File Content Extraction 25.3
Filter SDK C Programming Guide

# C API Programming Tutorial

The File Content Extraction Filter SDK allows you to embed File Content Extraction functionality into other services.

This tutorial helps you to:

- familiarize yourself with the Filter SDK C API

- create a sample program that replicates a common use case of the Filter SDK

## Setup

### Resources

You must download the following resources before you continue:

- source code for this tutorial: tutorial_file.c

- optional sample files for detection and various extraction options: sample files

### Environment and Compilers

To create a program that uses File Content Extraction, you need to install a supported compiler, and use it to build and link your program. See Supported Compilers.

NOTE: When building with the Visual Studio compiler, you must ensure you open the correct command prompt for the installed version. For example, if you install the `WINDOWS_X86_64` version of File Content Extraction, ensure you use x64 Native Tools Command Prompt.

### License Key

You need a license key to proceed with this tutorial.

## API Setup

### Building the sample program

This tutorial explains how to gradually build a working program that replicates a common use case of the Filter SDK. Sample code for this program is provided (see tutorial_file.c), but you must modify the `fpInit` call for the sample to compile.

Update the `tutorial.h` file with the following information:

- Replace the value of `YOUR_LICENSE_KEY` with your license.

- Change the `YOUR_BIN_DIR` variable to the location of the File Content Extraction bin directory.

NOTE: For compilation tips, refer to the `tutorial_file.c` source code.

### Linking Against Filter SDK

The easiest way to get access to File Content Extraction functionality is to link against the `kvfilter` shared library and place your executable in the same directory as the File Content Extraction binaries (that is, the directory containing `kvfilter.so` or `kvfilter.dll`).

TIP: Loading shared libraries can expose your application to attacks. For advice on avoiding DLL preloading attacks, see Security Best Practices.

- Linking using GCC

  On Linux, link against `kvfilter.so`, and also pass in the `–rpath $ORIGIN` option to the linker. For example:

       %KEYVIEW_HOME%/LINUX_X86_64/bin/kvfilter.so -Wl,-rpath,'$ORIGIN'

  NOTE: When you call it from inside a makefile, you might need to escape `$ORIGIN` to `$$ORIGIN`.

- Linking using Clang

  On MacOS, link against `kvfilter.so`, and also pass in the `–rpath @loader_path` option to the linker. For example:

      %KEYVIEW_HOME%/MACOS_X86_64/bin/kvfilter.so -Wl,-rpath,@loader_path

- Linking using Visual Studio

  On Windows, link against the import library for `kvfilter.dll`. This library is provided as part of the Filter SDK, under `{platform}/lib/kvfilter.lib`.

### Loading the Filter Interface

Now that you can access File Content Extraction functionality, you must include the required headers and load the interface functions from the kvfilter library, using `KV_GetFilterInterfaceEx()`.

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    #include "kvtypes.h"
    #include "kvfilt.h"

    KVFltInterfaceEx filter;

    KVErrorCode error = KV_GetFilterInterfaceEx(&filter, KVFLTINTERFACE_REVISION);

    if(error != KVError_Success)
    {
       //return error;
    }

This call fills out the interface structure, which contains the function pointers for the rest of Filter's functionality.

Like most Filter API functions, `KV_GetFilterInterfaceEx()` returns a `KVErrorCode`. If this is `KVError_Success`, then the function succeeded. Otherwise, the error code indicates the problem that occurred. The rest of this tutorial assumes that you check the error code after each function call.

Now that you have successfully loaded the interface, you are ready to use the Filter SDK.

### Creating a Filter Session

All Filter API functionality requires a session, which you must initialize at the start of processing, and shut down at the end of processing.

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    KVFilterSession session = NULL;

    KVFilterInitOptions options;
    KVStructInit(&options);
    options.outputCharSet = KVCS_UTF8;

    error = filter.fpInit(
       "/path/to/keyview/bin",
       YOUR_LICENSE_KEY,
       &options,
       &session);

    //Use KeyView

    filter.fpShutdown(session);

You initialize by using the function fpInit(), which you must provide with the path to the File Content Extraction bin folder, and your license key.

