Pass by pointer not updating memory address - c++

Question

I am wanting to read back data stored in a buffer. I have a function that takes a pointer to an unsigned char array as a parameter. I wanted this parameter to be populated (point to) the address of the buffer I wish to obtain.
Inside the function I can see while debugging that the parameter passed in is being updated correctly (Mock class), however once I return from this function to the calling method (Connection class), all the data is lost.
Please can anyone help me understand why this is?
Thanks in Advance
Unit Test
/// Unit test fails as expected data is not in the buffer
TEST(MemoryTest, TestWriteAndRead)
{
Connection* p_conneciton = new Connection();
/// Write
uint8_t txBuffer[_USB_PACKET_SIZE] = _CMD;
ASSERT_EQ(p_memory->Write(txBuffer), true);
/// Read
uint8_t* rxBuffer;
ASSERT_EQ(p_memory->Read((unsigned char*)&rxBuffer), true);
ASSERT_EQ(rxBuffer[0], 0xaa);
}
Connection Class
/// Information is lost here in _pu8_buffer
bool Connection::Read(uint8_t* _pu8_buffer)
{
int i_bytesRead = 0;
while(i_bytesRead != SIZE) {
i_bytesRead = read_timeout(_pu8_buffer, _PACKET_SIZE);
if ( i_bytesRead < _PACKET_SIZE) {
return false;
}
}
return true;
}
Mock Class
/// Information is copied correctly here from mpuc_data to data
int Mock:read_timeout(unsigned char* data, size_t length)
{
if (data == nullptr) {
return -1;
}
data = mpuc_data;
return 0;
}

Answer 1

You need to pass by reference parameters you want to modify. If you want to modify the value of a pointer, it's the pointer you need to pass by reference -that it is a reference itself (to something else) won't help.
Try
int Mock:read_timeout(unsigned char*& data, size_t length)
{
if (data == nullptr) {
return -1;
}
data = mpuc_data;
return 0;
}
this way, _pu8_buffer will actually point the same as mpuc_data after each call to read_timeout. You can do the same with the function Read.
In general, pay attention to levels of indirection. With
uint8_t* rxBuffer;
ASSERT_EQ(p_memory->Read((unsigned char*)&rxBuffer), true);
you're taking the address of a uint8_t*, obtaining a uint8_t**, pointer to a pointer. Then you cast it to unsigned char* -a simple, not double pointer! (and then implicitly to uint8_t* when it becomes a parameter) It can't end well... and it ends in trying to read an array of chars where the first 4 or 8 are in fact the bytes of your pointer, followed by other garbage.
Taking the address of a pointer is correct if you want to modify it, but then you need to expect a uint8_t** on the other side, and refer to it as *_pu8_buffer.

Answer 2

/// Information is lost here in _pu8_buffer
bool Connection::Read(uint8_t* _pu8_buffer)
{
int i_bytesRead = 0;
while(i_bytesRead != SIZE) {
i_bytesRead = read_timeout(_pu8_buffer, _PACKET_SIZE);
if ( i_bytesRead < _PACKET_SIZE) {
return false;
}
}
return true;
}
Calling this function gets you a copy of the pointer you passed to it. Meaning that if you change the location that the pointer is pointing to you only change the local copy.
You either change the way you set the data in read_timeout to:
*data = mpuc_data;
Or you pass a pointer to a pointer (uint8_t**) in Connection::Read and call read_timeout(&_pu8_buffer, _PACKET_SIZE);
Also, the way you currently have it &rxBuffer is not correct, it should be rxBuffer. Only pass the address if you change the signature to a uint8_t**.