Pass char[N] param to thread - c++

Question

i have a variable declared as
char myvariable[N] = "blablabla"
and i need to pass it to a thread
DWORD MyThread(LPVOID lpdwParam)
i tried to cast inside the thread function the lpdwParam in char* but it doesn't work. How can i fix it?
EDIT 1
Here the complete part of code
char myvariable[16];
GetDlgItemTextA(hDlg, IDC_IPADDRESS, &myvariable[0], 16);
myThreadHandle = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)MyThread, (LPVOID)&myvariable, 0, &myThreadId);
DWORD MyThread(LPVOID lpdwParam)
{
char *myvariable2 = (char *)lpdwParam;
....
}
EDIT 2
Doesn't work means that myvariable2 doesn't contains the text of myvariable
EDIT 3
char *myvariable = new char[16];
GetDlgItemTextA(hDlg, IDC_IPADDRESS, myvariable, 16);
myThreadHandle = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)MyThread, (LPVOID)myvariable, 0, &myThreadId);
and the thread function:
char *myvariable2 = (char *)lpdwParam;

Answer 1

If you're doing something like this:
// ...
char myvariable[N] = "blablabla";
HANDLE hThread=CreateThread(NULL, 0, MyThread, (LPVOID)myvariable, 0); // however you should use beginthread to avoid troubles with the CRT
// ...
DWORD MyThread(LPVOID lpdwParam)
{
char * mystring = (char *)lpdwParam;
// ...
return 0;
}
and you don't get the correct characters by reading mystring, my guess is that, being myvariable allocated on the stack, it goes out of scope before MyThread gets the chance to run, thus you get the same kind of problems you encounter when you return a pointer to a local variable.
The solution would be to allocate dynamically your string:
// ...
char * myvariable = new char[N];
*myvariable=0;
strncat(myvariable, "blablabla", N); // this is equivalent to what the nonstandard strlcpy would do
HANDLE hThread=CreateThread(NULL, 0, MyThread, (LPVOID)myvariable, 0); // however you should use beginthread to avoid troubles with the CRT
// ...
DWORD MyThread(LPVOID lpdwParam)
{
char * mystring = (char *)lpdwParam;
// ...
// when you're done with mystring, remember to free it; even better, use a smart pointer
delete [] mystring;
// ...
return 0;
}
By the way, notice that, as already said in the snippets, if you plan to use CRT functions inside your newly created thread you shouldn't use CreateThread directly, but you should instead use the _beginthread/_beginthreadex functions, because otherwise some CRT per-thread structures may not get initialized correctly.
Addendum
i edited my code to use new char[16] but it doesn't work. When i cast the variable from LPVOID to char* inside the thread function i get only strange symbols
If you changed the code so that myvariable is now a pointer, you should remove the & from the call to CreateThread; & was superfluous when it was used with the array (in that case just using its name was enough), but now that myvariable is a pointer it is plain wrong, since using it on myvariable will provide the address of myvariable instead of the address stored in myvariable.
Long story short, remove that ampersand.

Answer 2

If your doing c++ casting, you will need to use const char* var = reinterpret_cast<const char*>(lpdwParam);. Just be careful though, if its a stack string, it'll probably pass out of scope before the thread uses it, if its a static string, you should use const to preventing any accidental overwriting of data