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Asked: 2026-05-11T02:54:52+00:00

Is OutputDebugString(PAnsiChar(”)); thread safe? I/we have been using it in threads for debugging, and

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Is 

OutputDebugString(PAnsiChar(''));

thread safe?

I/we have been using it in threads for debugging, and it never occurred to me if I should be doing it a different way.

(Delphi 7)

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1 Answer

  1. Don’t worry, it is.

    When OutputDebugString() is called by an application, it takes these steps. Note that a failure at any point abandons the whole thing and treats the debugging request as a no-op (the string isn’t sent anywhere).

    1. Open DBWinMutex and wait until we have exclusive access to it.
    2. Map the DBWIN_BUFFER segment into memory: if it’s not found, there is no debugger running so the entire request is ignored.
    3. Open the DBWIN_BUFFER_READY and DBWIN_DATA_READY events. As with the shared memory segment, missing objects mean that no debugger is available.
    4. Wait for the DBWIN_BUFFER_READY event to be signaled: this says that the memory buffer is no longer in use. Most of the time, this event will be signaled immediately when it’s examined, but it won’t wait longer than 10 seconds for the buffer to become ready (a timeout abandons the request).
    5. Copy up to about 4kbytes of data to the memory buffer, and store the current process ID there as well. Always put a NUL byte at the end of the string.
    6. Tell the debugger that the buffer is ready by setting the DBWIN_DATA_READY event. The debugger takes it from there.
    7. Release the mutex
    8. Close the Event and Section objects, though we keep the handle to the mutex around for later.
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