ZipArchive: In-Memory Archive Processing

In-Memory Archive Processing

Applies To: All

Introduction
In-Memory Archive Creating and Opening
- Creating
- Opening
In-Memory Data Compressing and Extracting
See Also API Links

Introduction

The ZipArchive Library allows the following operations in memory:
- opening archives for creation and modification,
- compressing memory files,
- extracting files from archive to memory files.
This allows the whole archive processing to be performed entirely in memory.
The CZipMemFile object is not destroyed throughout the whole processing.
Note that it is not an error to set the file pointer in the CZipMemFile object to a position beyond the end of the file.
To avoid frequent memory re-allocations, depending on the situation, consider taking one of the following actions:
- initiate CZipMemFile with a large enough grow value in the nGrowBy parameter in the constructor
- attach a buffer with the CZipMemFile::Attach() method.
- set the length of the memory file with the CZipMemFile::SetLength() method (e.g. during extraction to memory, you can set it to the
  CZipFileHeader::m_uUncomprSize value).

In-Memory Archive Creating and Opening

To create or open an archive in memory, use the CZipArchive::Open(CZipAbstractFile&) method and pass CZipMemFile object as an argument.

Creating

If you are creating a new archive, the memory file will hold the resulting archive.

To clear the contents of the memory file before creating a new archive, open the archive using the CZipArchive::zipCreate mode.
To keep the contents of the memory file and append the archive at the end of it, open the archive using the CZipArchive::zipCreateAppend mode.

Sample Code

    CZipMemFile mf;
    CZipArchive zip;    
    // create the archive in memory
    zip.Open(mf, CZipArchive::zipCreate);
    // add a file
    zip.AddNewFile(_T("C:\\Temp\\file.dat"));
    zip.Close();
    // let's write the archive to the disk
    CZipFile f;
    if (f.Open(_T("C:\\Temp\\test.zip"), 
        CZipFile::modeWrite | CZipFile::modeCreate, false))
    {
        int iLen = (int)mf.GetLength();
        BYTE* b = mf.Detach();
        f.Write(b, iLen);
        f.Close();
        // we must free the detached memory
        free(b);
    }

Opening

If you are opening an existing archive, the memory file should already hold an archive.

Sample Code

    CZipFile f;
    if (f.Open(_T("C:\\Temp\\test.zip"), CZipFile::modeRead, false))
    {
        // read the contents of the file into the memory file
        int iLen = (int)f.GetLength();    
        BYTE* b = (BYTE*)malloc((UINT)iLen);
        f.Read(b, iLen);
        f.Close();
        CZipMemFile mf;
        mf.Attach(b, iLen);
        // open the archive in memory
        CZipArchive zip;
        zip.Open(mf);
        zip.ExtractFile(0, _T("C:\\Temp"));
        zip.Close();
    }

In-Memory Data Compressing and Extracting

To compress a memory file, use one of the following methods:
- CZipArchive::AddNewFile(CZipAbstractFile&, LPCTSTR),
- CZipArchive::AddNewFile(CZipAddNewFileInfo&)
To extract a file in an archive to a memory file, use the
CZipArchive::ExtractFile(ZIP_INDEX_TYPE, CZipAbstractFile&) method. When extracting to a memory file, it is good to initialize the memory file size to the size of the extracted data to avoid frequent memory re-allocations.

Sample Code

    LPCTSTR zipFileName = _T("C:\\Temp\\test.zip");
    CZipArchive zip;
    zip.Open(zipFileName, CZipArchive::zipCreate);
    CZipMemFile mf;
    // prepare the memory file
    LPCTSTR data1 = _T("Test data");
    mf.Write(data1, (DWORD)(_tcslen(data1) * sizeof(TCHAR)));
    // compress the memory data
    zip.AddNewFile(mf, _T("file1.txt"));
    zip.Close();    
    zip.Open(zipFileName);    
    CZipMemFile mfOut;
    // set the memory file size to the size of the uncompressed data to avoid 
    // frequent memory allocations
    mfOut.SetLength(zip[0]->m_uUncomprSize);
    ((CZipAbstractFile*)&mfOut)->SeekToBegin(); // may be needed when mfOut was used previously
    // extract the contents of the file into memory
    zip.ExtractFile(0, mfOut);
    zip.Close();
    // print the contents of the extracted data
    BYTE* b = mfOut.Detach();
    _tprintf((LPCTSTR)b);
    // we must free the detached memory
    free(b);

Introduction

In-Memory Archive Creating and Opening

Creating

Opening

In-Memory Data Compressing and Extracting

See Also API Links