This manual is for GNU Chess (version 6.0.0, 9 April 2011).
This manual is for GNU Chess (version 6.0.0, 9 April 2011), which is a complete chess program, frequently used as a chess engine.
Copyright © 2001–2011 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
GNU Chess (http://www.gnu.org/software/chess/) is a computer program for playing chess. It can be used to interactively play chess on a text terminal, but it is more often used in conjunction with a GUI program such as GNU XBoard.
Because it is protected by the GNU General Public License, users are free (in perpetuity) to share and change it.
The main author of GNU Chess version 6 is Fabien Letouzey, The original author is Stuart Cracraft.
We are the GNU Chess developers and you may reach us at:
Our official web page is:
We are indebted to our sponsor, the Free Software Foundation whose web page is:
and which also serves as our software depository for new versions of GNU and GNU Chess.
You can download the latest version from GNU's FTP site at:
The code is provided for the purpose of encouraging you to do the programming. If you lack the programming skills to do so, try dabbling in it. You might surprise yourself.
If you want to report a possible bug in GNU Chess, please send a message to the e-mail address indicated above, providing precise information about the conditions that led to the possible bug. As a general guideline, you can kindly include the follow information:
Log = true
The format for running the gnuchess program is:
gnuchess option ...
With no options, gnuchess starts in interactive mode and it is ready to start a chess game.
gnuchess supports the following options:
Option xboard is accepted without leading dashes for backward
Option post is accepted without leading dashes for backward
Moves are accepted either in standard algebraic notation (SAN) or in coordinate algebraic notation. Examples:
Nf3 g1f3 O-O e1g1
gnuchess supports the following commands:
(Same as gnuchess --help)
add - compiles book.dat from book.pgn
on - enables use of book
off - disables use of book
best - play best move from book
worst - play worst move from book
random - play any move from book
prefer (default) - choose a good move from book
(Method subject to variation)
(Same as gnuchess --version)
(Same as gnuchess --manual)
Note: not implemented in this version.
(White and black commands are mainly for icsDrone and will cause the current en-passant capture square to be forgotten).
Note: not implemented in this version.
(Same as gnuchess --easy)
If pondering (see hard) is on, the program will output it's thinking whilst the opponent is thinking.
(Same as gnuchess --post)
Note: not implemented in this version.
Note: not implemented in this version. Neither in v5
off - disables the memory hash table
off - disables using the null move heuristic
off - disables use of xboard/winboard
(Same as gnuchess --xboard)
Note: not implemented in this version.
board - displays the current board
time - displays the time settings
moves - shows all moves using one call to routine
escape - shows moves that escape from check using one call to routine
noncapture - shows non-capture moves
capture - shows capture moves
eval [or score] - shows the evaluation per piece and overall
game - shows moves in game history
pin - shows pinned pieces
Note: 'show eval' and 'show pin' not implemented in this version.
movelist - reads in an epd file and shows legal moves for its entries
capture - reads in an epd file and shows legal captures for its entries
movegenspeed - tests speed of move generator
capturespeed - tests speed of capture move generator
eval - reads in an epd file and shows evaluation for its entries
evalspeed tests speed of the evaluator
Note: not implemented in this version.
If GNUCHESS_PKGDATADIR is defined, it will be taken as the path for the config file and for the book, in case the files are not found in the current directory. If it is not defined, they will be taken from the package data directory, in case the files are not found in the current directory.
A file called gnuchess.ini is used to define configuration options. The file is applicable provided --uci is not used. The file is looked in three places according to the following precedence:
Sections are composed of variable = value lines.
Note: There can be spaces in variable names or values. Do not use quotes.
This section is used by the adapter only. The engine is unaware of these options. The list of available options is detailed below in this document.
This section contains engine UCI options. The PolyGlot-based adapter does not understand them, but sends the information to the engine at startup (converted to UCI form). You can add any UCI option that makes sense to the engine (not just the common options about hash-table size and tablebases).
Note: use INI syntax, not UCI. For example OwnBook = true is correct. It will be replaced by the adapter with setoption name OwnBook value true at engine startup.
Standard UCI options are Hash, NalimovPath, NalimovCache and OwnBook. Hidden options like Ponder or UCI_xxx are automatic and should not be put in the INI file.
These should be put in the [Adapter] section.
Whether the adapter should log all transactions with the interface and
the engine. This should be necessary only to locate problems.
The name of the log file. Note that it is put where the program was launched from, not into the engine directory.
WARNING: Log files are not cleared between sessions, and can become
very large. It is safe to remove them though.
Set this to "true" if you want the adapter to resign on behalf of the engine.
NOTE: Some engines display buggy scores from time to time although the
best move is correct. Use this option only if you know what you are
doing (e.g. you always check the final position of games).
