Arduino read string from serial

/*
* Serial.c
*
* Created: 18/10/2019 18:14:50
* Author : Leonardo Lisa
*/

#define F_CPU      16000000UL
#define BAUD       9600
#define MAX_BUFFER_LENGTH 64
#define USART_RX_MAX_BUFFER_LENGTH 20

#define BRC    ((F_CPU/16/BAUD) - 1)

#include <avr/io.h>
#include <stdlib.h>
#include <avr/eeprom.h>

void USART_Begin(unsigned int brc)
{
    // Set baud rate.
    UBRR0H = (brc >> 8);
    UBRR0L = brc;
    // Enable receiver and transmitter with their interrupt.
    UCSR0B |= (1 << TXEN0) | (1 << TXCIE0) | (1 << RXEN0) | (1 << RXCIE0);
    // Set frame format yo 8bit data.
    UCSR0C = (1 << UCSZ01) | (1 << UCSZ00);
}

// Send one byte.
void USART_Transmit(unsigned char data)
{
    /* Wait for empty transmit buffer */
    while (!(UCSR0A & (1<<UDRE0)));
    /* Put data into buffer, sends the data */
    UDR0 = data;
}

// Receive one byte.
unsigned char USART_Receive(void)
{
    /* Wait for data to be received */
    while (!(UCSR0A & (1<<RXC0)));
    /* Get and return received data from buffer */
    return UDR0;
}

void USART_Transmit_Buffer(char *buffer)
{
    char error_message[] = "ERROR: Empty message or null pointer\n";
    // Get message length.
    if (buffer == NULL)
    {
        buffer = error_message;
    }
    // Print buffer message.
    for (uint8_t size = 1; *buffer != '\0' && size != MAX_BUFFER_LENGTH; size++, buffer++)
    {
        USART_Transmit(*buffer);
    }
}

void *USART_Receive_Buffer(void)
{
    // Allocate n. byte for the message buffer + one byte for '\0' termination character.
    char *buffer = calloc((USART_RX_MAX_BUFFER_LENGTH + 1), 8);
    char *tmp_buffer = buffer;
    if (buffer == NULL)
    {
        return NULL;
    }
    for (uint8_t size = 1; size < USART_RX_MAX_BUFFER_LENGTH; size++, buffer++)
    {
        *buffer = USART_Receive();
        if (*buffer == '\0')
        {
            if (size == 1)
            {
                free(buffer);
                return NULL;
            }
            return realloc(tmp_buffer, size);
        }
    }
    // buffer++;
    // *buffer = '\0'; (remember that '\0' == 0).
    // Not necessary because calloc have already cleared the allocated memory to zero.
    return tmp_buffer;
}


int main(void)
{
    // Same as write: char message[]{'S','e','r','i','a','l',' ','O','N','\n'};
    // Compiler place '\0' at the end of the string.
    char message[] = "Serial On\n";
    // No '\0' character at the end of the string. DANGEROUS!
    char message2[] = {'1','2','3','4','5'};
    char *temp;

    USART_Begin(BRC);
    USART_Transmit_Buffer(message);
    USART_Transmit_Buffer(message2);
    while (1)
    {
        temp = USART_Receive_Buffer();
        USART_Transmit_Buffer(temp);
        // IMPORTANT! Clean your memory.
        free(temp);
    }
}