Timers are used to schedule a periodic event and execute some program code.  The SetTimer() API function creates a timer and sets its parameters. Once a timer is created, it will generate timer events until the timer is terminated or the application exits. The prototype of this function is

UINT_PTR SetTimer(HWND hWnd,UINT  ID,UINT uElapse,TIMERPROC lpTimerFunc);

hWnd – a handle to the window to be associated with the timer..
iD – specifies an id value associated with the timer.
uElapse – Sets the time between interrupts in milliseconds.
lpTimerFunc – If lpTimerFunc is NULL, the system posts a WM_TIMER message to the application queue. If a function is specified the system directly calls an application-defined TIMERPROC callback function for each timer event.

If the function succeeds, the return value is an integer identifying the new timer. If the function fails to create a timer, the return value is zero

The SetTimer function also allows a timer’s parameters to be amended. Both the window handle and the timer identifier must be specified when modifying an existing timer. SetTimer can also change a message-based timer into a callback timer. To terminate a timer use the KillTimer() function. The prototype is

BOOL KillTimer(HWND hWnd,UINT_PTR uIDEvent);

Where
HWnd is a handle to the window associated with the specified timer.
UINT_PTR is the identifier of the timer to be destroyed.

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

Example

The following short program creates a digital clock using the timer function to update the display

The string manipulation functions of the Win32 API allow an application to test and manipulate the contents of a string. A selection of these is listed below. For a full list of string manipulation functions – https://docs.microsoft.com/en-us/windows/win32/menurc/string-functions

CharLower

Translates a character string to lowercase. 

LPSTR CharLower(LPSTR lpsz);

Where lpsz is a null-terminated string or specifies a single character. If the operand is a character string, the function returns a pointer to the converted string.

CharNext

Retrieves a pointer to the next character in a string. The prototype for this function is

LPSTR CharNext(LPCSTR lpsz);

Where lpsz is a character in a null-terminated string. The return value is a pointer to the next character in the string, or to the terminating null character if at the end of the string.

CharPrev

Positions pointer to the previous character in a string. 

LPSTR CharPrev(LPCSTR lpszStart,LPCSTR lpszCurrent);

where
LpszStart – The beginning of the string.
LpszCurrent – A character in a null-terminated string.
The return value is a pointer to the preceding character in the string, or to the first character in the string

CharUpper

Converts a character string to uppercase. The prototype for this function is

LPSTR CharUpper(LPSTR lpsz);

Where lpsz is a null-terminated string or a single character. If the operand is a character string, the function returns a pointer to the converted string.

IsCharAlpha

Determines whether a character is an alphabetical character.

BOOL IsCharAlpha(CHAR ch);

Where ch is the character to be tested. If the character is alphabetical, the return value is nonzero. If the character is not alphabetical, the return value is zero.

IsCharAlphaNumberic

Determines whether a character is an alphanumeric character.

BOOL IsCharAlpha(CHAR ch);

Where ch is the character to be tested. If the character is alphanumeric, the return value is nonzero. If the character is not alphanumeric, the return value is zero.

IsCharLower

Determines whether a character is lowercase.

BOOL IsCharLower(CHAR ch);

Were ch is the character to be tested. If the character is lowercase, the return value is nonzero. If the character is not lowercase, the return value is zero.

IsCharUpper

Determines whether a character is uppercase.

BOOL IsCharLower(CHAR ch);

Where ch is the character to be tested. If the character is uppercase, the return value is nonzero. If the character is not uppercase, the return value is zero.

Lstrlen

Determines the length of the specified string excluding the terminating null character.

int lstrlen(LPCSTR lpString);

Where lpString is the null-terminated string to be checked.
The function returns the length of the string, in characters.

Example

The following short program demonstrates various API string manipulation functions

The Win32 API offers a set of functions for accessing and managing disk files. This is in addition to the I/O functions available as part of the C and C++ runtime libraries. A selection of these API functions is outlined below.

