When will GetMessage return -1?

Raymond Chen

A source of great consternation is the mysterious -1 return value from Get­Message:

If there is an error, the return value is −1. For example, the function fails if hWnd is an invalid window handle or lpMsg is an invalid pointer.

That paragraph has caused all sorts of havoc, because it throws into disarray the standard message pump:

MSG msg;
while (GetMessage(&msg, NULL, 0, 0)) {

But don’t worry, the standard message pump is safe. If your parameters are exactly

  • a valid pointer to a valid MSG structure,
  • a null window handle,
  • no starting message range filter,
  • no ending message range filter,

then Get­Message will not fail with -1.

Originally, the Get­Message function did not have a failure mode. If you passed invalid parameters, then you invoked undefined behavior, and you probably crashed.

Later, somebody said, “Oh, no, the Get­Message function needs to detect invalid parameters and instead of crashing, it needs to fail gracefully with some sort of error code.” (This was before “Fail-Fast” came into fashion.)

The problem is that Get­Message‘s return value of BOOL was already specified not as a success/failure code, but rather a “Has a WM_QUIT message been received?” code. So return FALSE wouldn’t work.

The solution (if that’s what you want to call it) was to have Get­Message return the not-really-a-BOOL-but-we’ll-pretend-it-is value -1 to signal an invalid parameter error.

And that’s what threw everybody into a tizzy, because now every message loop looks buggy.

But you can calm down. The standard message loop is fine. All the parameters are hard-coded (and therefore valid by inspection), save for the &msg parameter, which is still valid by inspection. So that case is okay. It has to be, for compatibility.

The people who need to worry are people who pass a variable as the window handle filter (because that window handle may no longer be valid), or pass dynamically-allocated memory as the lpMsg (because the pointer may no longer be valid), or who pass a nontrivial message filter (because the filter parameters may be invalid).

In practice, the memory for the lpMsg is nearly always a stack variable (so the pointer is valid), and the message range filters are hard-coded (so valid by inspection). The one to watch out for is the window handle filter. But we saw earlier that a filtered Get­Message is a bad idea anyway, because your program will not respond to messages that don’t meet the filter.


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