This function also takes a pointer to a `KVFilterInitOptions` structure, which you must initialize by using `KVStructInit()`. This macro ensures that a struct is correctly set up for use with the File Content Extraction interface, including version information for backwards compatibility. Any File Content Extraction struct that contains a KVStructHead member must be initialized with the `KVStructInit()` macro.

TIP: Performance considerations:

- Session Lifetime. You can process multiple files in a single session, which might improve performance by reducing costs associated with start-up and shutdown.

- Multi-threading. To maximize throughput when processing multiple files, you can call File Content Extraction from multiple threads. All File Content Extraction functions are thread-safe when called in this manner. Each thread using File Content Extraction must create its own session by calling `fpInit()`. You must not share filter sessions between threads.

TIP: Security considerations:

- Privilege Reduction. By default, File Content Extraction performs most of its operations out-of-process, creating a separate process to parse file data. This protects your main application from the effects of rare problems like memory leaks or crashes. You can include additional protection by running File Content Extraction with reduced privileges. See Run Filter with Minimal Privileges.

- Temp Directory. While processing, File Content Extraction might place sensitive data in the temporary directory. You might want to consider protecting the temporary directory. See Protect the Temporary Directory.

Now that you have set up the API, you can perform File Content Extraction Filter functionality on documents.

## Opening a Document

File Content Extraction functions operate on a `KVDocument` object, which is a representation of a document that is agnostic as to where the document actually lives. You can create a `KVDocument` from a file on disk by using `fpOpenDocumentFromFile`. You must close the `KVDocument` after use.

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    KVDocument document = NULL;

    error = filter.fpOpenDocumentFromFile(session, pathToInputFile, &document);

    //Pass document to KeyView functions

    filter.fpCloseDocument(document);

## Filter a File

One of the most important features of File Content Extraction is filtering text from a document. This section shows you how to get text from a `KVDocument` object, and output it to a file on disk.

### Filtering Text

You can filter text to an output file by using the fpFilterToFile() function.

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    error = filter.fpFilterToFile(document, pathToOutputFile);

TIP: Partial Filtering. The `fpFilterToFile()` function filters the entire file in one go, but you might want to filter only part of the file, or filter the file in chunks. The advanced tutorial covers how to do partial filtering.

TIP: Mail Files. Mail files, such as EML or MSG, are considered a form of container, and you cannot filter them directly. This tutorial covers how to filter mail files later, in Extracting Subfiles.

### Filtering Hidden Information

File Content Extraction provides a number of options that control what text to output, and how to convert or display that text. A common requirement of File Content Extraction is to display as much text as possible, including text that is not normally visible in the document, such as hidden cells or slides, or ancillary text like comments or notes. You can display this text by enabling the hidden text option in fpSetConfig().

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    error = filter.fpSetConfig(session, KVFLT_SHOWHIDDENTEXT, TRUE, NULL);

## Detecting and Using File Format Information

File Content Extraction enables you to reliably determine the file format of a huge range of documents. It does this by analyzing the internal structure and content of the file, rather than relying on file names or extensions. Detection prioritizes both accuracy and speed, only processing as much of the file as necessary to rule out false positives.

### Detecting the File Format

File format detection functionality is exposed through the API function fpGetDocInfo().

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    ADDOCINFO adInfo;
    error = filter.fpGetDocInfo(document, &adInfo);

TIP: Source Code Identification. File Content Extraction can optionally detect source code, attempting to identify the programming language that it is written in. You can learn more in Source Code Identification.

### Checking if a File is Supported

File Content Extraction provides a convenience function, fpCanFilter() that performs detection and determines if you can pass the file to fpFilter().

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    error = filter.fpCanFilter(document);

If this function does not return `KVError_Success`, the error returned explains why File Content Extraction cannot filter the format, for example, because File Content Extraction could not determine the format, because the file does not exist, or because the format is not supported for filtering.