For further reading on the full list of file management functions 

https://docs.microsoft.com/en-us/windows/win32/fileio/file-management-functions

Creating and Opening Files

All types of files can be created and opened with the API function CreateFile(). Windows assigns a file handle to each file that is opened or created. This handle is then used to access that file. File handles are valid until closed with the CloseHandle() function, which closes the file and flushes the buffers. The prototype of this function is

HANDLE CreateFile(LPCSTR lpFileName,DWORD dwAccess,DWORD dwShareMode,LPSECURITY_ATTRIBUTES lpSecurityAttributes,DWORD dwCreationDisposition,DWORD dwFlagsAndAttributes,HANDLE hTemplateFile);

Where
lpFileName – The name of the file or device to be created or opened with a backslash (\) to separate the components of a path.
dwAccess – The requested access to the file or device, which can be summarized as read, write, both or neither
dwShareMode – read, write, both, delete, all of these, or none.
lpSecurityAttributes – determines whether the child processes can be inherited the returned handle. This parameter can be NULL.
dwCreationDisposition – An action to take on a file or device that exists or does not exist.
dwFlagsAndAttributes – file or device attributes and flag
hTemplateFile – handle to a template file that supplies file attributes and extended attributes for the file that is being created. This parameter can be NULL.

If the function succeeds, the return value is an open handle to the specified file. If the function fails, the return value is INVALID_HANDLE_VALUE

Reading From and Writing to a File

When a file is first opened, Windows places a file pointer at the start of the file. Windows then advances the file pointer after the next read or write operation. An application can also move the file pointer position with the SetFilePointer() function. An application performs read and write operations with the ReadFile() and WriteFile() API functions. The prototype of these functions are –

BOOL ReadFile(HANDLE hFile,LPVOID lpBuffer,DWORD nNumberOfBytes,LPDWORD lpNumberOfBytes,LPOVERLAPPED lpOverlapped);

BOOL WriteFile(HANDLE hFile, LPCVOID lpBuffer, DWORD nNumberOfBytes, LPDWORD lpNumberOfBytes, LPOVERLAPPED lpOverlapped );

Where
hFile – A handle to the device
lpBuffer – A pointer to the buffer that holds the data to be read or written.
nNumberOfBytes – The maximum number of bytes to be read or written.
lpNumberOfBytes – Number of bytes read or written when using a synchronous hFile parameter. 
lpOverlapped – A pointer to an OVERLAPPED structure.

If the function succeeds, the return value is nonzero.  If the function fails the return value is zero.

When the file pointer reaches the end of the file any attempts to read any further data will return an error.

Windows allows more than one application to open a file and write to it. To prevent two applications from trying to write to the same file simultaneously, an application can lock the shared file area with the LockFile() function (see below). Locking part of a file prevents other processes from reading or writing anywhere in the specified area. When the application has completed its file operations it can unlock that region of the file using the UnlockFile() function. All locked regions of a file should be unlocked before closing a file.

The code section below demonstrates the API functions ReadFile() and WriteFile() by creating and then writing to a simple text file and then reading the contents before displaying them in a messagebox

#include <windows.h>
int APIENTRY WinMain( HINSTANCE hInst, HINSTANCE hPrev, LPSTR lpCmdLine, int nCmdShow )
{
 HANDLE hFile;
// create the file.
hFile = CreateFile( TEXT("FILE1.TXT"), GENERIC_READ | GENERIC_WRITE,FILE_SHARE_READ, NULL, OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL, NULL );
if ( hFile != INVALID_HANDLE_VALUE )
{
DWORD dwByteCount;
TCHAR szBuf[64]=TEXT("/0");
// Write a simple string to hfile.
WriteFile( hFile, "This is a simple message", 25, &dwByteCount, NULL );
// Set the file pointer back to the beginning of the file.
SetFilePointer( hFile, 0, 0, FILE_BEGIN );
// Read the string back from the file.
ReadFile( hFile, szBuf, 128, &dwByteCount, NULL );
// Null terminate the string.
szBuf[dwByteCount] = 0;
// Close the file.
CloseHandle( hFile );
//output message with string if successful
 MessageBox( NULL, TEXT("File created"), TEXT(""), MB_OK );
}
else
{
//output message if unsuccessful
 MessageBox( NULL, TEXT("File not created"), TEXT(""), MB_OK );
}
 return 0;
}