Number of consecutive moves with "resign" score (see below) before
the adapter resigns for the engine. Positions with only one legal move
This is the score in centipawns that will trigger resign "counting".
Show search information during engine pondering. Turning this off
might be better for interactive use in some interfaces.
Whether to kibitz when playing a move.
Whether to kibitz when the PV is changed (new iteration or new best move).
xboard command to use for kibitzing, normally "tellall" for kibitzing
or "tellothers" for whispering.
How many seconds to wait before starting kibitzing. This has an
affect only if "KibitzPV" is selected, move kibitzes are always sent
regardless of the delay.
Indicates whether the adapter should use a book. This has no effect
on the engine own book (which can be controlled with the UCI option
OwnBook in the [Engine] section). In particular, it is possible to
use both a PolyGlot book and an engine book. In that case, the engine
book will be used whenever PolyGlot is out of book. Remember that
PolyGlot is unaware of whether the engine is itself using a book or
The name of the (binary) book file. Note that PolyGlot will look for it in the directory it was launched from, not in the engine directory. Of course, full path can be used in which case the current directory does not matter.
If the file is not found in the current directory, it will be looked for in GNUCHESS_PKGDATADIR if the variable is defined, or in the package data directory otherwise.
Note that there is no option to control book usage. All parameters are fixed when compiling a PGN file into a binary book (see below). This is purposeful and is not likely to change.
Using a book does not require any additional memory, this can be important for memory-limited tournaments.
These are UCI options for the [Engine] section:
Default: Fail High
"Always" actually means the usual conditions (not in check, etc ...). "Fail High" adds the condition that the static evaluation fails high. Never use "Never" (ever)! OK you can use "Never" to test a Zugzwang problem.
I expect that this option has little effect (assuming the first two
choices only). It was only added because most engines do not use the
3 is rather aggressive, especially in the endgame. It seems better
than always using 2 though.
This tries to solve some Zugzwang-related problems. It is expected to
hardly have any effect in games. The default value should be
sufficient for most-common Zugzwang situations.
5 guarantees that the cost of verification search is negligible in
most cases. Of course it means Zugzwang problems need a lot of depth
to get solved, if ever! With such a reduction, verification search is
similar to Vincent Diepeveen's "double null move".
A bit dodgy, but fun to experiment with. It should
help in blitz, but it's possible it actually hurts play in longer
This is the thing, as it affects the search tree! Lower values are
safer, and higher values more aggressive. THIS VALUE HAS NOT BEEN
TUNED! There is a good chance Fruit's strength can be improved by
changing this option.
Very common but controversial. Makes the engine a tiny bit
better at tactics but slightly weaker positionally. It might be
beneficial by a very small amount, but has not been tested in
conjunction with history pruning yet.
This value is somewhat aggressive. It could lead to problems in
the endgame. Larger values prune less but will lead to fewer
Similar to futility pruning. Probably safer because it is used
mainly during the middlegame. Has not been tested with history
Same behaviour as futility margin. This one is probably safe.
Quiescence Check Plies
Fruit tries safe (SEE >= 0) checks at the first plies of the
quiescence search. 0 means no checks at all (as in most older
engines). 1 is the same as previous versions of Fruit. 2 is probably
not worth the extra cost. It could be interesting when solving mate
These options are evaluation-feature multipliers. You can modify Fruit's playing style to an extent or make Fruit weaker for instance by setting "Material" to a low value.
"Material" is obvious. It also includes the bishop-pair bonus. "Piece Activity": piece placement and mobility. "King Safety": mixed features related to the king during early phases "Pawn Structure": all pawn-only features (not passed pawns). "Passed Pawns": ... can you guess?
The following options were used in PolyGlot v1.4, but are deprecated in GNU Chess:
This is the name that will appear in the xboard interface. It is cosmetic only. You can use different names for tweaked versions of the same engine.
If no "Engine Name" is given, the UCI name will be used.
Full path of the directory where the engine is installed. You can use
"." (without the quotes) if you know that PolyGlot will be launched in
the engine directory or the engine is in the "path" and does not need
any data file.
NOTE: Unix users are recommended to prepend "./"; this is required on some secure systems.
Work arounds are identical to options except that they should be used only when necessary. Their purpose is to try to hide problems with various software (not just engines). The default value is always correct for bug-free software.
These workarounds are unlikely to be used or meaningful for GNU Chess.
The default value of 2 corresponds to UCI+. Use 1 to select plain
UCI for engines that have problems with UCI+.
The adapter now conforms to the documented UCI behaviour: the engine will
be allowed to ponder only if it (the engine) declares the Ponder UCI
option. However some engines which can actually ponder do not declare
the option. This work around lets The adapter know that they can ponder.