While this step is not strictly necessary, it can simplify many workflows.

## Retrieving Metadata

File formats can contain a variety of different metadata, and File Content Extraction makes it easy to access all of this information. File Content Extraction retrieves metadata from various sources in a file, such as:

- Format-specific standard metadata

- User-provided custom metadata

- Exif tags

- XMP elements

- MIP Labels

### Getting the Metadata List

You can retrieve metadata elements by using fpGetMetadataList(). This function fills out the KVMetadataList structure, which you must free by using its `fpFree` function.

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    const KVMetadataList* metadataList = NULL;
    error = filter.fpGetMetadataList(document, &metadataList);

    //Iterate through metadata

    metadataList->fpFree(metadataList);

### Iterating Through the List

You can retrieve individual metadata elements by iterating through the metadata list using the `fpGetNext()` function in `KVMetadataList`, which fills out the KVMetadataElement structure. The information that this structure returns is valid only while the session is still alive, and becomes invalid after you call `fpFree()`. The end of the list is indicated by the retrieved element being NULL.

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    while(1)
    {
       const KVMetadataElement* element = NULL;
       error = metadataList->fpGetNext(metadataList, &element);

       if(error != KVError_Success)
       {
          //Handle error
       }

       if(element == NULL)
       {
          break;
       }

       //Process metadata element
    }

### Interpreting a Metadata Element

Each metadata element is conceptually represented as a key-value pair, where `pKey` is the name of the metadata key, and `pValue` is the value of that piece of metadata. To know the type of the metadata object the `pValue` points to, you must first consult the `eType` member. Strings are output in the character set that you requested in the call to fpInit().

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    switch (element->eType)
    {
    case KVMetadataValue_Bool:
    {
       BOOL value = *(BOOL*)element->pValue;
       //Process Bool value
       break;
    }
    case KVMetadataValue_Int64:
    {
       int64_t value = *(int64_t*)element->pValue;
       //Process Int64 value
       break;
    }
    case KVMetadataValue_Double:
    {
       double value = *(double*)element->pValue;
       //Process Doube value
       break;
    }
    case KVMetadataValue_WinFileTime:
    {
       int64_t value = *(int64_t*)element->pValue;
       //Process WinFileTime value
       break;
    }
    case KVMetadataValue_String:
    {
       KVString value = *(KVString*)element->pValue;
       //Process String value
       break;
    }
    case KVMetadataValue_Binary:
    {
       KVBinaryData value = *(KVBinaryData*)element->pValue;
       //Process Binary value
       break;
    }
    case KVMetadataValue_MIPLabel:
    {
       KVMIPLabel value = *(KVMIPLabel*)element->pValue;
       //Process MIPLabel value
       break;
    }
    default:
       //Handle unrecognised type
       break;
    }

### Standardized Metadata Elements

Different file formats can store the same piece of information in different ways. For example, one file format might call the width of the image width, another image_width, and another x_size. This behavior is often unhelpful, because you then need to maintain a list of fields that correspond to a particular piece of information. File Content Extraction solves this problem by standardizing certain metadata fields. See Field Standardization.

## Extracting Subfiles

File Content Extraction Filter SDK allows you to access the subfiles of a document, from both pure containers (such as ZIP or TAR files) and from documents embedded inside other files (such as an Excel spreadsheet embedded in a Word document).

### Loading the Extract Interface

You can load the Extract interface by including the `kvxtract.h` header, and calling the fpGetExtractInterface() function in the `kvfilter` shared library.

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    #include "kvxtract.h"

    KVExtractInterfaceRec extract;

    KVStructInit(&extract);

    error = filter.fpGetExtractInterface(session, &extract);

The interface function takes the filter session you created earlier, as well as the extraction interface to fill out.

### Opening a Container

You must open a container file before you can access the subfiles. You open the container by using the fpOpenFileFromFilterSession() function. This function creates a file-specific handle that you can use with the other functions in the extract interface. You must close this handle after use.

