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distcc - distributed C/C++/ObjC compiler with distcc-pump extensions


distcc <compiler> [COMPILER OPTIONS]





distcc distributes compilation of C code across several machines on a network. distcc
should always generate the same results as a local compile, it is simple to install and
use, and it is often much faster than a local compile.

This version incorporates plain distcc as well as an enhancement called pump mode or

For each job, distcc in plain mode sends the complete preprocessed source code and
compiler arguments across the network from the client to a compilation server. In pump
mode, distcc sends the source code and recursively included header files (excluding those
from the default system header directories), so that both preprocessing and compilation
can take place on the compilation servers. This speeds up the delivery of compilations by
up to an order of magnitude over plain distcc.

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 (if
distcc's pump mode is not used), the linker, and other stages of the build process. Any
number of volunteer machines act as compilation servers and help the client to build the
program, by running the 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
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 -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. As a rule of thumb, the -j value
should be set to about twice the total number of available server CPUs but subject to
client limitations. This setting allows for maximal interleaving of tasks being blocked
waiting for disk or network IO. Note that distcc can also work with other build control
tools, such as SCons, where similar concurrency settings must be adjusted.

The -j setting, especially for large values of -j, must take into account the CPU load on
the client. Additional measures may be needed to curtail the client load. For example,
concurrent linking should be severely curtailed using auxiliary locks. The effect of
other build activity, such as Java compilation when building mixed code, should be
considered. The --localslots_cpp parameter is by default set to 16. This limits the
number of concurrent processes that do preprocessing in plain distcc (non-pump) mode.
Therefore, larger -j values than 16 may be used without overloading a single-CPU client
due to preprocessing. Such large values may speed up parts of the build that do not
involve C compilations, but they may not be useful to distcc efficiency in plain mode.

In contrast, using pump mode and say 40 servers, a setting of -j80 or larger may be
appropriate even for single-CPU clients.

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


1 For each machine, download distcc, unpack, and install.

2 On each of the servers, run distccd --daemon with --allow options to restrict

3 Put the names of the servers in your environment:
$ export DISTCC_HOSTS='localhost red green blue'

4 Build!
$ make -j8 CC=distcc


Proceed as above, but in Step 3, specify that the remote hosts are to carry the burden of
preprocessing and that the files sent over the network should be compressed:

$ export DISTCC_HOSTS='--randomize localhost red,cpp,lzo green,cpp,lzo

The --randomize option enforces a uniform usage of compile servers. While you will get
some benefit from distcc's pump mode with only a few servers, you get increasing benefit
with more server CPUs (up to the hundreds!). Wrap your build inside the pump command,
here assuming 10 servers:

$ distcc-pump make -j20 CC=distcc


distcc only ever runs the compiler and assembler remotely. With plain distcc, 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 distributed.


In pump mode, distcc runs the preprocessor remotely too. To do so, the preprocessor must
have access to all the files that it would have accessed if had been running locally. In
pump mode, therefore, distcc gathers all of the recursively included headers, except the
ones that are default system headers, and sends them along with the source file to the
compilation server.

In distcc-pump mode, the server unpacks the set of all source files in a temporary
directory, which contains a directory tree that mirrors the part of the file system that
is relevant to preprocessing, including symbolic links.

The compiler is then run from the path in the temporary directory that corresponds to the
current working directory on the client. To find and transmit the many hundreds of files
that are often part of a single compilation, pump mode uses an incremental include
analysis algorithm. The include server is a Python program that implements this
algorithm. The distcc-pump command starts the include server so that throughout the build
it can answer include queries by distcc commands.

The include server uses static analysis of the macro language to deal with conditional
compilation and computed includes. It uses the property that when a given header file has
already been analyzed for includes, it is not necessary to do so again if all the include
options (-I's) are unchanged (along with other conditions).

For large builds, header files are included, on average, hundreds of times each. With
distcc-pump mode each such file is analyzed only a few times, perhaps just once, instead
of being preprocessed hundreds of times. Also, each source or header file is now
compressed only once, because the include server memoizes the compressed files. As a
result, the time used for preparing compilations may drop by up to an order of magnitude
over the preprocessing of plain distcc.

Because distcc in pump mode is able to push out files up to about ten times faster, build
speed may increase 3X or more for large builds compared to plain distcc mode.


Using pump mode requires both client and servers to use release 3.0 or later of distcc and
distccd (respectively).

The incremental include analysis of distc-pump mode rests on the fundamental assumption
that source and header files do not change during the build process. A few complex build
systems, such as that for Linux kernel 2.6, do not quite satisfy this requirement. To
overcome such issues, and other corner cases such as absolute filepaths in includes, see
the include_server(1) man page.

Another important assumption is that the include configuration of all machines must be
identical. Thus the headers under the default system path must be the same on all servers
and all clients. If a standard GNU compiler installation is used, then this requirement
applies to all libraries whose header files are installed under /usr/include or
/usr/local/include/. Note that installing software packages often lead to additional
headers files being placed in subdirectories of either.

If this assumption does not hold, then it is possible to break builds with distcc-pump
mode, or worse, to get wrong results without warning. Presently this condition is not
verified, and it is on our TODO list to address this issue.

An easy way to guarantee that the include configurations are identical is to use a cross-
compiler that defines a default system search path restricted to directories of the
compiler installation.

See the include_server(1) manual for more information on symptoms and causes of violations
of distcc-pump mode assumptions.


Most options passed to distcc are interpreted as compiler options. The following options
are understood by distcc itself. If any of these options are specified, distcc will not
invoke the compiler.

--help Displays summary instructions.