When a ponder miss occurs, the adapter interrupts the engine and
immediately launches a new search. While there should be no problem
with this, some engines seem confused and corrupt their search board.
SyncStop forces the adapter to wait for the (now useless) ponder search
to finish before launching the new search.
Some engines do not specify a promotion piece, e.g. they send "e7e8" instead of the correct "e7e8q". This work around enables the incorrect form (and of course promotes into a queen).
GNU Chess produces several output files:
LogFile = FILENAME
The file is produced if option --uci is not specified and if the following variable is set in gnuchess.ini:
Typically, this file is named adapter.log, but any other name will do the job.
Log = true
opponent-name wins loses draws
./configure make make install
Use the following commands:
./configure CFLAGS=-DDEBUG make make install
When analysing a bug, this file could be very helpful. Users are encouraged to provide it.
Running the program with the "–xboard" command line parameter sets it to produce output acceptable to and accept input suitable for XBoard and WinBoard, the graphical display front-ends with mouse interface.
For historical reasons the option "xboard" does not need to be preceeded by "–", however we would encourage the new syntax.
How to run XBoard with GNU Chess as chess backend:
xboard -fcp 'gnuchess --xboard' xboard -fd . -fcp './gnuchess --xboard'
For GNU Chess to behave as a pure UCI chess engine, execute the following command:
In this mode, configuration file gnuchess.ini is ignored.
For GNU Chess to run in Internet, Zippy is required. See Zippy documentation in the XBoard/WinrBoard distribution:
this is an example of how to run GNU Chess on FICS using XBoard as frontend and Zippy as connector:
xboard -zp -ics -icshost freechess.org -icshelper timeseal -fcp 'gnuchess --xboard'
See options Book and Bookfile in Running GNU Chess - Configuration file - Options
GNU Chess 6 has been tested on the Free Internet Chess Server (http://www.freechess.org) with XBoard.
The .bin file format is a simple binary format for the compiled book which is read by the program when it is using book. See the book section for more detail.
EPD and PGN require little introduction. These are the uniformly accepted standards for position recording and game recording.
Note that log.nnn and game.nnn files are written at the end of a game when you use the name command to give the computer your name. It is highly recommended to do this since the resulting two files that match in a monotonically-increasing extension numbered suffix may be used for reporting bugs and keeping track of your games.
The first version of GNU Chess was written by Stuart Cracraft back in 1984. Versions from 2 to 4 were written by John Stanback. Version 5 was written by Chua Kong-Sian. Version 6 was written by Fabien Letouzey.
These are known problems of PolyGlot v1.4 as described by Fabien Letouzey.
The addition of Chess960 support lead to a change in internal-move representation for castling. This slightly affected the opening-book format. We recommend that you recompile books with this version.
Several users reported engines losing on time. The playing conditions always mixed playing on an Internet server with pondering.
It is not yet clear what the source of the problem is, but there seems to be a forever incompatibility between the xboard and UCI protocol regarding a complex pondering/remaining-time relation.
Since version 5.07 of the program was released in 2003, GNU Chess has basically remained unchanged. Then Fabien Letouzey developped Fruit, which proved to be a stronger chess engine and shaked the computer chess world. At some point in time, he kindly assigned copyright to FSF, which allowed us to make Fruit 2.1 (the latest free version) the base for GNU Chess v6.
Therefore, it can be fairly said that Fabien Letouzey is remarkably the main contributor to GNU Chess v6.
The idea was to use Fruit 2.1 as the base for GNU Chess v6, keeping external interfaces backwards compatible. This is particularly important since GNU Chess has been world-wide used for long.
GNU Chess can be used in two modes: interactively using the command-line interface on a text console, and used as backend engine from a graphical frontend. Both cases rely on almost identical grammar. The former is described in GNU Chess help.
When GNU Chess is used as a pure chess engine, for instance, as backend for XBoard, it uses the Chess Engine Communication Protocol (aka XBoard protocol), which is described here:
Since Fruit uses the Universal Chess Interface (UCI), keeping the former interface was the main challenge of GNU Chess v6. UCI is described here:
GNU Chess can also run as a UCI chess engine, if the --uci command-line option is specified.
UCI is very different from the Chess Engine Communication Protocol. PolyGlot is a UCI-to-xboard adapter developed by Fabien. It connects a UCI chess engine to an xboard interface such as WinBoard. UCI2WB is another such adapter (for Windows).
Standalone PolyGlot can be used, along with Fruit, as chess engine for chess frontends, suchs as XBoard. In that case, PolyGlot and Fruit run as two independent single-threaded processes. PolyGlot starts first, and it forks Fruit. Both processes get communicated by by means of pipes: PolyGlot captures Fruit's standrad input and output.