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    void* fileHandle = NULL;
    KVOpenFileArgRec        openArg;

    KVStructInit(&openArg);
    openArg.extractDir = "path/to/extract/dir";
    openArg.document = document;

    error = extract.fpOpenFileFromFilterSession(session, &openArg, &fileHandle);

        //Use File Handle

    extract.fpCloseFile(fileHandle);

You can then get information about the container itself by using the function fpGetMainFileInfo(). Most importantly, this tells you the number of subfiles. You must free this structure after use.

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    KVMainFileInfo          fileInfo = NULL;

    error = extract.fpGetMainFileInfo(fileHandle, fileInfo);

    //Use main file info

    extract.fpFreeStruct(fileHandle, fileInfo);

### Extracting Subfiles

Before you extract the subfile itself, you can first get some information about the subfile. You get this information by calling the fpGetSubFileInfo() function, using the index to identify the subfile. You must free this structure after use.

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    for(int ii = 0; ii < fileInfo->numSubFiles; ++ii)
    {
       KVSubFileInfo subFileInfo = NULL;
       error = extract.fpGetSubFileInfo(fileHandle, ii, &subFileInfo);

       //Use sub file info

       extract.fpFreeStruct(fileHandle, subFileInfo);
    }

After you have this subfile info, you can use it to construct the necessary arguments for extraction.

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    KVSubFileExtractInfo extractInfo = NULL;
    KVExtractSubFileArgRec extractArg;
    KVStructInit(&extractArg);

    if(subFileInfo->subFileType == KVSubFileType_Folder ||
       subFileInfo->subFileType == KVSubFileType_External)
    {
       goto skipfile;
    }

    extractArg.index = index;
    extractArg.filePath = subFileInfo->subFileName;
    extractArg.extractionFlag =
       KVExtractionFlag_CreateDir |
       KVExtractionFlag_Overwrite |
       KVExtractionFlag_GetFormattedBody |
       KVExtractionFlag_SanitizeAbsolutePaths;

    error = extract.fpExtractSubFile(fileHandle, &extractArg, &extractInfo);

    //Do more processing, such as filtering the sub file

    extract.fpFreeStruct(fileHandle, extractInfo);

The fpExtractSubFile() function fills out the `KVSubFileExtractInfo` pointer, which tells you more about what the function actually did. For example, it tells you the location it extracted the file to.

TIP: Mail Files. File Content Extraction treats mail files as containers, where the first subfile is the contents of the mail file, and subsequent subfiles are the attachments.

NOTE: Security. `KVExtractionFlag_SanitizeAbsolutePaths` mitigates against certain path traversal attacks. See Sanitize Absolute Paths.

By default, File Content Extraction does not extract images when extracting subfiles. You can enable image extraction by using the fpSetConfig() function. This option is set globally for the session, so you can set it outside of the loop that you use to process files.

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    error = filter.fpSetConfig(session, KVFLT_EXTRACTIMAGES, TRUE, NULL);

### Retrieving Mail Metadata

You can retrieve mail metadata for a particular subfile by using the function fpGetSubFileMetadataList(). This function fills out the same `KVMetadataList` structure that you used in Retrieving Metadata, and can be handled in the same way. You must initialize KVGetSubFileMetadataListArg by using `KVStructInit()`.

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    const KVMetadataList* metadataList = NULL;

    KVGetSubfileMetadataListArgRec metaArgs;
    KVStructInit(&metaArgs);
    metaArgs.index = index;
    metaArgs.trgCharset = KVCS_UTF8;

    error = extract.fpGetSubfileMetadataList(fileHandle, &metaArg, &metadataList);

    //Process metadata using metadataList->fpGetNext()

    metadataList->fpFree(metadataList);

## Conclusion

You have now built a basic sample program, processing documents using file mode. To learn about more advanced features, such as processing files in stream mode, take a look at the advanced tutorial.

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Version 25.3 | Last updated July 2025

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Product: File Content Extraction 25.3

Topic Title: C API Programming Tutorial