CopyFile

Copies an existing file to a new file but not the security attributes. The prototype of the copyfile() API function is

BOOL CopyFile(LPCTSTR lpExistingFileName,LPCTSTR lpNewFileName,BOOL bFailIfExists);

Where
lpExistingFileName – The name of an existing file.
lpNewFileName – The name of the new file.
bFailIfExists – If this parameter is TRUE and the new file already exists the function fails. If this parameter is FALSE and the new file already exists, the function overwrites the existing file and returns true.

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

CreateDirectory

Creates a new directory. The function applies a specified security descriptor to the new directory if the underlying file system is NTFS or one that supports security on files and directories. The prototype of this function is

BOOL CreateDirectory(LPCSTR lpPathName,LPSECURITY_ATTRIBUTES lpSecurityAttributes);

lpPathName – The path of the directory to be created
lpSecurityAttributes – A pointer to a SECURITY_ATTRIBUTES structure

Returns TRUE if successful; otherwise, the return value is FALSE.

DeleteFile

Deletes an existing file. If an application attempts to delete an open file or a file that does not exist, the function fails. The prototype of this function is

BOOL DeleteFile(LPCSTR lpFileName);

Where LpFileName is the name of the file to be deleted.  If the function succeeds, the return value is nonzero. If the function fails, the return value is zero (0).

GetFileAttributes

Retrieves file system attributes for a specified file or directory. The prototype of this function is

DWORD GetFileAttributes(LPCSTR lpFileName);

Where lpFileName is the name of the file or directory.  If the function succeeds, the return value contains the attributes of the specified file or directory. If the function fails, the return value is INVALID_FILE_ATTRIBUTES.

MoveFile

Moves an existing file or a directory to a new location on a volume. MoveFile will fail on directory moves when the destination is on a different volume. The prototype for this function is

BOOL MoveFile(LPCTSTR lpExistingFileName,LPCTSTR lpNewFileName);

LpExistingFileName – The name of the file or directory on the local computer.
LpNewFileName – The new name for the file or directory.

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

RemoveDirectory.

Deletes the specified empty directory. The prototype of this function is

BOOL RemoveDirectory( LPCTSTR lpszDir );

Where lpszDir is a pointer to a null-terminated string that contains the path of the directory to be removed. The directory must be empty and the calling process must have delete access to the directory.  Returns true if successful; otherwise, the return value is FALSE. 

LockFile

Locks a region in an open file. The prototype for this function is

BOOL LockFile(HANDLE hFile,DWORD dwFileOffsetLow,DWORD dwFileOffsetHigh,DWORD nNumberOfBytesToLockLow,DWORD nNumberOfBytesToLockHigh);

where
hFile – A handle to the file.
dwFileOffsetLow – The low-order 32 bits of the starting byte offset in the file where the lock should begin.
dwFileOffsetHigh – The high-order 32 bits of the starting byte offset in the file where the lock should begin.
nNumberOfBytesToLockLow– The low-order 32 bits of the length of the byte range to be locked.
nNumberOfBytesToLockHigh -The high-order 32 bits of the length of the byte range to be locked.

If the function succeeds, the return value is nonzero (TRUE). If the function fails, the return value is zero (FALSE). 

UnlockFile

Unlocks a region in an open file

BOOL UnlockFile(HANDLE hFile,DWORD dwFileOffsetLow,DWORD dwFileOffsetHigh,DWORD nNumberOfBytesToUnlockLow,DWORD nNumberOfBytesToUnlockHigh);

where
hFile – A handle to the file that contains a region locked with LockFile.
dwFileOffsetLow – The low-order word of the starting byte offset in the file where the locked region begins.
dwFileOffsetHigh – The high-order word of the starting byte offset in the file where the locked region begins.
nNumberOfBytesToUnlockLow – The low-order word of the length of the byte range to be unlocked.
nNumberOfBytesToUnlockHigh – The high-order word of the length of the byte range to be unlocked.

