summaryrefslogtreecommitdiffstats
path: root/doc/pkg/basic-expat.mdwn
blob: e2a1d44570af32a784a371b973944ec6ef9a730e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
[[!meta title="Basic Packaging Tutorial"]]

In this introduction to software packaging, we will package the Expat XML parser
library.  This is a pretty simple but complete package, consisting of a shared
library and its development files plus an executable utility and some
documentation.

We will build a package in [Source Package Format 2.0][spf] (SPF 2.0) with the
assistance of [[opkhelper_3.0.x|dev/opkhelper/]].

This tutorial assumes some knowledge of the UNIX shell command language and
utilities (see the "Shell and Utilities" volume of [POSIX.1-2008][posix]) and at
least basic familiarity with [makefile syntax][posix-makefile].

This tutorial presents one possible packaging workflow that seems to work well.
There is no mandatory workflow to packaging.  The only requirements are those
made by the source package format and any build helper utilities that are used.

[opkbuild]: http://git.proteanos.com/opkbuild/opkbuild.git/
[spf]: http://specs.proteanos.com/spf-2.0/
[opkhelper]: http://git.proteanos.com/opkhelper/opkhelper.git/
[posix]: http://pubs.opengroup.org/onlinepubs/9699919799/
[posix-makefile]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/make.html#tag_20_76_13


Getting Started
===============

Source Package Directory
------------------------

First, make a *source package directory*.  This is the directory that will
contain all of our source package files.  SPF 2.0 makes no requirements on the
name of this directory, but using the name of the source package is recommended.

    $ mkdir expat
    $ cd expat

We need [a file called `format`][spf-format] to identify the format of our
source package.  For SPF 2.0, it should simply contain the string `2.0`.

    $ echo '2.0' >format

[spf-format]: http://specs.proteanos.com/spf-2.0/overview.html#files-format

Upstream Source Archive
-----------------------

Obviously we need the source code of the software to be packaged.  Go to
[Expat's Web site][expat], find the expat 2.1.0 archive, and download it into
the source package directory.

    $ wget 'http://downloads.sourceforge.net/project/expat/expat/2.1.0/expat-2.1.0.tar.gz'

SPF 2.0 requires that an [upstream source archive][spf-upstream-source] be named
`<pkgname>-<pkgver>.orig.tar<ext>`, where `<pkgname>` is the name of the source
package, `<pkgver>` is the upstream version of the source package, and `<ext>`
is an optional file extension to indicate compression.  So, rename the archive
accordingly.

    $ mv 'expat-2.1.0.tar.gz' 'expat-2.1.0.orig.tar.gz'

[expat]: http://expat.sourceforge.net/
[spf-upstream-source]: http://specs.proteanos.com/spf-2.0/overview.html#files-src-src-ver-tar-ext


Source Package Metadata
=======================

Now we need some metadata for our source package.

Control File
------------

First we'll make a `control` file.  The format of this file is not yet
documented in the SPF 2.0 specification, but it is documented [in the Debian
Policy Manual][dpm-control].  The [source package fields][spf-fields-src] are
`Maintainer` (required), `Build-Depends` (optional), and `Homepage` (optional).
We'll fill in the fields whose values we know right now: `Maintainer` and
`Homepage`.

`Maintainer` is the name and e-mail address of the person or team responsible
for the package (i.e. usually you when you are making a package).  The value
must follow the syntax of the `mailbox` symbol in [RFC 5322 section
3.4][rfc-5322-3.4].  That is, the value must be of the form `name <address>`.
If `name` contains any of the following characters, it must be in double quotes:

    ( ) < > [ ] : ; @ \ , .

`Homepage` is the URL of the Web site for the package, if such a site exists.

