#include <windows.h>

# Preprocessor directives are lines included in the code of our programs that are not program statements but directives for the preprocessor. These lines are always preceded by a hash sign (#). The preprocessor is executed before the actual compilation of code begins, therefore the preprocessor digests all these directives before any code is generated by the statements.

These preprocessor directives extend only across a single line of code. As soon as a newline character is found, the preprocessor directive is considered to end. No semicolon (;) is expected at the end of a preprocessor directive. The only way a preprocessor directive can extend through more than one line is by preceding the newline character at the end of the line by a backslash (\).

WINDOWS.H is a master include file that includes other Windows header files, some of which also include other header files. The most important and most basic of these header files are:

• WINDEF.H Basic type definitions.
• WINNT.H Type definitions for Unicode support.
• WINBASE.H Kernel functions.
• WINUSER.H User interface functions.
• WINGDI.H Graphics device interface functions.

These header files define all the Windows data types, function calls, data structures, and constant identifiers. They are an important part of Windows documentation. You might find it convenient to use the Find In Files option from the Edit menu in the Visual C++ Developer Studio to search through these header files. You can also open the header files in the Developer Studio and examine them directly.

The entry point to a Windows program is WinMain

Looks Like this

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)

WINAPI must be included because windows expects a WinMain that is of the _stdcall calling convention, not the default _cdecl.

The first parameter HINSTANCE hInstance is a instance handle.  Handle is simply a number that an application uses to identify something. In this case, the handle uniquely identifies the program. It is required as an argument to some other Windows function calls. In early versions of Windows, when you ran the same program concurrently more than once, you created multiple instances of that program. All instances of the same application shared code and read-only memory (usually resources such as menu and dialog box templates). A program could determine if other instances of itself were running by checking the hPrevInstance Second parameter. It could then skip certain chores and move some data from the previous instance into its own data area.

In the 32-bit versions of Windows, this concept has been abandoned. The second parameter to WinMain is always NULL (defined as 0).

The third parameter to PSTR szCmdLine is the command line used to run the program. Some Windows applications use this to load a file into memory when the program is started.

The fourth parameter to int iCmdShow indicates how the program should be initially displayed—either normally or maximized to fill the window, or minimized to be displayed in the task list bar.