PolyGlot tries to solve known problems with other adapters. For instance, it detects and reports draws by fifty-move rule, repetition, etc.
PolyGlot 1.4 has been adapted and incorporated to GNU Chess v6 as chess engine protocol adapter. It connects Fruit-based GNU Chess engine to the good old GNU Chess frontend.
We want to keep three loosely-coupled modules in GNU Chess v6:
The main program contains the frontend and starts two addtional threads, one for the adapter, and another one for the engine. The three components comprise a chain, thus there are two links:
The links are based on pipes. There is no need for additional synchronization mechanisms such as mutex. The changes in PolyGlot and Fruit are minimal, since they were already using the same mechanism to communicate with each other.
Another technical problem was the fact that GNU Chess v5 was written in C, whereas PolyGlot and Fruit are written in C++. In GNU Chess v6 the main program, the adapter and the engine are in C++, but the frontend remains in C.
Source code was placed under a single src in GNU Chess v5, as usual. Three additional directories have been created for GNU Chess v6, so the code is organized in four directories:
The sources in each src subdirectory are compiled as a static library. When link with the sources in top src, they result into the binary gnuchess. The module libraries are named after their respective directory name:
libfrontend.a libadapter.a libengine.a
There is no problem in having both C and C++ in the same program. However, there is no technical reason for this heterogeneity. The only reason is the different origins of the code. It is desirable to get a more homogeneous program in the future.
PolyGlot and Fruit have a common origin, which means that there is a lot of shared code. The point is that the shared code is very similar but not identical, which would make hard an eventual unification. In order to avoid massive name clash, all the code has been wrapped in namespaces, one for the adapter, one for the engine. The frontend remains compiled as C code.
Fruit was designed to help with the study of game-tree search algorithms, when applied to chess. It is now released as a chess engine, which is a somewhat different category of programs. Therefore the source code contains entire files and also functions that are either not used by the engine, or could be replaced with a much simpler (although somewhat less efficient) equivalent.
As a chess engine, Fruit combines a "robust" search algorithm with a "minimalist" evaluation function. The latter is not a design choice, and will hopefully change in the future.
The following description is only a very incomplete description. Please consult the source code for an absolute definition.
The search algorithm was designed to accommodate with heavy forward-pruning eccentricities (such as search inconsistencies).
Fruit uses the 16x12 board. Although this structure is not very popular, it can be seen as simply combining 10x12 (mailbox) with 16x8 (0x88).
0x88 was picked in Fruit because of the small memory requirements of vector calculations (much smaller tables). It is possible that Fruit uses bitboards for pawns in the future.
The main search algorithm is a classical PVS with iterative deepening. Search enhancements such as a transposition table and null-move pruning are also used (see below).
A few details in the PVS implementation are not-so-standard and are there to supposedly enhance the stability of the search (like reducing the consequences of search inconsistencies). For example the re-search window after a scout fail high of score "value" (with value > alpha) is [alpha,beta], not [value,beta]. As another example, I only allow null move when the static evaluation fails high (i.e. eval() >= beta). Whether these features improve the strength of the engine is an open question.
The main search function is full_search() in search_full.cpp
Fruit uses 4 probes and replaces the shallowest entry. Time stamping is used so that entries from previous searches are considered available for overwriting.
Enhanced Transposition Cutoff (ETC) is also used 4 plies (and more) away from the horizon.
Fruit uses R=3 recursive null move, even in the endgame.
In Fruit, a precondition to using null move is that the static eval fails high. One of the consequences of this is that no two null moves can be played in a row (this is because the evaluation is symmetrical). This is a usual condition but notice that in Fruit the null-move condition is "pure" (independent of move paths). The fail-high condition was selected for other reasons however.
Also, a verification search is launched in the endgame.
The move ordering is rather basic:
The evaluation function includes:
Fruit is not fast (in nodes per second) given the little it is calculating. Some "optimisations" could be undone in order to make the code shorter and more flexible.
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If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
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If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
--easy: Invoking gnuchess
--help: Invoking gnuchess
--manual: Invoking gnuchess
--memory: Invoking gnuchess
--post: Invoking gnuchess
--quiet: Invoking gnuchess
--silent: Invoking gnuchess
--uci: Invoking gnuchess
--version: Invoking gnuchess
--xboard: Invoking gnuchess
-e: Invoking gnuchess
-h: Invoking gnuchess
-m: Invoking gnuchess
-M: Invoking gnuchess
-p: Invoking gnuchess
-q: Invoking gnuchess
-u: Invoking gnuchess
-v: Invoking gnuchess
-x: Invoking gnuchess