Our expat `control` file looks like this:

    Maintainer: "J. Random Hacker" <jrandom@example.com>
    Homepage: http://expat.sourceforge.net/

[dpm-control]: http://www.debian.org/doc/debian-policy/ch-controlfields.html#s-controlsyntax
[spf-fields-src]: http://specs.proteanos.com/spf-2.0/fields.html#fields-src
[rfc-5322-3.4]: https://tools.ietf.org/html/rfc5322#section-3.4

Change Log
----------

Now we'll make a `changelog` file.  The format of this file is documented [in
the SPF 2.0 specification][spf-changelog].  We're making version "2.1.0-1" of
the "expat" source package for the "trunk" distribution.  We can get the current
date and time in the RFC 5322 format using the **date**(1) command:

    $ LC_ALL='POSIX' date '+%a, %d %b %Y %H:%M:%S %z'

Our expat `changelog` file looks like this:

    expat (2.1.0-1) trunk
    
      * Initial release.
    
     -- "J. Random Hacker" <jrandom@example.com>  Sun, 18 Nov 2012 11:58:19 -0500

Be careful – the changelog format is quite strict and tends to confuse many new
package maintainers.  There are no spaces before the package name, version, and
distribution (called the "header"); at least two spaces before each line of
change details; one space before the line starting with `--` (called the
"trailer"); and two spaces between the maintainer and the date.

[spf-changelog]: http://specs.proteanos.com/spf-2.0/metadata.html#changelog


Building the Software
=====================

We can now write our `build` makefile to try to get the Expat software to build.
[The `build` makefile][spf-build] "directs the process of building and
installing data files to be provided by binary packages".

Looking Through the Source
--------------------------

With a "[no-op][no-op]" target in `build`, we can make **opkbuild**(1) prepare a
*[build work area][spf-work-area]* with the unpacked source code and stop.  This
target isn't required by SPF 2.0, but it seems to facilitate a nice workflow.
So begin writing `build` as follows:

    #!/usr/bin/make -f
    
    nop:
            @:

Note that, due to makefile syntax, the line after `nop:` must begin with a tab
character.  This line is called a "command line" in makefile syntax.  The [`:`
utility][posix-colon] is a "null utility" that returns an exit status of zero.
A command prefix of `@` tells **make**(1) to not write the command to standard
output before executing it.

The `build` makefile must be executable, so set its file mode:

    $ chmod 755 build

We can now make **opkbuild**(1) prepare our build work area.

    $ opkbuild -b -c -T nop

The options are explained in the help output of opkbuild, obtained by running
`opkbuild -h`.  The `-b` option tells **opkbuild**(1) to build only binary
packages (no source package).  The `-c` option tells it to not clean up the work
area after building packages.  The `-T` option specifies a target to be built
instead of the standard `build` and `install` targets.

Now look in `tmp/src/`, the location of the source code within the build work
area.

    $ ls tmp/src/

Look for some documentation file that might tell us how to build Expat.  This
kind of information is usually kept in a file called `INSTALL` or `README`.
Expat's `README` file says to run `./configure`, then `make` and `make install`.

Looking at `tmp/src/configure`, we see that it is "[g]enerated by GNU Autoconf
2.68 for expat 2.1.0".  The `tmp/src/README` file reports that the makefile
supports the use of either the `DESTDIR` or `INSTALL_ROOT` macro to install
Expat somewhere other than in the root of the filesystem.  So, we should be able
to use opkhelper's buildsystem utilities to automatically configure, build, and
install Expat for us.

Building
--------

So let's add a `build` target to our `build` makefile.  The makefile should now
look like this:

    #!/usr/bin/make -f
    
    nop:
            @:
    
    build:
            oh-autoconfigure
            oh-autobuild
            touch $@

Read the manual pages and/or source code of **oh-autoconfigure**(1) and
**oh-autobuild**(1) to learn more about what they do.

The `touch $@` command is recommended by SPF 2.0:

> The build target should create a file named build in the build work area to
> prevent configuration and compilation from being performed multiple times.

We can now build Expat.

    $ opkbuild -b -c -T build

[spf-build]: http://specs.proteanos.com/spf-2.0/buildsys.html#build
[no-op]: https://en.wiktionary.org/wiki/no-op
[spf-work-area]: http://specs.proteanos.com/spf-2.0/buildsys.html#work-area
[posix-colon]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#colon


Installing the Software
=======================

We can now finish our `build` makefile to install the Expat software and make
some binary packages.

