_str=**>*>

SENT (18 bytes): <rand_len_str=***>
RECD (19 bytes): <rand_len_str=***>>

SENT (18 bytes): <rand_len_str=***>
RECD (19 bytes): <rand_len_str=***>>

As long as the string is growing, all is well - but see the error in the 4th pair, with the extra *> characters, etc.

The following is my Arduino code; I've tried to reset the receiving string using memset, but this seems not to be working.

#include "SerialTransfer.h"

SerialTransfer myTransfer;

const int CMD_LEN = 200;
char buff[CMD_LEN];
char received_str[CMD_LEN];

void setup()
{
  Serial.begin(115200);
  myTransfer.begin(Serial);
}

void loop()
{
  if(myTransfer.available())
  {
    //////////////////////////////////////////////
    // handle call from Python

    memset(received_str, '\0', CMD_LEN*sizeof(char));
    myTransfer.rxObj(received_str, CMD_LEN);

    sprintf(buff, "%s", received_str);
    //////////////////////////////////////////////

    //////////////////////////////////////////////
    // send response
    myTransfer.txObj(buff, strlen(buff));
    myTransfer.sendData(strlen(buff));
    //////////////////////////////////////////////
  }
  else if(myTransfer.status < 0)
  {
    Serial.print("ERROR: ");

    if(myTransfer.status == -1)
      Serial.println(F("CRC_ERROR"));
    else if(myTransfer.status == -2)
      Serial.println(F("PAYLOAD_ERROR"));
    else if(myTransfer.status == -3)
      Serial.println(F("STOP_BYTE_ERROR"));
  }
}

I didn't see anything in SerialTransfer.h about clearing the rxObj; I can always just use the first terminating > character to parse the command - but I imagine there is a cleaner / better solution in clearing the buffer, or at least, I'd like to understand what's going on! Thanks in advance.

Answer 1

Instead of clearing your buffers, make use of the bytesRead property. Have a look at how they print the received string in the example code from the library using a for loop. I don't know that it is null terminated or anything.

Example from the library's github page:

#include "SerialTransfer.h"

SerialTransfer myTransfer;

void setup()
{
  Serial.begin(115200);
  Serial1.begin(115200);
  myTransfer.begin(Serial1);
}

void loop()
{
  if(myTransfer.available())
  {
    Serial.println("New Data");
    for(byte i = 0; i < myTransfer.bytesRead; i++)
      Serial.write(myTransfer.rxBuff[i]);
    Serial.println();
  }
  else if(myTransfer.status < 0)
  {
    Serial.print("ERROR: ");

    if(myTransfer.status == -1)
      Serial.println(F("CRC_ERROR"));
    else if(myTransfer.status == -2)
      Serial.println(F("PAYLOAD_ERROR"));
    else if(myTransfer.status == -3)
      Serial.println(F("STOP_BYTE_ERROR"));
  }

Answer 2

Thanks for the pointer - that did the trick. Applying it to my original code, if I update the few lines around handle call from Python to the following, it works as intended.

    //////////////////////////////////////////////
    // handle call from Python

    myTransfer.rxObj(received_str, CMD_LEN);
    strncpy(buff, received_str, myTransfer.bytesRead);
    buff[myTransfer.bytesRead] = '\0';

    //////////////////////////////////////////////

Thanks again!