distcc.1   [plain text]

.TH distcc 1 "28 July 2004"
distcc \- distributed C/C++/ObjC compiler
.B distcc 
.I <compiler> [COMPILER OPTIONS] 
.B distcc
.B <compiler>
distcc distributes compilation of C code across several machines on a
network.  distcc should always generate the same results as a local
compile, is simple to install and use, and is often much faster than a
local compile.
distcc sends the complete preprocessed source code and compiler
arguments across the network for each job, so the machines do not need
to share a filesystem, have the same headers or libraries installed,
or have synchronized clocks.
Compilation is driven by a "client" machine, which is typically the
developer's workstation or laptop.  The distcc client runs on this
machine, as does make, the preprocessor, the linker, and other stages
of the build process.  Any number of "volunteer" machines help the
client to build the program, by running the 
.B distccd(1) 
daemon, C compiler and assembler
as required.
distcc can run across either TCP sockets (on port 3632 by default), or
through a tunnel command such as ssh(1).  For TCP connections the
volunteers must run the distccd(1) daemon either directly or from inetd.
For SSH connections distccd must be installed but should
.B not
be listening for connections.  
TCP connections should only be used on secure networks because there
is no user authentication or protection of source or object code.  SSH
connections are typically 25% slower because of processor overhead for
encryption, although this can vary greatly depending on CPUs, network
and the program being built.
distcc is intended to be used with GNU Make's
.B -j 
option, which runs several compiler processes concurrently.  distcc
spreads the jobs across both local and remote CPUs.  Because distcc is
able to distribute most of the work across the network a higher
concurrency level can be used than for local builds.  The
.B -j 
value should normally be set to about twice the total number of
available CPUs, to allow for some tasks being blocked waiting for disk
or network IO.  distcc can also work with other build control tools
such as SCons.
It is strongly recommended that you install the same compiler version
on all machines participating in a build.  Incompatible compilers may
cause mysterious compile or link failures.
For each machine, download distcc, unpack, and install.
On each of the servers, run 
.B distccd --daemon
optionally with 
.B --allow
options to restrict access.
Put the names of the servers in your environment:
$ export DISTCC_HOSTS='localhost red green blue'
$ make -j8 CC=distcc
distcc only ever runs the compiler and assembler remotely.
The preprocessor must always run locally because it needs to
access various header files on the local machine which may
not be present, or may not be the same, on the volunteer.
The linker similarly needs to examine libraries and object
files, and so must run locally.
The compiler and assembler take only a single input file (the
preprocessed source) and produce a single output (the object file).
distcc ships these two files across the network and can therefore run
the compiler/assembler remotely.
Fortunately, for most programs running the preprocessor is
relatively cheap, and the linker is called relatively
infrequent, so most of the work can be distributed.
distcc examines its command line to determine which of these
phases are being invoked, and whether the job can be
Most options passed to distcc are interpreted as compiler options.
Two options are understood by distcc itself:
.B --help
Displays summary instructions.
.B --version
Displays the distcc client version.
There are three different ways to call distcc, to suit different
distcc can be installed under the name of the real compiler, to
intercept calls to it and run them remotely.  This "masqueraded"
compiler has the widest compatibility with existing source trees, and
is convenient when you want to use distcc for all compilation.  The
fact that distcc is being used is transparent to the makefiles.
distcc can be prepended to compiler command lines, such as "distcc cc
-c hello.c" or CC="distcc gcc".  This is convenient when you want to
use distcc for only some compilations or to try it out, but can cause
trouble with some makefiles or versions of libtool that assume $CC
does not contain a space.
Finally, distcc can be used directly as a compiler.  "cc" is always
used as the name of the real compiler in this "implicit" mode.  This
can be convenient for interactive use when "explicit" mode does not
work but is not really recommended for new use.
Remember that you should not use two methods for calling distcc at the
same time.  If you are using a masquerade directory, don't change CC and/or
CXX, just put the dirirectory early on your PATH.  If you're not using
a masquerade directory, you'll need to either change CC and/or CXX, or
modify the makefile(s) to call distcc explicitly.
