Using MFC, posting variable to thread along with message - c++

Question

I am using CWinThread. And I have made the main GUI send to the thread an array in LPARAM. Example, this code WORKS:
//On GUI
char *headData = "L1";
PostThreadMessage(threadID,SEND_HEAD, 0, (LPARAM)head);
On the thread:
void CMyThread::OnSendhead( WPARAM, LPARAM lParam){
char *head = (char*)lParam;
if (strcmp(head,"L1")==0){
//This line is reached.
}
return;
}
But when I make a little change here:
char *head = "L1"
unsigned char byteHead[3];
memcpy(byteHead, head, 3);
PostThreadMessage(threadID,SEND_HEAD, 0, (LPARAM)byteHead);
On the thread:
void CMyThread::OnSendhead( WPARAM, LPARAM lParam){
unsigned char* byteHead = (unsigned char*)lParam;
char head[3];
memcpy(head, byteHead,3);
head[3] = '\0';
if (strcmp(head,"L1")==0){
//This line is nerver reached.
}
return;
}
The line inside if is not reach. I have moved the code on thread to GUI for testing (without casting to LPARAM), and everthing works. So I guess I can't cast between lParam and unsigned char*? Why, and how can I do this? Thank you guys

Answer 1

PostMessage doesn't process the message immediately, but leaves it for the message loop to pick up later. If the data pointed to by lparam has been destroyed in the meantime, it's going to fail.
In your first example, you're passing a string literal. String literals tend to stay valid for the full lifetime of the program, so this is OK.
In the second example, you're using a local variable that presumably gets destroyed soon after you do the PostMessage.
To make this work, use SendMessage instead; it gets processed immediately. Or use a variable that is guaranteed to be valid until the message is processed.

Answer 2

With PostMessage or PostThreadMessage you should always use heap allocated memory to send data, as small as bool and in the message handler function you delete it. You can wrap a message in some struct, for example:
struct MyMsg
{
int msgCode; // specific to you, and you know underlying datatype
char buffer[1024]; // or have a `vector` or dynamic-array you'd manage
};
Sender:
MyMsg* msg = new MyMsg;
msg->msgCode = 0x1; // It's a string
strcpy_s(msg->buffer, "abc");
PostThreadMessage(threadID,SEND_HEAD, 0, (LPARAM)msg);
Receiver:
void CMyThread::OnSendhead( WPARAM, LPARAM lParam)
{
MyMsg* msg= (MyMsg*)lParam;
// use msg
delete msg;
}