If you write a program in C/C++ for Windows, you are very close to the interface with the operating system. For this reason, it is also interesting to have some idea of what it is doing.

There are two main types of Windows programs: graphical programs and console-based programs. The way Windows determine what kind of program you are trying to create is by a flag it stores inside the executable. This is also called the “subsystem” in which the program will run.

To determine the subsystem in visual C++, one has to set an option in the linker section. This will determine what Windows will see as the subsystem for that program.

Depending on the subsystem, Windows will call one form or another of the C library function that starts a program. The name of this function is also configurable (usually it is __tmainCRTStartup), and it can be changed from the linker section of VC++.

First Steps of a Program

The initialization function in the C library does just a little bit of work that is required by the C run time. One of the most important is to initialize the heap, so functions like malloc and the operator new can work properly.

The initialization function also sets some common values used by the system, such as environmental variables, and the version number of Windows.

Finally, __tmainCRTStartup calls the main function declared by the Windows program. Usually this is called WinMain, but it can be some variation of this depending of the version of Windows and if you are using ANSI or Unicode.

When a program returns from the main function, the initialization function cleans up the heap and returns to the operating system.

Reference

• Windows Via C/C++ (by Jeffrey M. Richter)

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