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円', 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.

• Natan Jimenez: what mean '0円' in this line? memset(received_str, '0円', CMD_LEN*sizeof(char))

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円';
 //////////////////////////////////////////////

Thanks again!