Why is "This program cannot be started in DOS mode" present in DLL files?

Question

I recently opened a DLL file created by Visual Studio 9 when compiling my own C ++ DLL project, and was surprised to see the text "This program cannot be started in DOS mode" at the beginning.

Why is this text needed in DLL files?

Answer 1

A DLL is very similar to an executable with a different extension. The text you saw is part of the standard executable header in windows. It (was) used to gracefully stop trying to run an executable from DOS.

Answer 2

The Portable Executable format specification states the following:

The MS-DOS stub is a valid application that runs under MS-DOS. This is a front-facing EXE image. The compiler places a default stub here, which prints the message "This program cannot be started in DOS mode" when the image is launched in MS-DOS. The user can specify another stub using the / STUB linker option.

At 0x3c, the stub has a file offset to the PE signature. This information allows Windows to execute the image file correctly, although it has an MS-DOS stub. This file offset is placed at 0x3c during linking.

Answer 3

Win32 programs launched from DOS mode (i.e. one user, without graphics) print this text. DLLs also probably print this message if you try to use them without starting Windows.