Installing
----------

Add a basic `install` target to the `build` makefile.  The makefile should now
look like this:

    #!/usr/bin/make -f
    
    nop:
            @:
    
    build:
            oh-autoconfigure
            oh-autobuild
            touch $@
    
    install: build
            oh-autoinstall

The `install` target is declared as depending on the `build` target:

    install: build

Read the manual page and/or source code of **oh-autoinstall**(1) to learn more
about what it does.

Install Expat:

    $ opkbuild -b -c -T install

Splitting Files Into Binary Packages
------------------------------------

Look in the *installation destination directory* `tmp/dest/` for files installed
by Expat's build system.  This can be done with the **find**(1) command, which
results in the following when building for the `amd64-linux-glibc` architecture:

    $ find tmp/dest -exec ls -Fd '{}' ';' | sed 's|^tmp/dest||'
    /
    /usr/
    /usr/bin/
    /usr/bin/xmlwf*
    /usr/share/
    /usr/share/man/
    /usr/share/man/man1/
    /usr/share/man/man1/xmlwf.1
    /usr/lib/
    /usr/lib/amd64-linux-glibc/
    /usr/lib/amd64-linux-glibc/pkgconfig/
    /usr/lib/amd64-linux-glibc/pkgconfig/expat.pc
    /usr/lib/amd64-linux-glibc/libexpat.so@
    /usr/lib/amd64-linux-glibc/libexpat.a
    /usr/lib/amd64-linux-glibc/libexpat.la*
    /usr/lib/amd64-linux-glibc/libexpat.so.1@
    /usr/lib/amd64-linux-glibc/libexpat.so.1.6.0*
    /usr/include/
    /usr/include/expat_external.h
    /usr/include/expat.h

We have the `libexpat.so.1.6.0` shared library and two symbolic links to it:
`libexpat.so.1` and `libexpat.so`.  We have the `libexpat.a` static library and
associated `libexpat.la` library metadata file generated by GNU libtool.  We
have a pkg-config file and two header files.  We have an executable utility and
an associated manual page.

We should therefore split these files into four binary packages: one for the
shared library, one for the library development files, one for the utility, and
one for the utility's documentation.

To find out what we should call the library package, we can use **objdump**(1)
to get the *SONAME* of the library:

    $ objdump -p tmp/dest/usr/lib/amd64-linux-glibc/libexpat.so.1.6.0 | grep SONAME
      SONAME               libexpat.so.1

We should name our library package after the SONAME of the shared library,
without `.so`.  The binary package shall be named **`libexpat.1`**.

The versionless `libexpat.so` link is only needed by **ld**(1) when linking a
just-compiled object with the `-lexpat` linker flag.  So this can be provided by
our library development package.  Also provided by that package will be the
header files, the pkg-config file, and the static library.  The development
package can be called **`libexpat.1-dev`**.

The `xmlwf` utility can be provided by a package called simply **`xmlwf`**.

The `xmlwf.1` manual page can be provided by a package called **`xmlwf-doc`**.


Binary Packages
===============

Binary Package Metadata
-----------------------

Each binary package to be built needs to have [a directory for its
metadata][spf-binpkg.pkg].  So let's create directories for our packages.

    $ mkdir libexpat.1.pkg libexpat.1-dev.pkg xmlwf.pkg xmlwf-doc.pkg

SPF 2.0 requires a `control` file for each binary package.  The format of this
file is the same as that of the source package `control` file.  The required
[binary package fields][spf-fields-bin] are `Architecture`, `Platform`, and
`Description`.

None of these binary packages are platform-specific, so they will all have a
`Platform: all` field.  All of the binary packages except `xmlwf-doc` are
architecture-specific; that is, they provide files whose contents depend on the
host architecture (files like executable and linkable objects).  So `xmlwf-doc`
will have an `Architecture: all` field while the others will have `Architecture:
any` fields.

Let's start with the `libexpat.1.pkg/control` file:

    Architecture: any
    Platform: all
    Description: XML parser library
     Expat is an XML parser library written in C. It is a stream-oriented parser in
     which an application registers handlers for things the parser might find in the
     XML document (like start tags).

That's fairly simple.

Now let's write a `control` file for `libexpat.1-dev`.  Because it provides
development files for `libexpat.so.1`, `libexpat.1-dev` should depend on the
`libexpat.1` package.  This should be a versioned dependency, because the
`libexpat.so` symbolic link points to a specific version of `libexpat.so`.