Displays the distcc client version.

Displays the host list that distcc would use. See the Host Specifications section.

Displays the list of files that distcc would send to the remote machine, as
computed by the include server. This is a conservative (over-)approximation of the
files that would be read by the C compiler. This option only works in pump mode.
See the "How Distcc-pump Mode Works" section for details on how this is computed.

The list output by distcc --scan-includes will contain one entry per line. Each
line contains a category followed by a path. The category is one of FILE, SYMLINK,

FILE indicates a source file or header file that would be sent to the distcc server

SYMLINK indicates a symbolic link that would be sent to the distcc server host.

DIRECTORY indicates a directory that may be needed in order to compile the source
file. For example, a directory "foo" may be needed because of an include of the
form #include "foo/../bar.h". Such directories would be created on the distcc
server host.

SYSTEMDIR indicates a system include directory, i.e. a directory which is on the
compiler's default include path, such as "/usr/include"; such directories are
assumed to be present on the distcc server host, and so would not be sent to the
distcc server host.

-j Displays distcc's concurrency level, as calculated from the host list; it is the
maximum number of outstanding jobs issued by this client to all servers. By
default this will be four times the number of hosts in the host list, unless the
/LIMIT option was used in the host list. See the Host Specifications section.


There are three different ways to call distcc, to suit different circumstances:

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 directory 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

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.

Recursion errors can be avoided by using shell scripts instead of links. For example, in
/usr/lib/distcc/bin create a file cc which contains:

distcc /usr/bin/gcc "$@"

In this way, we are not dependent on distcc having to locate the real gcc by investigating
the PATH variable. Instead, the compiler location is explicitly provided.


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 upgrades.

ccache can then be run using either a masquerade directory or by setting

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.

distcc's pump mode is not compatible with ccache.


A "host list" tells distcc which machines to use for compilation. In order, distcc looks
in the $DISTCC_HOSTS environment variable, the user's $DISTCC_DIR/hosts file, and the
system-wide host file. If no host list can be found, distcc emits a warning and compiles

The host list is a simple whitespace separated list of host specifications. The simplest
and most common form is a host names, such as

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 (but see --randomize below).

Placing 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.

If you have a large shared build cluster and a single shared hosts file, the above rules
would cause the first few machines in the hosts file to be tried first even though they
are likely to be busier than machines later in the list. To avoid this, place the keyword
--randomize into the host list. This will cause the host list to be randomized, which
should improve performance slightly for large build clusters.

There are two special host names --localslots and --localslots_cpp which are useful for
adjusting load on the local machine. The --localslots host specifies how many jobs that
cannot be run remotely that can be run concurrently on the local machine, while
--localslots_cpp controls how many preprocessors will run in parallel on the local
machine. Tuning these values can improve performance. Linking on large projects can take
large amounts of memory. Running parallel linkers, which cannot be executed remotely,
may force the machine to swap, which reduces performance over just running the jobs in
sequence without swapping. Getting the number of parallel preprocessors just right
allows you to use larger parallel factors with make, since the local machine now has some
machanism for measuring local resource usage.

Finally there is the host entry

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 performance.

The syntax is

LOCAL_HOST = localhost[/LIMIT]
| --localslots=<int>
| --localslots_cpp=<int>
OPTION = lzo | cpp
GLOBAL_OPTION = --randomize
ZEROCONF = +zeroconf

Here are some individual examples of the syntax:

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.)

IPV6 A literal IPv6 address enclosed in square brackets, such as [::1]

IPV4 A literal IPv4 address, such as

A hostname to be looked up using the resolver.

:PORT Connect to a specified decimal port number, rather than the default of 3632.

Connect to the host over SSH, rather than TCP. Options for the SSH connection can
be set in ~/.ssh/config

USER@ 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.

/LIMIT 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.

,lzo Enables LZO compression for this TCP or SSH host.

,cpp Enables distcc-pump mode for this host. Note: the build command must be wrapped in
the distcc-pump script in order to start the include server.

Randomize the order of the host list before execution.

This option is only available if distcc was compiled with Avahi support enabled at
configure time. When this special entry is present in the hosts list, distcc will
use Avahi Zeroconf DNS Service Discovery (DNS-SD) to locate any available distccd
servers on the local network. This avoids the need to explicitly list the host
names or IP addresses of the distcc server machines. The distccd servers must have
been started with the "--zeroconf" option to distccd. An important caveat is that
in the current implementation, pump mode (",cpp") and compression (",lzo") will
never be used for hosts located via zeroconf.

Here is an example demonstrating some possibilities:

localhost/2 @bigman/16:/opt/bin/distccd oldmachine:4200/1
# cartman is down

Comments are allowed in host specifications. Comments start with a hash/pound sign (#)
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.


The 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 added CPU time is insignificant for pump mode. 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.

Pump mode requires the servers to have the lzo host option on.


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:

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 DISTCC_VERBOSE environment variable on the client, and the --verbose
option on the server. For troubleshooting, examine both the client and server error


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

distcc internal errors cause an exit code between 100 and 127. In particular

100 General distcc failure.

101 Bad arguments.

102 Bind failed.

103 Connect failed.

104 Compiler crashed.

105 Out of memory.

106 Bad Host SPEC

107 I/O Error

108 Truncated.

109 Protocol Error.

110 The given compiler was not found on the remote host. Check that $CC is set
appropriately and that it's installed in a directory on the search path for

111 Recursive call to distcc.

112 Failed to discard privileges.

113 Network access denied.

114 In use by another process.

115 No such file.

116 No hosts defined and fallbacks disabled.

118 Timeout.

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