Developer FAQ

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Mercurial repository layout for developers

  1. In your home dir on, you will find a new directory named "repositories/username". e.g.: /home/tomh/repositories/tomh. If you create a repository in this directory, it will appear automatically on
    • Note: To enable this for new users, edit /var/www/cgi-hg/hgweb.config
  2. You can obviously ssh to your personal account and manage these repositories
  3. You can push to these repositories with the command: hg push ssh://
  4. You can pull with the usual commands: hg clone
  5. If you want to allow another user to push into your repository, all you have to do is change the unix permissions of your repository to allow this user write permissions. This means that if you want to give everyone write permissions, you can "chmod -R g +rw /home/tomh/repositories/tomh/ns-3-com/". If you want to allow only a smaller subset of users to push, we will need to create unix group which matches this subset
  6. The push command for the main tree is still: hg push ssh://
  7. New developers, please read this: When creating a new repository, do not "hg clone" it into your directory on (which will generate a big ns-commits mail message); instead, just copy (cp -r) or rsync it to your local "/home/username/repositories/username" directory.

Mercurial tips

  1. How to undo a commit: Let's suppose you are working on a private repository and you check something in, but some other files were inadvertently checked in, and you want to revert and start over. There are two ways to do this:
    1. hg revert: This can be used to revert the repository to a previous revision number. For example, to revert to changeset number #1000, type hg revert -r 1000 --all. This does not remove your checkin from the repository history. For example, if your mistaken checkin was number 1001, and you revert back to 1000 and then commit, you will be at changeset number 1002 now even though the code matches what was in there at changeset 1000.
    2. hg rollback: This can be used to completely wipe clean the last transaction only (commit, import, push, pull). Use with care-- cannot be undone.
  2. How to rename a file: hg rename old-file-name new-file-name This is preferable to adding the new file name and removing the old file name, because it preserves revision history. Don't forget to commit once you are done.
  3. How to merge a branch: If you have forked a branch repository, have worked on it, and are ready to merge it back to ns-3-dev, here are the steps to take (Also, read this chapter to better understand how the mercurial source tree is structured when merging is occurring):
    1. cd into your branch
    2. hg pull
    3. hg merge
    4. resolve all of the merge issues, if any, and confirm that it builds and validates
    5. hg ci -m"merge your-branch-name with tip"
    6. hg push ssh://
  4. How to create patches for circulation: hg export tip Suppose you don't have write access to the main repositories, but have some patches you'd like to circulate. This command outputs out all the diffs for the latest changeset you committed into your local repository. Simply redirect this output to a file, and you can circulate your patch for consideration. This is for when you have committed changesets. If you would like to export uncommitted changes as a patch, use: hg diff This gets the diffs of all the uncommitted changes versus what is checked into the repository. Redirect to a file for circulation among developers, or for inclusion with a bug report, etc.

The WAF build system

See also the Waf User FAQ.

Obtaining WAF

A snapshot of WAF is distributed with ns-3 releases and mercurial branches. This snapshopt has been tested to work correctly with ns-3, although the trunk version from the main WAF repository usually works equally well.

Documentation resources

There is some incomplete documentation near the bottom of the WAF home page. Some useful tips can be found in the WAF Wiki. Finally, there is a plethora of examples distributed in WAF itself, in the 'demos' directory.

How to add new ns-3 modules

Ns-3 is organized as a set of modules. Each module is built as a set of object files, has a name, may depend on other modules, and installs a specific set of public header files.

To add a new ns-3 module to the WAF build system, begin by creating a directory under the src/ subtree, with the source files inside. We will use p2p module as example here. Each module needs to define a wscript file. For instance let us see what src/devices/point-to-point/wscript contains:

## -*- Mode: python; py-indent-offset: 4; indent-tabs-mode: nil; coding: utf-8; -*-

def build(bld):
    module = bld.create_ns3_module('point-to-point', ['node'])
    module.source = [
    headers = bld.create_obj('ns3header')
    headers.module = 'point-to-point'
    headers.source = [

A wscript file is basically a special python module. The main entry point to this module is the build(bld) python function.

In the code above, the module variable represents a ns-3 module; internally it is a WAF 'objects' build object that will be linked to be come part of the ns3 library. It is created by calling a special method bld.create_ns3_module, whose first parameter is the name of the module, and the second parameter is a list of other modules that this module depends on. Additionally, module.sources has to be set to the list of source files (excluding header files) that constitute the module. Warning: beware that the name of the module must match the name of the directory where it is built. In this case, the module is in 'src/devices/point-to-point', so the module name must be 'point-to-point'.