The basic idea is to create a "masquerade directory" which contains
links from the name of the real compiler to the distcc binary.  This
directory is inserted early on the PATH, so that calls to the compiler
are intercepted and distcc is run instead.  distcc then removes itself
from the PATH to find the real compiler.
For example:
# mkdir /usr/lib/distcc/bin
# cd /usr/lib/distcc/bin
# ln -s ../../../bin/distcc gcc
# ln -s ../../../bin/distcc cc
# ln -s ../../../bin/distcc g++
# ln -s ../../../bin/distcc c++
Then, to use distcc, a user just needs to put the directory
/usr/lib/distcc/bin early in the PATH, and have set a host list in
DISTCC_HOSTS or a file.  distcc will handle the rest.  
Note that this masquerade directory must occur on the PATH earlier
than the directory that contains the actual compilers of the same
names, and that any auxiliary programs that these compilers call (such
as as or ld) must also be found on the PATH in a directory after the
masquerade directory since distcc calls out to the real compiler with
a PATH value that has all directory up to and including the masquerade
directory trimmed off.
It is possible to get a "recursion error" in masquerade mode, which
means that distcc is somehow finding itself again, not the real
compiler.  This can indicate that you have two masquerade directories
on the PATH, possibly because of having two distcc installations in
different locations.  It can also indicate that you're trying to mix
"masqueraded" and "explicit" operation.
ccache is a program that speeds software builds by caching the results
of compilations.  ccache is normally called before distcc, so that
results are retrieved from a normal cache.  Some experimentation may
be required for idiosyncratic makefiles to make everything work together.
The most reliable method is to set
This tells ccache to run distcc as a wrapper around the real
compiler.  ccache still uses the real compiler to detect compiler
ccache  can then be run using either a masquerade directory 
.I or 
.B CC="ccache gcc"
As of version 2.2, ccache does not cache compilation from preprocessed
source and so will never get a cache hit if it is run from distccd or
distcc.  It must be run only on the client side and before distcc to
be any use.
A "host list" tells distcc which machines to use for compilation.  In
order, distcc looks in the 
environment variable, the user's 
.B $DISTCC_DIR/hosts
file, and the system-wide host 
file.  If no host list can be found, distcc emits a warning and
compiles locally.
The host list is a simple whitespace separated list of host
specifications.  The simplest and most common form is a host names,
such as
.B localhost red green blue
distcc prefers hosts towards the start of the list, so machines should
be listed in descending order of speed.  In particular, when only a
single compilation can be run (such as from a configure script), the
first machine listed is used.  
.I localhost
at the right point in the list is important to getting good
performance.  Because overhead for running jobs locally is low,
localhost should normally be first.  However, it is important that the
client have enough cycles free to run the local jobs and the distcc
client.  If the client is slower than the volunteers, or if there are
many volunteers, then the client should be put later in the list or
not at all.  As a general rule, if the aggregate CPU speed of the
client is less than one fifth of the total, then the client should be
left out of the list.
Performance depends on the details of the source and makefiles used
for the project, and the machine and network speeds.  Experimenting
with different settings for the host list and -j factor may improve
The syntax is
  LOCAL_HOST = localhost[/LIMIT]
  OPTION = lzo
Here are some individual examples of the syntax:
.B localhost
The literal word "localhost" is interpreted specially to cause
compilations to be directly executed, rather than passed to a daemon
on the local machine.  If you do want to connect to a daemon on the
local machine for testing, then give the machine's IP address or real
hostname.  (This will be slower.)
A literal IPv4 address, such as 
A hostname to be looked up using the resolver.
Connect to a specified decimal port number, rather than the default of
Connect to the host over SSH, rather than TCP.  Options for the SSH
connection can be set in 
.B ~/.ssh/config
Connect to the host over SSH as a specified username.
Connect over SSH, and use a specified path to find the distccd
server.  This is normally only needed if for some reason you can't
install distccd into a directory on the default PATH for SSH
connections.  Use this if you get errors like "distccd: command not
found" in SSH mode.     
