File header format

Here is the file header format as of release 6.22.06. It is never compressed.

If END, SeekFree, or SeekInfo are located past the 32 bit file limit (> 2000000000) then these fields will be 8 instead of 4 bytes and 1000000 is added to the file format version.

 -----------------------------------
 byte  0->3  "root"               = Identifies this file as a ROOT file.
       4->7  Version              = File format version                         TFile::fVersion
                                  |  (10000*major+100*minor+cycle (e.g. 62206 for 6.22.06))
       8->11          BEGIN       = Byte offset of first data record (100)      TFile::fBEGIN
      12->15 [12->19] END         = Pointer to first free word at the EOF       TFile::fEND
                                  | (will be == to file size in bytes)
      16->19 [20->27] SeekFree    = Byte offset of FreeSegments record          TFile::fSeekFree
      20->23 [28->31] NbytesFree  = Number of bytes in FreeSegments record      TFile::fNBytesFree
      24->27 [32->35] nfree       = Number of free data records
      28->31 [36->39] NbytesName  = Number of bytes in TKey+TNamed for TFile at creation TDirectory::fNbytesName
      32->32 [40->40] Units       = Number of bytes for file pointers (4)       TFile::fUnits
      33->36 [41->44] Compress    = Zip compression level (i.e. 0-9)            TFile::fCompress
      37->40 [45->52] SeekInfo    = Byte offset of StreamerInfo record          TFile::fSeekInfo
      41->44 [53->56] NbytesInfo  = Number of bytes in StreamerInfo record      TFile::fNbytesInfo
      45->46 [57->58] UUID vers   = TUUID class version identifier              TUUID::Class_Version()
      47->62 [59->74] UUID        = Universally Unique Identifier               TUUID::fTimeLow through fNode[6]
      63->99 [75->99] Extra space to allow END, SeekFree, or SeekInfo to become 64 bit without moving this header

Here is the file header format as of release 3.02.06. It is never compressed.

 -----------------------------------
 byte  0->3  "root"      = Identifies this file as a ROOT file.
       4->7  Version     = File format version                        TFile::fVersion
                         |  (10000*major+100*minor+cycle (e.g. 30203 for 3.2.3))
       8->11 BEGIN       = Byte offset of first data record (64)      TFile::fBEGIN
      12->15 END         = Pointer to first free word at the EOF      TFile::fEND
                         | (will be == to file size in bytes)
      16->19 SeekFree    = Byte offset of FreeSegments record         TFile::fSeekFree
      20->23 NbytesFree  = Number of bytes in FreeSegments record     TFile::fNBytesFree
      24->27 nfree       = Number of free data records
      28->31 NbytesName  = Number of bytes in TKey+TNamed for TFile at creation TDirectory::fNbytesName
      32->32 Units       = Number of bytes for file pointers (4)      TFile::fUnits
      33->36 Compress    = Zip compression level (i.e. 0-9)           TFile::fCompress
      37->40 SeekInfo    = Byte offset of StreamerInfo record         TFile::fSeekInfo
      41->44 NbytesInfo  = Number of bytes in StreamerInfo record     TFile::fNbytesInfo
      45->63             = Unused??