If the function succeeds, the return value is nonzero. – If the function fails, the return value is zero.

The multiple-document interface (MDI) enables a user to work with more than one document with multiple windows contained within a single parent window. An MDI application must register two window classes: one for its frame window and one for its child windows. At any one time, only one document window is active (indicated by a highlighted title bar), and it appears in front of all the other document windows.

Creating an MDI Application

The first step in creating an MDI application is registering the window classes. One class will be required for the MDI frame window, and another class will be required for each type of MDI child window. The class structure for an MDI frame window is like a normal application main window. The class structure for an MDI child window is filled in like the class structure for child windows with two exceptions. First, the class structure for an MDI child window should specify an ICON since the user can minimize MDI child windows within the MDI frame window. Second, the menu name should be NULL, since an MDI child window can not have its own menu.

Example

The following short program demonstrates how to create a Multiple document interface. Clicking the Menubar item file followed by the menu item new creates a new MDI child window containing a textbox.

Toolbars can be created using the CreateWindowEx function, specifying TOOLBARCLASSNAME as the window class name, or by using the depreciated API CreateToolbarEx function. A TBBUTTON structure contains information about the individual buttons and this is added to the toolbar via the sendmessage() function using the TB_ADDBUTTONS or TB_INSERTBUTTON message parameters. A bitmap is used to hold the toolbar image. Each time a button is clicked a WM command message will be generated with the ID of each button being held in the low order word of the wparam.

In the following example, a simple 3-button toolbar is defined and created with two standard buttons divided by a separator. A separator provides a small gap between button groups and does not receive user input. The API function LoadImage loads the toolbar bitmap from the local c drive into a handle of type HBITMAP. The toolbar image bitmap is then created by a call to the function ImageList_Create() and the imagelist is then added to the toolbar control by the function ImageList_Add(). The individual image associated with the toolbar button is set in the TBBUTTON structure.  Clicking either of these buttons will generate a message box indicating the button clicked

For in-depth reading on the creation of toolbars
https://docs.microsoft.com/en-us/windows/win32/controls/toolbar-control-reference

The following short program creates a simple toolbar with two buttons using the following two images. Clicking either button will produce a message box.

Windows supports two types of bitmap: device-independent (DIB) and device-dependent(DDB). A device-independent bitmap is an external format, which allows bitmaps to be moved from one device to another. Device-dependent bitmaps are designed and optimised for use with a specific device (device-dependent) and hence are unsuitable for use in an environment for which they were not created. A typical example would be a bitmap created for video memory optimised for displaying screen output.

Creating and Loading Device-Independent Bitmaps

Device-independent bitmaps are typically created using an image editor. To load a bitmap for use in an application use the LoadImage() function. This supersedes the LoadBitmap() function. The prototype for this function is

HANDLE LoadImage(HINSTANCE hInst, LPCSTR name, UINT type, int cx, int cy, UINT fuLoad);

Where
hInst – is a handle to the module of either a DLL or executable (.exe) that contains the image to be loaded.
name – the image to be loaded.
UINT – The type of image to be loaded.
cx – The width, in pixels, of the icon or cursor.
cy– The height, in pixels, of the icon or cursor.
fuLoad – can be one of the following values: LR_CREATEDIBSECTION; LR_DEFAULTCOLOR;LR_DEFAULTSIZE; LR_LOADFROMFILE; LR_LOADMAP3DCOLORS; LR_LOADTRANSPARENT; LR_MONOCHROME; LR_SHARED; LR_VGACOLOR

If the function succeeds, the return value is the handle of the newly loaded image. If the function fails, the return value is NULL.