A decimal limit can be added to any host specification to restrict the
number of jobs that this client will send to the machine.  The limit
defaults to four per host (two for localhost), but may be further
restricted by the server.  You should only need to increase this for
servers with more than two processors.
.B ,lzo
Enables LZO compression for this TCP or SSH host.
Here is an example demonstrating some possibilities:
.B localhost/2 @bigman/16:/opt/bin/distccd oldmachine:4200/1
.B  # cartman is down
.B distant/3,lzo
Comments are allowed in host specifications.  Comments start with a
hash/pound sign (\fB#\fP) and run to the end of the line.
If a host in the list is not reachable distcc will emit a warning and
ignore that host for about one minute.
.B lzo 
host option specifies that LZO compression should be used for data
transfer, including preprocessed source, object code and error
messages.  Compression is usually economical on networks slower than
100Mbps, but results may vary depending on the network, processors and
source tree.
Enabling compression makes the distcc client and server use more CPU
time, but less network traffic.  The compression ratio is typically
4:1 for source and 2:1 for object code.
Using compression requires both client and server to use at least
release 2.9 of distcc.  No server configuration is required: the
server always responds with compressed replies to compressed requests.
If the compiler name is an absolute path, it is passed verbatim to the
server and the compiler is run from that directory.  For example:
.B distcc /usr/local/bin/gcc-3.1415 -c hello.c
If the compiler name is not absolute, or not fully qualified,
distccd's PATH is searched.  When distcc is run from a masquerade
directory, only the base name of the compiler is used.  The client's
PATH is used only to run the preprocessor and has no effect on the
server's path.
Both the distcc client and server impose timeouts on transfer of data
across the network.  This is intended to detect hosts which are down
or unreachable, and to prevent compiles hanging indefinitely if a
server is disconnected while in use.  If a client-side timeout
expires, the job will be re-run locally.
The timeouts are not configurable at present.
Error messages or warnings from local or remote compilers are passed
through to diagnostic output on the client.
distcc can supply extensive debugging information when the verbose
option is used.  This is controlled by the 
environment variable on the client, and the
.B --verbose
option on the server.  For troubleshooting, examine both the client
and server error messages.
The exit code of distcc is normally that of the compiler:
zero for successful compilation and non-zero otherwise.
distcc distinguishes between "genuine" errors such as a syntax error
in the source, and "accidental" errors such as a networking problem
connecting to a volunteer.  In the case of accidental errors, distcc
will retry the compilation locally unless the DISTCC_FALLBACK option
has been disabled.
If the compiler exits with a signal, distcc returns an exit code of
128 plus the signal number.
distcc internal errors cause an exit code between 100 and 127.  In
General distcc failure.
Out of memory.
Compiler not found.
Recursive call to distcc.
No hosts defined and fallbacks disabled.
(Others are listed in exitcode.h.)
If $DISTCC_HOSTS is not set, distcc reads a host list from either 
.B $DISTCC_DIR/hosts
or a system-wide configuration file set at compile time.  The file
locations are shown in the output from
.B distcc --help
distcc creates a number of temporary and lock files underneath the
temporary directory.
distcc's behaviour is controlled by a number of environment variables.
For most cases nothing need be set if the host list is stored in a
Space-separated list of volunteer host specifications.
If set to 1, distcc produces explanatory messages on the standard
error stream or in the log file.  This can be helpful in debugging
problems.  Bug reports should include verbose output.
Log file to receive messages from distcc itself, rather
than stderr.
By default distcc will compile locally if it fails to distribute a job
to the intended machine, or if no host list can be found.  If this
variable is set to 0 then fallbacks are disabled and those
compilations will simply fail.  Note that this does not affect jobs
which must always be local such as linking.
If set to 1, temporary files are not deleted after use.  Good for
debugging, or if your disks are too empty.
If set to 0, disable use of "TCP corks", even if they're present on
this system.  Using corks normally helps pack requests into fewer
packets and aids performance.  This should normally be left enabled.
