Archived:Getting trailer and header info from GZIP files using Symbian C++

ID		Creation date	May 6, 2008
Platform	S60 3rd Edition, FP1	Tested on devices	Nokia N93
Category	Symbian C++	Subcategory	Files/Data

Keywords (APIs, classes, methods, functions): RFile, EZGZipFile, TEZGZipHeader, TEZGZipTrailer, RFile::Open(),EZGZipFile::IsGzipFileL(), EZGZipFile::LocateAndReadTrailerL(), EZGZipFile::ReadHeaderL(),EZGZipFile::EFText,EZGZipFile::EFHcrc, EZGZipFile::EFExtra,EZGZipFile::EFName,EZGZipFile::EFComment

Overview

This snippet shows how to get the trailer and the header info from gzip files.

The trailer part of gzip file should contain following data:

• CRC32
• input size (calculated from the original uncompressed data)

The actual gzip header is defined to contain following fields:

• fixed value for ID1 = 31
• fixed value for ID2 = 139
• compression method (8 is the standard)
• flags (0 = no flags are set)
• modification time (time stamp in Unix format)
• extra flags (maximum compression - fastest algorithm)
• Operating System

This snippet can be self-signed.

MMP file

The following libraries are required:

LIBRARY efsrv.lib
LIBRARY ezlib.lib

Source file

#include <ezgzip.h>
#include <f32file.h>
 
 
void GetGZipFileTrailerAndHeaderInfoL(RFs& aRfs, const TDesC& aFileName)
  {
  RFile file;
  TEZGZipHeader header;
  TEZGZipTrailer trailer;
 
  if (!EZGZipFile::IsGzipFileL(aRfs,aFileName))
    {
    _LIT(KIsNotGZipFile,"File:%S is not in GZip format\n");
    console->Printf(KIsNotGZipFile,&aFileName);
    console->Getch();
    User::Leave(KErrGeneral);
    }
 
  EZGZipFile::LocateAndReadTrailerL(aRfs, aFileName, trailer);
  _LIT(KTrailer,"CRC32 = %d size = %d\n");
  console->Printf(KTrailer,trailer.iCrc32,trailer.iSize);
 
  User::LeaveIfError(file.Open(aRfs,aFileName,EFileStream | EFileRead | EFileShareAny));
  EZGZipFile::ReadHeaderL(file, header);
 
  _LIT(KFileId1,"ID1 = %d\n");    
  console->Printf(KFileId1,header.iId1);
  _LIT(KFileId2,"ID2 = %d\n");
    console->Printf(KFileId2,header.iId2);    
  _LIT(KCompressionMethod,"compression method = %d\n");
  console->Printf(KCompressionMethod,header.iCompressionMethod);
  _LIT(KFlags,"flags = %d\n");
  console->Printf(KFlags,header.iFlags);
  _LIT(KTimeStamp,"time stamp = %d\n");
  console->Printf(KTimeStamp,header.iTime);
  _LIT(KExtraFlags,"extra flags %d\n");
  console->Printf(KExtraFlags,header.iExtraFlags);
  _LIT(KOS,"OS %d\n");
  console->Printf(KOS,header.iOs);
 
  if (header.iFlags & EZGZipFile::EFText)
    {
    _LIT(KText,"text flag set\n");
    console->Printf(KText);
    }
 
  if (header.iFlags & EZGZipFile::EFHcrc)
    {
    _LIT(KCrc16,"CRC16 = %d\n");
    console->Printf(KCrc16,header.iCrc);
    }
 
  if (header.iFlags & EZGZipFile::EFExtra)
    {
    _LIT(KExtraLength,"extra length %d\n");
    console->Printf(KExtraLength,header.iXlen);
    HBufC *buf = HBufC::NewLC(header.iExtra->Length());
    buf->Des().Copy(*header.iExtra);
    console->Printf(*buf);
    CleanupStack::PopAndDestroy(); //buf
    }
 
  if (header.iFlags & EZGZipFile::EFName)
    {
    _LIT(KFName,"fname: %S\n");
    HBufC *buf = HBufC::NewLC(header.iFname->Length());
    buf->Des().Copy(*header.iFname);
    console->Printf(KFName,buf);
    CleanupStack::PopAndDestroy(); //buf
    }
 
  if (header.iFlags & EZGZipFile::EFComment)
    {
    _LIT(KComment,"comment: %S\n");
    HBufC *buf = HBufC::NewLC(header.iComment->Length());
    buf->Des().Copy(*header.iComment);
    console->Printf(KComment,buf);
    CleanupStack::PopAndDestroy(); //buf
    }
  }

Postconditions

The trailer and header info from the given gzip file is printed to the console.

See also

Compressing_and_decompressing_gzip_files

The gzip format is specified in the page http://www.faqs.org/rfcs/rfc1952.html.