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Read and Write Exif Tags and Comments - Windows C DLL

This tutorial shows how to read and write Exif tags and comments in a Windows C/C++ API application using the LEADTOOLS SDK.

Overview
Summary	This tutorial covers how to work with Exif tags and comments in a Windows C DLL application.
Completion Time	30 minutes
Visual Studio Project	Download tutorial project (20 KB)
Platform	Windows C DLL Application
IDE	Visual Studio 2017, 2019
Development License	Download LEADTOOLS
Try it in another language	C#:.NET Framework (Console) Apple:macOS C API:C DLL (Windows)

Required Knowledge

Get familiar with the basic steps of creating a project and loading/displaying an image by reviewing theAdd References and Set a LicenseandLoad, Display, and Save Imagestutorials, before working on theRead and Write Exif Tags and Comments - Windows C DLLtutorial.

Create the Project and Add LEADTOOLS References

Start with a copy of the project created in theLoad, Display, and Save Imagestutorial. If the project is not available, create it by following the steps in that tutorial.

Open the pre-compiled header file (eitherpch.horstdafx.h, depending on the version of Visual Studio used) and ensure the below lines are added.

#define LTV22_CONFIG#include"C:\LEADTOOLS22\Include\L_Bitmap.h"// use the actual path where LEADTOOLS is installed#pragma comment (lib,"C:\\LEADTOOLS22\\Lib\\CDLL\\x64\\Ltkrn_x.lib")#pragma comment (lib,"C:\\LEADTOOLS22\\Lib\\CDLL\\x64\\Ltfil_x.lib")// file loading and saving#pragma comment (lib,"C:\\LEADTOOLS22\\Lib\\CDLL\\x64\\Ltdis_x.lib")// image display

Note

For a complete list of DLLs that are required for specific application features, refer toFiles to be Included with your Application - C API

Set the License File

The License unlocks the features needed for the project. It must be set before any toolkit functionality is called. For details, including tutorials for different platforms, refer toSetting a Runtime License.

There are two types of runtime licenses:

Evaluation license, obtained at the time the evaluation toolkit is downloaded. It allows the toolkit to be evaluated.
Deployment license. If a Deployment license file and developer key are needed, refer toObtaining a License.

Note

Adding LEADTOOLS references and setting a license are covered in more detail in theAdd References and Set a Licensetutorial.

Add the Read/Write Exif Comments Code

With the project created, the references added, the license set, and the load image code added, coding can begin.

In theSolution Explorer, double-click the resources file (.rc).

Add a newExif &Commentmenu item to theFiledrop-down menu, between theOpenandSaveitems. Leave the new menu item's ID asID_FILE_EXIFCOMMENT.

Open the project's CPP file and navigate to theWndProcfunction. Under theswitch (wmId)statement that is below theWM_COMMANDcase, add a new case and the code below.

switch(wmId){caseID_FILE_EXIFCOMMENT:ReadAndWriteExifComments(hWnd);break;// Keep rest of the code as is

Add the ReadAndWriteExifComments function below to flip the loaded image, write a comment to the file stating that the image has been flipped, and then read the user comment created.

voidReadAndWriteExifComments(HWND hWnd){L_TCHAR szSrcFile[260] = TEXT(""), szDestFile[260] = TEXT("");if(SUCCESS != GetBitmapLoadingName(hWnd, szSrcFile, ARRAYSIZE(szSrcFile)))return;if(LEADBmp.Flags.Allocated)L_FreeBitmap(&LEADBmp);L_LoadBitmap(szSrcFile, &LEADBmp,sizeof(LEADBmp), 24, ORDER_BGR, NULL, NULL);InvalidateRect(hWnd, NULL, TRUE);MessageBox(hWnd, TEXT("Image loaded and will be flipped.\nSelect output file"), TEXT("LEADTOOLS Demo"), MB_ICONINFORMATION);// Flip the image.L_FlipBitmap(&LEADBmp);InvalidateRect(hWnd, NULL, TRUE);// Specify output fileif(SUCCESS != GetBitmapSavingName(hWnd, szDestFile, ARRAYSIZE(szDestFile)))return;// Load the current comments from the filefor(inti = 0; i < CMNT_LAST; i++){// Clear the commentL_SetComment(i, NULL, 0);// Use the return value to get the length of a comment in the fileL_INT CommentLength = L_ReadFileComment(szSrcFile, i, NULL, 0, NULL);if(CommentLength > 0){L_UCHAR* pCommentToGet =newL_UCHAR[CommentLength]();// parentheses initialize array to zeros// Get the comment from the fileL_ReadFileComment(szSrcFile, i, pCommentToGet, CommentLength, NULL);// Set the comment to be savedL_SetComment(i, pCommentToGet, CommentLength);delete[] pCommentToGet;}}// Create the CMNT_USERCOMMENT comment as a byte array// When writing a user comment, the first 8 bytes must contain the "ASCII" word.L_SetComment(CMNT_USERCOMMENT, NULL, 0);// IMPORTANT: initializing CommentToSet array causes remainder of array to contain null characters, which is needed for rest of code to workL_UCHAR CommentToSet[80] ="ASCII";constL_CHAR* pUserComment ="The image has been flipped!";strcpy_s((char*)CommentToSet + 8,sizeofCommentToSet - 8, pUserComment);// Set the CMNT_USERCOMMENT commentL_SetComment(CMNT_USERCOMMENT, CommentToSet, (L_UINT)strlen(pUserComment) + 9);L_SaveBitmap(szDestFile, &LEADBmp, FILE_EXIF, 24, 0, NULL);L_UCHAR CommentCheck[80] ="";L_ReadFileComment(szDestFile, CMNT_USERCOMMENT, CommentCheck,sizeofCommentCheck, NULL);charszMessageA[256] ="Comment read back from file:\n";strcat_s(szMessageA,sizeofszMessageA, (constchar*)CommentCheck + 8);// The returned string is in ASCII, so the "MessageBoxA" function is used to display itMessageBoxA(hWnd, szMessageA,"LEADTOOLS Demo", MB_OK);}