For further reading
https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-loadimagea

To convert a DIB to a DDB, use the API function CreateDIBitmap. 
https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-createdibitmap

Displaying Bitmaps

To display a bitmap, two device contexts must be declared: one will hold the current device context while the other will create a second copy device context that will be used to store the bitmap until it is ready to be drawn in the window. This copy device context is created using a call to API function CreatCompatibleDC().   A newly created or existing bitmap can then be selected into the copy device context using the SelectObject() API function and finally copied to the screen using the BitBlt() API function. The sequence of events is as follows –

DC=GetDC(hwnd);
memDC=CreateCompatibleDC(DC);
SelectObject(memDC,bitmap1);
BitBlt(DC,x.y,cx,cy, memdc,x1,y1,SRCCOPY);

The syntax for the CreateCompatibleDC function is

HDC CreateCompatibleDC(HDC hdc);

Where hdc is a handle to an existing DC. If this handle is NULL, the function creates a memory DC compatible with the application’s current screen. If the function succeeds, the return value is the handle to a memory DC. If the function fails, the return value is NULL.

The syntax for this Bitblt() copy function is

BOOL BitBlt(HDC hdc,int x,int y,int cx, int cy,HDC hdcSrc,int x1,int y1,DWORD rop);

where
hdc – handle to the destination device context.
x – The x-coordinate upper-left corner of the destination rectangle.
y – The y-coordinate of the upper-left corner of the destination rectangle.
cx – The width of the source and destination rectangles.
cy – The height, in logical units, of the source and the destination rectangles.
HdcSrc – A handle to the source device context.
x1 – The x-coordinate of the upper-left corner of the source rectangle.
y1 – The y-coordinate of the upper-left corner of the source rectangle.
rop – A raster-operation code. Define how the colour data for the source rectangle will be combined with the destination rectangle. The ‘vanilla’ operating code SRCCOPY will copy the source rectangle directly onto the destination rectangle. For additional raster code information see the link below

Bltbmp returns zero if successful and non-zero otherwise.

When a bitmap is no longer needed it must be destroyed using the DeleteObject() API function.

For further information 
https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-bitblt

Repainting the Screen Using Device Dependent Bitmaps

Using a bitmap to create a copy of the device context offers one solution to the problem of saving the contents of the client area before a repaint request. All screen output is directed to this virtual window and each time a repaint is received, the contents of the virtual window are then copied to the screen’s device context.

Creating Device-Dependent Bitmaps

The API function CreateCompatibleBitmap() creates a bitmap compatible with the current device context handle. This function is best used for creating colour bitmaps. The prototype of this API function is 

HBITMAP CreateCompatibleBitmap (hdc, cx, cy) ;

Where 
hdc is a handle to a device context
cx is the bitmap width, in pixels
cy is the bitmap height, in pixels. 

If the function succeeds, the return value is a handle to the compatible bitmap (DDB). If the function fails, the return value is NULL.

In addition, a Windows application can also create a device-dependent bit using the CreateBitmap API function. This function is best used for creating monochrome bitmaps. The syntax for this function is

HBITMAP CreateBitmap(int nWidth,int nHeight,UINT nPlanes,UINT nBitCount,const VOID *lpBits);

where
nWidth – The bitmap width, in pixels.
nHeight – The bitmap height, in pixels.
nPlanes – The number of colour planes used by the device.
nBitCount – The number of bits required to identify the colour of a single pixel.
lpBits – Set the colours in a rectangle of pixels. 

If the function succeeds, the return value is a handle to a bitmap. If the function fails, the return value is NULL.

Example

In the code section below, Windows creates a device context of the application window and a compatible device context. The application then creates and fills this virtual copy with 200 random lines. Each time a repaint request is received, the contents of the virtual window are copied to the screen device context by the BitBlt function.

Copying a Bitmap Using the StretchBlt Function

The StretchBlt() function copies a bitmap from a source rectangle into a destination rectangle, stretching or compressing the bitmap to fit the dimensions of the destination rectangle.  In the code section below Windows creates a bitmap of the existing screen display and then copies it to the application window using StretchBlt API function. Clicking the left mouse button anywhere with the client Window will initiate the screen copy action.

Example

In the code section below Windows creates a bitmap of the existing screen display and then copies it to the application window using StretchBlt API function. Clicking the left mouse button anywhere with the client Window will initiate the screen copy action.

