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In-Memory Archive Processing
Applies To: All


  • 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.


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
// let's write the archive to the disk
CZipFile f;
if (f.Open(_T("C:\\Temp\\"),
CZipFile::modeWrite | CZipFile::modeCreate, false))
int iLen = (int)mf.GetLength();
BYTE* b = mf.Detach();
f.Write(b, iLen);
// we must free the detached memory


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\\"), 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);
CZipMemFile mf;
mf.Attach(b, iLen);
// open the archive in memory
CZipArchive zip;
zip.ExtractFile(0, _T("C:\\Temp"));

In-Memory Data Compressing and Extracting

Sample Code
LPCTSTR zipFileName = _T("C:\\Temp\\");
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"));
CZipMemFile mfOut;
// set the memory file size to the size of the uncompressed data to avoid
// frequent memory allocations
((CZipAbstractFile*)&mfOut)->SeekToBegin(); // may be needed when mfOut was used previously
// extract the contents of the file into memory
zip.ExtractFile(0, mfOut);
// print the contents of the extracted data
BYTE* b = mfOut.Detach();
// we must free the detached memory

See Also API Links

Article ID: 0610231924
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