The code for theGetBitmapLoadingName()andGetBitmapSavingName()functions, called inside the code above, is listed in theLoad, Display, and Save Imagestutorial.

Add the Read/Write Exif Tags Code

Using theSolution Explorer, navigate back to the resources file (.rc).

Add a newEXIF &Tagmenu item to theFiledrop-down menu, between theOpenandSaveitems. Leave the new menu item's ID asID_FILE_EXIFTAG.

Go to theWndProcfunction and under theswitch (wmId)statement that is below theWM_COMMANDcase, add a new case and the code below.

switch(wmId){caseID_FILE_EXIFTAG:ReadAndWriteExifTags(hWnd);break;// Keep rest of the code as is

Add the code of theReadAndWriteExifTagsfunction, which adds a custom text tag to the EXIF file then reads all the tags back from the file.

voidReadAndWriteExifTags(HWND hWnd){L_TCHAR szExifFile[260] = TEXT("");if(SUCCESS != GetBitmapLoadingName(hWnd, szExifFile, ARRAYSIZE(szExifFile)))return;FILEINFO info = { 0 };// Important: Must initialize to zeroinfo.uStructSize =sizeofinfo;L_FileInfo(szExifFile, &info,sizeofinfo, 0, NULL);if(!L_TagsSupported(info.Format)){MessageBox(hWnd, TEXT("File Format Does Not Support Tags"), TEXT("LEADTOOLS Demo"), MB_ICONERROR);return;}// Add a private (custom) "Phone Number" tagL_UINT16 PhoneNumber = 0x8001;charTagData[] ="+1-800-637-4699";// Set the tag data to be savedL_SetTag(PhoneNumber, TAG_ASCII,sizeofTagData, TagData);// Save the the tag into the fileL_WriteFileTag(szExifFile, NULL);// Clear the tag from memoryL_SetTag(0, 0, 0, NULL);// Read the tags from the file and display them in the debugger windowL_UINT uTagCount = 0;pLEADFILETAG pTags = NULL, pCurrentTag;L_SIZE_T uTagDataSize = 0;L_UCHAR* pTagsData = NULL;L_ReadFileTags(szExifFile, 0, &uTagCount, &pTags, &uTagDataSize, &pTagsData, NULL);if(uTagCount > 0){// Show the tagsOutputDebugString(TEXT("Tags\n"));for(L_UINT n = 0; n < uTagCount; n++){pCurrentTag = &pTags[n];// If this tag is of type ASCII, get its dataL_CHAR* pszAscii = NULL;if(pCurrentTag->uType == TAG_ASCII){pszAscii =newL_CHAR[pCurrentTag->uDataSize + 1]();memcpy(pszAscii, pTagsData + pCurrentTag->uDataOffset, pCurrentTag->uDataSize);}else{pszAscii = NULL;}CHAR szMessage[1024];wsprintfA(szMessage,"Id: 0x%X, data length: %u, data: %s\n", pCurrentTag->uTag, pCurrentTag->uDataSize, pszAscii != NULL ? pszAscii :"Binary data");if(pszAscii)delete[] pszAscii;OutputDebugStringA(szMessage);}L_FreeFileTags(uTagCount, pTags, uTagDataSize, pTagsData);}}

Run the Project

Run the project by pressingF5, or by selectingDebug -> Start Debugging.

If the steps are followed correctly, the application runs and gives the user the ability to execute the following commands:

Write a comment to an EXIF file then read that comment back and display it.
Write a custom EXIF tag then read it back, along with other tags, from the same file.

Wrap-up

This tutorial showed how to use theL_SetComment,L_ReadFileComment,L_ReadFileTags,L_SetTagandL_WriteFileTagfunctions with EXIF files.