The Common Dialog Box Library consists of a collection of functions for invoking standard dialog boxes for performing common tasks. There are standard Windows dialogs for opening and saving files, choosing a printer, finding and replacing text, setting font options, picking a colour, and using a help file. Using the common control dialog enables an application to maintain a look of consistency. The common dialog box library files are found in COMMDLG.H header file. To use these functions, the application must first initialise the fields of a structure and then pass a structure pointer to the function. When the user terminates the dialog box, control returns to the calling application with any relevant information passed back to the application via the dialog box structure.

For further reading https://docs.microsoft.com/en-us/windows/win32/dlgbox/dialog-box-types

The examples below demonstrate 3 common dialog boxes: text find and replace, the file open and save and colour select. 

Select Colour Common Dialog Box

The Select Colour Dialog Box displays a basic set of available colours in addition to allowing a user to create custom colours by specifying RGB values

File Open/Save Common Dialog Box 

The ‘Common File-Open/Save’ dialog offers a consistent way to deal with file management operations, using the standard dialog interface that Windows users should be familiar with.

The Open dialog box lets the user specify the drive, directory, and the name of a file or set of files to open.

The Save As dialog box lets the user specify the drive, directory, and name of a file to save.

Find Common Dialog Box

The Find and Replace Common Dialog Box displays a modeless dialog box that allows the user to specify a string to search for within a text document, as well as additional options for text editing

A dialog box is a temporary popup window that prompts the user for additional information. Dialog boxes are usually created using using dialog editor and defined within a programs resource file. A dialog box will usually contain one or more controls (child windows) with which the user can enter text, choose options, or direct the action of the application.

In addition to user-defined dialogue boxes, Windows also provides predefined dialog boxes that support common menu items such as colour selection and an open file interface.

Modal v Modeless Dialogue Box

Dialog boxes can be either modeless or modal. A modal dialogue box will not allow the user to switch between the dialog box and any other window without explicitly closing the dialog box. A modeless dialog box does not prevent the user from switching to other parts of the application.

Creating a Modeless Dialog Box

Modeless dialog boxes are created using the API function CreateDialog(). The syntax is below

hDlgModeless = CreateDialog (hInstance, lpTemplate, hWndParent, lpDialogFunc) ;

hInstance – A handle to the current application.
lpTemplate – specifies the resource identifier of the dialog box template.
hWndParent – A handle to the parent window that owns the dialog box.
lpDialogFunc – A pointer to the dialog box procedure.

Return value – If the function succeeds, the return value is the dialog box handle. If the function fails, the return value is NULL.

Deactivating a Modeless Dialog Box

To destroy the modeless dialog box, use the DestroyWindow() function.

BOOL DestroyWindow(HWND hWnd);

Where HWnd is a handle to the window to be destroyed.  If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

Creating a Modal Dialog Box

Modal dialog boxes are created using the APO function DialogBox(). The syntax is

void DialogBox(hInstance,lpTemplate,hWndParent,lpDialogFunc);

hInstance – A handle to the current application.
lpTemplate – Specifies the resource identifier of the dialog box template.
hWndParent – A handle to the parent window that owns the dialog box.
lpDialogFunc – A pointer to the dialog box procedure.

Deactivating a Modal Dialog Box

To deactivate and close a modal dialogue box use the EndDialog() API function call. The syntax of this function is

BOOL EndDialog(HWND hDlg,INT_PTR nResult);

HDlg – A handle to the dialog box to be destroyed.
nResult – The value to be returned to the application from the function that created the dialog box.

Return value – If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

Processing Dialog Messages

Each dialogue box will have its version of the window call-back function (wndproc) and receive messages in the same fashion as the main window. The dialog callback function, however, differs from its main Windows counterpart in that it returns a boolean value. 

Example

The code example demonstrates a modal and modeless dialog box. The modal dialog box displays a simple message. The modeless dialog box has 3 radio buttons. Selecting any of the radiobutton changes the background colour of the parent window.