Specifies the command used for opening SSH connections.  Defaults to
"ssh" but may be set to a different connection command such as "lsh"
or "tsocks-ssh" that accepts a similar command line.  The command is
not split into words and is not executed through the shell. 
Per-user configuration directory to store lock files and state files.
By default 
.B ~/.distcc/ 
is used.
Directory for temporary files such as preprocessor output.  By default
/tmp/ is used.
If set and if DISTCC_LOG is not set, distcc errors are written to the
file descriptor identified by this variable.  This variable is
intended mainly for automatic use by ccache, which sets it to avoid
caching transient errors such as network problems.
Cross compilation means building programs to run on a
machine with a different processor, architecture, or
operating system to where they were compiled.  distcc
supports cross compilation, including teams of
mixed-architecture machines, although some changes to the
compilation commands may be required.
The compilation command passed to distcc must be one that
will execute properly on every volunteer machine to produce
an object file of the appropriate type.  If the machines
have different processors, then simply using 
.B distcc cc
will probably not work, because that will normally invoke the
volunteer's native compiler.
Machines with the same CPU but different operating systems may not
necessarily generate compatible .o files.
Several different gcc configurations can be installed
side-by-side on any machine.  If you build gcc from source,
you should use the 
.B --program-suffix configuration
options to cause it to be installed with a name that encodes
the gcc version and the target platform.
The recommended convention for the gcc name is
such as
.B i686-linux-gcc-3.2
\&.  GCC 3.3 will install itself
under this name, in addition to 
and, if it's native, 
.I gcc
The compiler must be installed under the same name on the
client and on every volunteer machine.
If you think you have found a  distcc bug, please see the file
.I reporting-bugs.txt
in the documentation directory for information on how to report it.
Some makefiles have missing or extra dependencies that cause incorrect
or slow parallel builds.  Recursive make is inefficient and can leave
processors unnecessarily idle for long periods.  (See
.I Recursive Make Considered Harmful
by Peter Miller.)  Makefile bugs are the most common cause of trees
failing to build under distcc.  Alternatives to Make such as
.I SCons
can give much faster builds for some projects.
Using different versions of gcc can cause confusing build problems
because the header files and binary interfaces have changed over time,
and some distributors have included incompatible patches without
changing the version number.  distcc does not protect against using
incompatible versions.  Compiler errors about link problems or
declarations in system header files are usually due to mismatched or
incorrectly installed compilers.
Due to limitations in gcc, gdb may not be able to automatically find
the source files for programs built using distcc in some
circumstances.  The gdb 
.B directory
command can be used.  This should be fixed in gcc 3.4.
.B -MD 
option can produce output in the wrong directory if the source and
object files are in different directories and the 
.B -MF 
option is not used.  There is no perfect solution because of
incompatible changes between gcc versions.  Explicitly specifying the
dependency output file with 
.B -MF
will fix the problem.
TCP mode connections should only be used on trusted networks.
Including slow machines in the list of volunteer hosts can slow the
build down.
When distcc or ccache is used on NFS, the filesystem must be exported
with the
.B no_subtree_check 
option to allow reliable renames between directories.
The compiler can be invoked with a command line
.B gcc hello.c
to both compile and link.  distcc doesn't split this into separate
parts, but rather runs the whole thing locally.
Other known bugs may be documented on 
.I http://distcc.samba.org/
distcc was written by Martin Pool <mbp@sourcefrog.net>, with the
co-operation of many scholars including Wayne Davison, Frerich Raabe,
Dimitri Papadopoulos and others noted in the NEWS file.  Please report
bugs to <distcc@lists.samba.org>.
You are free to use distcc.  distcc (including this manual) may be
copied, modified or distributed only under the terms of the GNU
General Public Licence version 2 or later.  distcc comes with
absolutely no warrany.  A copy of the GPL is included in the file
distccd(1), ccache(1), gcc(1), make(1)
.I http://distcc.samba.org/
.I http://ccache.samba.org/