5.3. Structure of a Source Package

5.3.1. Format

A source package is usually comprised of three files, a .dsc, a .orig.tar.gz, and a .debian.tar.gz or .diff.gz. They allow creation of binary packages (.deb files described above) for the program(s) from their source code, written in a programming language.

The .dsc (Debian Source Control) file is a short text file containing an RFC 2822 header (just like the control file studied in Section 5.2.1, “Description: the control File”) which describes the source package and indicates which other files are part thereof. It is signed by its maintainer, which guarantees authenticity. See Section 6.5, “Checking Package Authenticity” for further details on this subject.
Example 5.1. A .dsc file
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Format: 3.0 (quilt)
Source: zim
Binary: zim
Architecture: all
Version: 0.48-1
Maintainer: Emfox Zhou <emfox@debian.org>
Uploaders: Raphaël Hertzog <hertzog@debian.org>
Homepage: http://zim-wiki.org
Standards-Version: 3.9.0
Vcs-Browser: http://svn.debian.org/wsvn/collab-maint/deb-maint/zim/trunk?op=log
Vcs-Svn: svn://svn.debian.org/collab-maint/deb-maint/zim/trunk
Build-Depends: debhelper (>= 7.4.12), python-support (>= 0.8), xdg-utils, python (>= 2.5), libgtk2.0-0 (>= 2.6), python-gtk2, python-xdg, python-simplejson | python (>= 2.6)
Checksums-Sha1: 
 bd84fa5104de5ed85a49723d26b350856de93217 966899 zim_0.48.orig.tar.gz
 352111ff372a20579664416c9abd4970839835b3 9615 zim_0.48-1.debian.tar.gz
Checksums-Sha256: 
 77d8df7dc89b233fdc3aab1a8ad959c6888881ae160770f50bf880a56e02f895 966899 zim_0.48.orig.tar.gz
 0fceab5d3b099075cd38c225fa4002d893c1cdf4bbcc51d1391a34248e1e1a22 9615 zim_0.48-1.debian.tar.gz
Files: 
 88cfc18c0c7339528d5f5f463647bb5f 966899 zim_0.48.orig.tar.gz
 608b6e74aa14252dfc6236ab184bdb0c 9615 zim_0.48-1.debian.tar.gz

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)
Comment: Signed by Raphael Hertzog

iQEcBAEBCAAGBQJMSUAfAAoJEAOIHavrwpq5qjUIAKmM8p86GcHYTxMmKENoBUoW
UPi5R7DzrLMbFrUXKgXWLvEKQTXpmkJhh2aSWq2iY+5piBSHwMiITfaBTpdTRvzU
5nT/n9MlF8sJFESet/NgZaMPFDzWUbIy5aYbuG1TXmn/7XiDrBaQGiVqKkVLPrqc
yWhsotn3JNKIjbPDW/DjImYyKD5RZpXrbVjuIgDT1E6yxtNYwUyBlK0cx/GITNep
uV48hsT8cj0paqVXl5+P9Ww8XIE3clxNpE/45/tvKvkqGOeysc6OPAqsIw6HYFY9
0EnvMTfMpeQOA68ZqsNpUjomv5r/EGwdCbAWo5iJDsZzXQ1Feh6iSNrjv3yeRzg=
=qnbh
-----END PGP SIGNATURE-----

Note that the source package also has dependencies (Build-Depends) completely distinct from those of binary packages, since they indicate tools required to compile the software in question and construct its binary package.
CAUTION Distinct namespaces

It is important to note here that there is no strong correspondence between the name of the source package and that of the binary packages that they generate. It is easy enough to understand if you know that each source package may generate several binary packages. This is why the .dsc file has fields for Source and Binary to explicitly name the source package and store the list of binary packages that it generates.
CULTURE Why divide into several packages

Quite frequently, a source package (for a certain group of programs) can generate several binary packages. The reasons are multiple: a program may often be used in different contexts, so a shared library may be installed to make an application work (for example, libc6), or when it can be installed to develop a new program (libc6-dev will then be the correct package). We find the same logic for client/server services where we want to install the server part on one machine and the client part on others (this is the case, for example, of openssh-server and openssh-client).

Just as frequently, the documentation is provided in a dedicated package: the user may install it independently from the software, and may at any time choose to remove it to save disk space. Additionally, this also saves disk space on the Debian mirrors, since the documentation package will be shared amongst all of the architectures (instead of having the documentation duplicated in the packages for each architecture).
PERSPECTIVE Different source package formats

Originally there was only one source package format. This is the 1.0 format, which associates an .orig.tar.gz archive to a .diff.gz “debianization” patch (there is also a variant, consisting of a single .tar.gz archive, which is automatically used if no .orig.tar.gz is available).

Since Debian Squeeze, Debian developers have the option to use new formats that correct many problems of the historical format. Format 3.0 (quilt) can combine multiple upstream archives in the same source package: in addition to the usual .orig.tar.gz, supplementary .orig-component.tar.gz archives. This is useful with software that are distributed in several upstream components but for which a single source package is desired. These archives can also be compressed with bzip2 rather than gzip (lzma and xz are supported by dpkg-source but not accepted into the official archive), which saves disk space and network resources. Finally, the monolithic patch, .diff.gz is replaced by a .debian.tar.gz archive containing the compiling instructions and a set of upstream patches contributed by the package maintainer. These last are recorded in a format compatible with quilt, a tool that facilitates the management of a series of patches.

The .orig.tar.gz is an archive containing the program source code as provided by the original developer. Developers are asked to not modify this archive in order to be able to easily check the source and integrity of the file (by simple comparison with a checksum) and to respect the wishes of some authors.

The .debian.tar.gz contains all of the modifications made by the Debian maintainer, especially the addition of a debian directory containing instructions to execute to construct a Debian package.
TOOL Decompressing a source package

If you have a source package, you can use the dpkg-source (from the dpkg-dev package) to decompress it: 
$ dpkg-source -x package_0.7-1.dsc

You can also use apt-get to download a source package and unpack it right away. It requires that the appropriate deb-src lines be present in the /etc/apt/sources.list file, however (for further details, see Section 6.1, “Filling in the sources.list File”). These are used to list the “sources” of source packages (meaning the servers on which a group of source packages are hosted). 
$ apt-get source package

5.3.2. Usage within Debian

The source package is the foundation of everything in Debian. All Debian packages come from a source package, and each modification in a Debian package is the consequence of a modification made to the source package. The Debian maintainers work with the source package, knowing, however, the consequences of their actions on the binary packages. The fruits of their labors are found, thus, in the source packages available from Debian: you can easily go back and follow everything.

When a new version of a package (source package and one or more binary packages) arrives on the Debian server, the source package is the most important. Indeed, it will then be used by a network of machines of different architectures for compilation of the various architectures supported by Debian. The fact that the developer also sends one or more binary packages for a given architecture (usually i386 or amd64) is relatively unimportant, since these could just as well have been automatically generated.