ZipArchive: Requesting Information, Predicting Names and Sizes

Requesting Information, Predicting Names and Sizes

Applies To: All

Requesting Information
- File Information
- Archive Information
Predicting Names and Sizes

Requesting Information

File Information

You can request different types of information about files inside an archive. To find out about types of information you may retrieve, examine members of the CZipFileHeader class. To request the CZipFileHeader object for a particular file, call one of the following methods:

CZipArchive::GetFileInfo(CZipFileHeader&, ZIP_INDEX_TYPE) - this method copies an information about the file to the provided CZipFileHeader object. You can modify this object if you want to.
CZipArchive::GetFileInfo(ZIP_INDEX_TYPE) - this method returns a pointer to the entry in the central directory. This method is provided for fast access to avoid information copying. You should normally not modify this object unless:
- you want to modify extra fields in the central directory (see Providing Custom Data: Extra Fields for more information),
- you want to change the file header time stamp (see Modification of Archives: Replacing, Renaming, Deleting and Changing Data for more information).
CZipArchive::operator[]() - the same as above.

Filename Conversion

When calling the CZipFileHeader::GetFileName() method for the first time, the conversion takes place which converts the filename from an internal representation to the CZipString object. This conversion takes into account filename code pages (see Unicode Support: Using Non-English Characters in Filenames and Comments) and directory separators (to match the current platform's default directory separator).

Local Header Information

If you wish to obtain the data that is stored in a local header, you need to make sure that the local information has been already read before. The local information is read at the moment when you start extracting a file. If you don't want to extract a file, but you still want to read the local information, call the CZipArchive::ReadLocalHeader() method. The local information can be one of the following:

Sample Code

    CZipArchive zip;
    // open an existing archive
    zip.Open(_T("C:\\Temp\\test.zip"));
    for (ZIP_INDEX_TYPE i = 0; i < zip.GetCount(); i++)
    {
        CZipFileHeader* info = zip[i];
        LPCTSTR name = info->GetFileName();
        ZIP_SIZE_TYPE uncomprSize = info->m_uUncomprSize;
        ZIP_SIZE_TYPE comprSize = info->m_uComprSize;        

        // read some sample local information
        zip.ReadLocalHeader(i);
        int extraSize = info->m_aLocalExtraData.GetTotalSize();
#ifdef _ZIP64
        _tprintf(_T("%I64u: Name: %s Size: %I64u Compressed Size: %I64u\r\n\
    Size of Local Extra Data: %d\r\n"),
            i, name, uncomprSize, comprSize, extraSize);
#else
        _tprintf(_T("%u: Name: %s Size: %u Compressed Size: %u\r\n\
    Size of Local Extra Data: %d\r\n"),
            i, name, uncomprSize, comprSize, extraSize);
#endif
    }

Iterating Over Items

To find out about the number of files in an archive, use the CZipArchive::GetCount() method. This method and the GetSize() and GetCount() methods (collections' members) in some implementations, return an unsigned type. To check the data type used by collections, examine the ZIP_ARRAY_SIZE_TYPE definition. You should pay special attention when reverse-iterating over such collections. The code below represents an invalid code. If ZIP_ARRAY_SIZE_TYPE is represented by an unsigned type, the iteration variable will never be negative and the archive will report an invalid index when an overflow happens. If the number of items is the maximum value for the unsigned type, the loop may run forever.

Sample Code

    /* THIS IS A BAD CODE. */
    /* IT WILL THROW AN EXCEPTION OR RUN FOREVER */

    CZipArchive zip;
    zip.Open(_T("C:\\Temp\\test.zip"));
    ZIP_INDEX_TYPE uMax = zip.GetCount();
    // incorrect reverse iteration
    for (ZIP_INDEX_TYPE i = uMax - 1; i >= 0; i--)
    {
        CZipFileHeader info;
        zip.GetFileInfo(info, i);
         //... do something
    }
    zip.Close();

The correct way for reverse iteration illustrates the code below.

Sample Code

    CZipArchive zip;
    zip.Open(_T("C:\\Temp\\test.zip"));
    ZIP_INDEX_TYPE i = zip.GetCount();
    // correct reverse iteration
    while (i > 0)
    {
        i--;
        CZipFileHeader info;
        zip.GetFileInfo(info, i);
        // ... do something
    }    
    zip.Close();

Please pay attention when you use your code in both MFC and STL versions, because you will use both signed and unsigned types as ZIP_ARRAY_SIZE_TYPE. Operations that work on these types and produce negative results will be interpreted differently in both versions.

When you don't use Zip64 extensions (see Zip64 Format: Crossing the Limits of File Sizes and Number of Files and Segments), you can use a signed type (int) for the file index and volume number types. Just make sure that _ZIP_STRICT_U16 is not defined in _features.h file.

Archive Information

The ZipArchive Library provides the following methods for requesting archive-related information:

Predicting Names and Sizes

The ZipArchive Library provides the following methods for predicting names and sizes:

Article ID: 0610242128