Dialog-Based Applications

Creating a program where the main window is a dialog box is referred to as a dialog-based application. This method simplifies the application build process by making it easier to place controls onto the main window using the system resource dialog editor. A dialog box procedure can then be added to react to events that occur in the dialog box. Dialog-based applications are much simpler, architecturally, than other designs therefore dialog applications are best suited for creating rudimentary apps.

The following example creates a simple dialog-based application. The dialog box title can be changed to the value in the textbox by clicking the button control.

Drawing Pixels

A pixel is the smallest image element that can be represented on screen. To draw a point within the client area of a window use the API function SetPixel(). The prototype for this function is

COLORREF SetPixel(HDC hdc,int x,int y,COLORREF color);

Where
hdc – the device context.
X – The x-coordinate, in logical units, of the point to be set.
Y – The y-coordinate, in logical units, of the point to be set.
Colour – is a COLORREF to paint the point. If the colour cannot be created on the video display, Windows will use the nearest pure non-dithered colour and then return that value from the function.

If the function is successful, the return value is the RGB colour. If the function fails, the return value is -1.

Drawing Lines

The LineTo() function draws a line within the client area from the current graphics drawing point. The prototype is

BOOL LineTo(HDC hdc, int x, int y);

where
hdc – the device context.
x – specifies the x-coordinate of the line’s ending point.
y – specifies the y-coordinate of the line’s ending point. 

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero. The new starting point will then become the endpoint of the previous line.

MoveToEx

The initial starting position for graphics output will be the screen coordinate position 0,0 however this can be set by the application with a call to the API function MoveToEx(). The prototype for the MoveToEx function is –

BOOL MoveToEx(HDC hdc,int x,int y,LPPOINT lppt);

where
hdc – handle to a device context.
x – specifies the x-coordinate of the new position, in logical units.
y – specifies the y-coordinate of the new position, in logical units.
Lppt – is a pointer to a POINT structure that receives the previous current position.

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

GetCurrentPosition

Retrieves the current logical graphics starting position.  The prototype of this function is

BOOL GetCurrentPositionEx(HDC hdc,LPPOINT lppt);

Where
hdc – handle to the device context
lppt – is a pointer to a POINT structure that receives the logical coordinates of the current position. 

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

Drawing Rectangles

Rectangles are drawn with the currently selected pen and brush. To draw a rectangle, use the Rectangle() function. The prototype is

BOOL Rectangle(HDC hdc,int left,int top,int right,int bottom);

hdc – is a handle to the device context.
Left – is the x-coordinate of the upper-left corner of the rectangle.
Top – is the y-coordinate of the upper-left corner of the rectangle.
Right – the x-coordinate of the lower-right corner of the rectangle.
Bottom – the y-coordinate of the lower-right corner of the rectangle.
If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

To display rectangles with rounded corners using the API function RoundRect(). The prototype for this function is

BOOL RoundRect(HDC hdc, int left,int top,int right,int bottom,int width,int height);

where
hdc – A handle to the device context.
Left – The x-coordinate of the upper-left corner of the rectangle.
Top– The y-coordinate of the upper-left corner of the rectangle.
Right – The x-coordinate of the lower-right corner of the rectangle.
Bottom – The y-coordinate of the lower-right corner of the rectangle.
Width – The width, of the ellipse used to draw the rounded corners.
Height – The height of the ellipse used to draw the rounded corners.

If the function succeeds, the return value is nonzero.If the function fails, the return value is zero.

Drawing an Ellipse

To draw an ellipse or circle using the current pen and filled by the current brush, use the Ellipse() function. The prototype is –

BOOL Ellipse(HDC hdc,int left,int top,int right,int bottom);

where
hdc – A handle to the device context.
left – is the x-coordinate of the upper-left corner of the bounding rectangle.
Top – is the y-coordinate of the upper-left corner of the bounding rectangle.
Right – is the x-coordinate of the lower-right corner of the bounding rectangle.
Bottom – is the y-coordinate of the lower-right corner of the bounding rectangle.

If the function succeeds, the return value is nonzero.  If the function fails, the return value is zero.

