Gpart is a small tool which tries to guess what partitions are on a PC type, MBR-partitioned hard disk in case the primary partition table was damaged.

Gpart works by scanning through the device (or file) given on the command line on a sector basis. Each guessing module is asked if it thinks a filesystem it knows about could start at a given sector. Several filesystem guessing modules are built in.

Consult the manual page for command line options and usage.

See file INSTALL.

Each guessing module must provide three functions callable from gpart:

int xxx_init(disk_desc *d,g_module *m)

Initialisation function. Will be called before a scan. It should return the minimum number of bytes it wants to receive for a test. The module should set the description of the filesystem/partition type it handles in g_module.m_desc. If the filesystem/partition type included a partition table like first sector (like the *BSD disklabels do), the flag m_hasptbl should be set. Another flag is m_notinext which means the tested type cannot reside in a logical partition.

int xxx_term(disk_desc *d)

Termination/cleanup function, called after the scanning of the device has been done.

int xxx_gfun(disk_desc *d,g_module *m)

The actual guessing function, called from within the scan loop. It should test the plausibility of the given sectors, and return its guess in m->m_guess (a probability between 0 and 1). See existing modules for examples.

The given file descriptor d->d_fd can be used for seeking and reading (see e.g. gm_ext2.c which tries to read the first spare superblock). If a module is convinced that it has found a filesystem/partition start it should fill in the assumed begin and size of the partition.

The test performed should not be too pedantic, for instance it should not be relied upon that the file- system is clean/was properly unmounted. On the other hand too much tolerance leads to misguided guesses, so a golden middle way must be found.

Here is a sample gpart -v run on my first IDE hard disk (comments in block-quotes):

dev(/dev/hda) mss(512) chs(1232/255/63)(LBA) #s(19792080) size(9664mb)

mss is the medium sector size, chs the geometry retrieved from the OS (or from the command line), #s is the total sector count.

Primary partition(1)
   type: 006(0x06)(Primary 'big' DOS (> 32MB)) (BOOT)
   size: 502mb #s(1028097) s(63-1028159)
   chs:  (0/1/1)-(63/254/63)d (0/1/1)-(63/254/63)r
   hex:  80 01 01 00 06 FE 3F 3F 3F 00 00 00 01 B0 0F 00

size: the size of the partition in megabytes, number of sectors and the sector range. chs: the partition table chs range (d) and the real one (r). If the number of cylinders is less than 1024, both are identical. hex: the hexadecimal representation of the partition entry as found in the partition table.

...

Begin scan...
Possible partition(DOS FAT), size(502mb), offset(0mb)
   type: 006(0x06)(Primary 'big' DOS (> 32MB))
   size: 502mb #s(1028097) s(63-1028159)
   chs:  (0/1/1)-(63/254/63)d (0/1/1)-(63/254/63)r
   hex:  00 01 01 00 06 FE 3F 3F 3F 00 00 00 01 B0 0F 00

Possible extended partition at offset(502mb)
    Possible partition(Linux ext2), size(31mb), offset(502mb)
      type: 131(0x83)(Linux ext2 filesystem)
      size: 31mb #s(64196) s(1028223-1092418)
      chs:  (64/1/1)-(67/254/62)d (64/1/1)-(67/254/62)r
      hex:  00 01 01 40 83 FE 3E 43 7F B0 0F 00 C4 FA 00 00

   Possible partition(Linux swap), size(125mb), offset(533mb)
      type: 130(0x82)(Linux swap or Solaris/x86)
      size: 125mb #s(256976) s(1092483-1349458)
      chs:  (68/1/1)-(83/254/62)d (68/1/1)-(83/254/62)r
      hex:  00 01 01 44 82 FE 3E 53 83 AB 10 00 D0 EB 03 00

During the scan phase all found partitions are listed by their real type names. The Linux swap partition above is recognized as Linux swap but will get the 0x82 partition identifier which can be both a Solaris disklabel or a Linux swap partition.

When examining the hex values of the first primary partition it can be seen that they are identical to the values of the actual partition table (good guess) except for the first value (0x80 vs. 0x00). This entry denotes the partition 'boot' flag which cannot be guessed.

...

End scan.

Checking partitions...
Partition(Primary 'big' DOS (> 32MB)): primary
   Partition(Linux ext2 filesystem): logical
   Partition(Linux swap or Solaris/x86): logical
   Partition(Linux LVM physical volume): logical
   Partition(Linux ext2 filesystem): logical
   Partition(DOS or Windows 95 with 32 bit FAT, LBA): logical
Partition(FreeBSD/NetBSD/386BSD): primary
Partition(Linux LVM physical volume): primary
Ok.

During the scan phase gpart gathers a simple list of possible partitions, the check phase now tries to decide if found extended partitions seem consistent, if partitions do not overlap etc. Overlapping partitions are silently discarded, all remaining ones are given an attribute 'primary', 'logical', 'orphaned' or 'invalid'. If gpart is called like gpart -vv ..., it also tells why it thinks a partition guess is invalid.

If any inconsistencies are found, gpart prints the number of remaining inconsistencies, otherwise it says 'Ok.'

Guessed primary partition table:
Primary partition(1)
   type: 006(0x06)(Primary 'big' DOS (> 32MB))
   size: 502mb #s(1028097) s(63-1028159)
   chs:  (0/1/1)-(63/254/63)d (0/1/1)-(63/254/63)r
   hex:  00 01 01 00 06 FE 3F 3F 3F 00 00 00 01 B0 0F 00

Primary partition(2)
   type: 005(0x05)(Extended DOS)
   size: 6157mb #s(12611025) s(1028160-13639184)
   chs:  (64/0/1)-(848/254/63)d (64/0/1)-(848/254/63)r
   hex:  00 00 01 40 05 FE FF 50 40 B0 0F 00 D1 6D C0 00

Primary partition(3)
   type: 165(0xA5)(FreeBSD/NetBSD/386BSD)
   size: 1396mb #s(2859570) s(13639185-16498754)
   chs:  (849/0/1)-(1023/254/63)d (849/0/1)-(1026/254/63)r
   hex:  00 00 C1 51 A5 FE FF FF 11 1E D0 00 32 A2 2B 00

Primary partition(4)
   type: 254(0xFE)(Linux LVM physical volume)
   size: 1608mb #s(3293325) s(16498755-19792079)
   chs:  (1023/254/63)-(1023/254/63)d (1027/0/1)-(1231/254/63)r
   hex:  00 FE FF FF FE FE FF FF 43 C0 FB 00 8D 40 32 00

This is a resulting primary partition table. Note that the logical partition guesses were only used to create the extended partition entry. Up to now gpart cannot reconstruct a damaged logical partition chain itself.

If a guessed primary partition table should be written to some file or device the user must specify (via the -W option) which partition gets the active (bootable) one.

gpart README, Aug 1999, Michail Brzitwa mb@ichabod.han.de