There is usually also a headers object. It is used to declare public header files. These files are copied to the build directory, and can be included from any module or program with #include "ns3/header-name.h".

A final step, after the wscript file is created, is to register it. Open the file src/wscript and add the new module to the all_modules list variable:

all_modules = (
    'devices/point-to-point', # <---- example here

Adding programs

Use the special method bld.create_ns3_program(name, [...dependencies...]). Example:

    obj = bld.create_ns3_program('main-simple',
                                 ['node', 'internet-node', 'applications'])
    obj.source = ''

Generating new python bindings

See the ns-3 python wiki page.

Submitting patches for consideration

When you send a tree without a detailed summary of your changes, it would help if you could send a list of the changesets you want to merge. To generate it, first merge with ns-3-dev and then, from your modified directory, run "hg outgoing -p"

Also, avoid spurious coding style and whitespace changes when preparing such a patch, as it distracts from the readability of your proposed technical changes.

If you use VIM you should add the following lines to your ~/.vimrc:

set ts=2
set sw=2
set sta
set et
set ai
set si
set cin

And the following lines to realize white space errors (trailing white spaces):

let c_no_bracket_error=1
let c_no_curly_error=1
let c_comment_strings=1
let c_gnu=

Checking in code

just before you do a checkin you should run the regression tests. Change into the top level directory and type

 ./waf --regression

If you do this, you will see a bunch of passing tests and then, if something goes wrong:

 Traces differ in test: test-tcp-large-transfer
 Reference traces in directory: regression/ns-3-dev-ref-traces/tcp-large-transfer.ref
 Traces in directory: traces
 Rerun regression test as: "./waf --regression --regression-tests=test-tcp-large-transfer"
 Then do "diff -u regression/ns-3-dev-ref-traces/tcp-large-transfer.ref regression/traces" for details
 FAIL test-tcp-large-transfer

Now, this "failure" is most likely due to another developer checking in a change that altered the behavior of an underlying model. You can follow the directions in the failure message to see what has changed for ASCII trace files. If the problem is with pcap traces, it is usually convenient to "tcpdump -nn -tt -r trace-file-name" in order to translate the output to human-readable form before doing the diff.

If you yourself have made a change and broken the regression output, and you expect the changes, you'll need to update the reference traces for the new behavior. All you have to do is to run the individual regression test:

 ./waf --regression-generate --regression-tests=test-tcp-large-transfer

This will write the new trace bits into a directory in a subdirectory of regression/ns-3-dev-ref-traces (assuming you are in a repository with VERSION set to 3-dev) corresponding to the test name. You will need to verify that the new traces are what you expect to see given your change. If you don't expect to see any changes, you may have caused a real regression and you'll need to debug it before checking in.

Assuming that the new traces represent new expected behavior, you just need to push the new bits into the ns-3-dev-ref-traces repository.

 cd ns-3-dev-ref-traces
 hg push ssh://

Using gcov and lcov code coverage tools

Here is a brief howto for using the lcov front-end to gcc's code coverage tool gcov

You must have lcov installed on your machine to do this. e.g.

 sudo apt-get install lcov

or install from source.

Then, for instance, to see the coverage of the ns-3 unit tests, type:

 ./waf configure --enable-gcov
 ./waf --lcov-report
 ./waf check
 ./waf --lcov-report

You will find a file "index.html" in the directory build/debug-gcov/lcov-report/ that you can look at with your browser.

Note that the first time you type ./waf --lcov-report, it may complain that no .gcda files were found. Just ignore this and run the program you want to test coverage of.

The preferred way to create a private repository

Let's say that developer "alice" wants to create a new repository "ns-3-dev-new-feature" that will exist on the code server as alice/ns-3-dev-new-feature. Suppose she wants to fork from the tip of ns-3-dev.

 cd /home/alice/repositories/alice
 cp -r /home/code/repos/ns-3-dev ns-3-dev-new-feature
 cd ns-3-dev-new-feature/.hg

At this point, edit the "hgrc" file to provide contact/description information:

 default =
 description = alice's new feature
 contact = <>

Note: A common minor problem is if you do an "hg clone" into the new directory instead of a "cp -r", there will be a huge ns-commits mail message generated. This is why "cp -r" is preferred way to do this.

Testing code on hosts

Some times a compilation error can only reproduced in certain architectures. has some machines that can be accessed remotely for testing purposes, if you have an appropriate account.

Ubuntu x86_64


Mac OS X PowerPC

sudo -u nsnam bash
<enter your password>
ssh darwin-ppc