To draw a circle the bounding rectangle must be a square. For example, to draw a circle that has a centre (50,50) with a radius of 10, use the following function parameters – Ellipse(hdc,10,10,50,50);

Drawing a Semi-Circular Wedge

To draw a semi-circular wedge using the current pen and fill it with the current brush, use the API function Pie(). The prototype of this function is

BOOL Pie(HDC hdc,int left,int top,int right,int bottom,int xr1,int yr1, int xr2,int yr2);

where
hdc – A handle to the device context.
Left – The x-coordinate of the upper-left corner of the bounding rectangle.
Top – The y-coordinate of the upper-left corner of the bounding rectangle.
Right – The x-coordinate of the lower-right corner of the bounding rectangle.
Bottom – The y-coordinate of the lower-right corner of the bounding rectangle.
xr1 – The x-coordinate of the endpoint of the first radial.
yr1 -The y-coordinate of the endpoint of the first radial.
xr2 – The x-coordinate of the endpoint of the second radial.
yr2 – The y-coordinate of the endpoint of the second radial.

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero.

Drawing a Chord

To draw a Chord using the current pen and fill it using the current brush, use the API function Chord(). The prototype of this function is –

BOOL Chord( HDC hdc,int x1,int y1,int x2,int y2,int x3,int y3,int x4, int y4);

hdc – handle to the device context.
x1 – The x-coordinate of the upper-left corner of the bounding rectangle.
y1 – The y-coordinate of the upper-left corner of the bounding rectangle.
x2 – The x-coordinate of the lower-right corner of the bounding rectangle.
y2 – The y-coordinate of the lower-right corner of the bounding rectangle.
x3 – The x-coordinate of the endpoint of the radial defining the beginning of the chord.
y3 – The y-coordinate of the endpoint of the radial defining the beginning of the chord.
x4 – The x-coordinate of the endpoint of the radial defining the end of the chord.
y4 – The y-coordinate of the endpoint of the radial defining the end of the chord.

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero

Drawing Polygons

The API function used to draw a polygon is Polygon(). The prototype of this function is

BOOL Polygon(HDC hdc,const POINT *apt,int cpt);

hdc – A handle to the device context.
apt – A pointer to an array of POINT structures that specify the vertices of the polygon
cpt – The number of vertices in the array. This value must be greater than or equal to 2.

If the function succeeds, the return value is nonzero.  If the function fails, the return value is zero.

Bézier Curves

The PolyBezier() function draws one or more Bézier curves. The prototype of this function is

BOOL PolyBezier(HDC hdc,const POINT *apt,DWORD cpt);

where
hdc – A handle to a device context.
apt – A pointer to an array of POINT structures that contain the endpoints and control points of the curve(s), in logical units.
cpt – The number of points in the lppt array. This value must be one more than three times the number of curves to be drawn.

If the function succeeds, the return value is nonzero.  If the function fails, the return value is zero.

Example

The following code builds on the basic window to display Windows graphics

A message box displays a simple message to the user but may include some selection options. They are typically used to inform the user that an event has taken place. To create a message box use the API function call MessageBox. The prototype for this function is

int MessageBox(HWND hWnd,LPCTSTR lpText,LPCTSTR lpCaption,UINT uType);

where
hWnd – is a handle to the owner window of the message box to be created. If this parameter is NULL, the message box has no owner window.
LpText – The message to be displayed.
LpCaption – Contains dialog box title. If this parameter is NULL, the default title is Error
UType – defines the contents and behaviour of the dialog box and will be a combination of a number of  different flag value but some of the more common values are

The return value will depend on the type of message box selected but will be one of the following: IDABORT, IDCANCEL, IDCONTINUE, IDIGNORE, IDNO, IDOK, IDRETRY, IDYES

For further detailed reading
https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-messagebox

The following short program displays a very simple message box

#include <windows.h>
int APIENTRY WinMain( HINSTANCE hInst, HINSTANCE hPrev, LPSTR lpCmdLine, int nCmdShow )
{
    MessageBox( NULL, "Hello, World!", "Hi!", MB_OK );
    return 0;
}