array in C/C++ for AVR keeps appending - c++

Question

In C, I have an array waiting to receive bytes from a sensor, save in a buffer, and then print out like this:
unsigned char responseFrame[300];
int main(void) {
UART_init();
while(1) {
receive(responseFrame);
myLog(responseFrame, sizeof(responseFrame));
}
}
I populate the array by doing the following:
void receive(unsigned char *rcv_buff) {
uint8_t recv_data;
for (int i=0; i<300; i++){
USART1_Flush();
rcv_buff[i] = USART1_RX();
}
}
Then I print out what's in the buffer using the following:
// Logs this output to the serial port; used for debugging
void myLog(unsigned char *msg, int size) {
for (int i=0; i<size; i++) {
USART0_TX(msg[i]);
}
}
This prints out the array, but when another iteration of bytes is received, everything is appended so let's say I receive {0xFF, 0xFF} first my output for the first iteration is:
0xFF 0xFF, 0x00, 0x00 ... 0x00
But upon the next iteration let's say {0x0A, 0x0A} is received instead, in the output I see this:
0xFF, 0xFF, 0x0A, 0x0A, 0x00, 0x00 ... 0x00
NOTE: The ellipses is just saying there's more 0x00s which are printed out until we basically reach the size of the array.
Why is this appending and not overwriting from the start of the array?
Here's my USART0_TX and USART1_RX functions:
void USART0_TX(uint8_t myData) {
// Wait if a byte is being transmitted
while( !(UCSR0A & (1<<UDRE0)) );
// Transmit data
UDR0 = myData;
};
uint8_t USART1_RX(void) {
// Wait until recv buffer is full
while( !(UCSR1A & (1<<RXC1)) );
// Return recvd data
return UDR1;
};
Here's the code I'm using to flush my USART1 RX:
//USART1 flush, clears USART1 buffer
void USART1_Flush( void )
{
unsigned char dummy;
while ( UCSR1A & (1<<RXC1) ) dummy = UDR1;
}

Answer 1

I believe your function called "Flush" is really a "Poll" function, looking for a character to appear (this would be normal usage, wait for the char to appear). The logic where you use RXC1 appears inverted. Try looking at this quite professional looking AVR driver (that has a poll option just as you are doing):
usart.c
Another nicely commented polled driver (and small with lots of comments): avr uart driver