    Architecture: any
    Platform: all
    Depends: libexpat.1 (= 2.1.0-1)
    Description: XML parser library - development files
     Expat is an XML parser library written in C. It is a stream-oriented parser in
     which an application registers handlers for things the parser might find in the
     XML document (like start tags).
     .
     This package provides development files for Expat.

Next is `xmlwf`, which should also depend on `libexpat.1` since the `xmlwf`
utility is dynamically linked against the `libexpat.so.1` library.

    Architecture: any 
    Platform: all 
    Depends: libexpat.1
    Description: XML parser library - example application
     This package provides an example application of Expat that determines if an XML 
     document is well-formed.

Finally, we can write metadata for `xmlwf-doc`, which should depend on `xmlwf`
since it documents the `xmlwf` utility.

    Architecture: all 
    Platform: all 
    Depends: xmlwf
    Description: XML parser library - example application documentation files
     This package provides the manual page for xmlwf, an example application of
     Expat that determines if an XML document is well-formed.

Binary Package Data Files
-------------------------

The **oh-installfiles**(1) utility of opkhelper, which we'll be using to install
files into *binary package data directories*, requires a `files` file for each
binary package that is to provide data files.

Recall how we decided to split files between packages.  We will now write
pathname patterns to do this.

Again, let's start with `libexpat.1`.  We can write the following pattern in
`libexpat.1.pkg/files`:

    /usr/lib/*/libexpat.so.*

This will match `/usr/lib/amd64-linux-glibc/libexpat.so.1` and
`/usr/lib/amd64-linux-glibc/libexpat.so.1.6.0`; these two files will be provided
by `libexpat.1`.

The patterns for `libexpat.1-dev` are a little more complicated:

    /usr/include
    /usr/lib/*/libexpat.so
    /usr/lib/*/libexpat.a
    /usr/lib/*/pkgconfig

The first pattern simply matches the directory containing header files.  The
second matches the versionless symbolic link; remember this is used by **ld**(1)
to link a just-compiled object against `libexpat.so.1.6.0`.  The third matches
the static library, and the fourth matches the directory containing the
`expat.pc` pkg-config file.

`xmlwf.pkg/files` need only contain a pattern to match the directory containing
the `xmlwf` utility.

    /usr/bin

`xmlwf-doc.pkg/files` is similarly simple:

    /usr/share/man/man1

With these pathname patterns done, we can add **oh-installfiles**(1) to our
`build` makefile:

    #!/usr/bin/make -f
    
    nop:
            @:
    
    build:
            oh-autoconfigure
            oh-autobuild
            touch $@
    
    install: build
            oh-autoinstall
            oh-installfiles

Now run **opkbuild**(1) again:

    $ opkbuild -b -c -T install

You can verify that all files were installed where they should be:

    $ find tmp/*.data -exec ls -Fd '{}' ';'
    tmp/libexpat.1.data/
    tmp/libexpat.1.data/usr/
    tmp/libexpat.1.data/usr/lib/
    tmp/libexpat.1.data/usr/lib/amd64-linux-glibc/
    tmp/libexpat.1.data/usr/lib/amd64-linux-glibc/libexpat.so.1@
    tmp/libexpat.1.data/usr/lib/amd64-linux-glibc/libexpat.so.1.6.0*
    tmp/libexpat.1-dev.data/
    tmp/libexpat.1-dev.data/usr/
    tmp/libexpat.1-dev.data/usr/lib/
    tmp/libexpat.1-dev.data/usr/lib/amd64-linux-glibc/
    tmp/libexpat.1-dev.data/usr/lib/amd64-linux-glibc/pkgconfig/
    tmp/libexpat.1-dev.data/usr/lib/amd64-linux-glibc/pkgconfig/expat.pc
    tmp/libexpat.1-dev.data/usr/lib/amd64-linux-glibc/libexpat.so@
    tmp/libexpat.1-dev.data/usr/lib/amd64-linux-glibc/libexpat.a
    tmp/libexpat.1-dev.data/usr/include/
    tmp/libexpat.1-dev.data/usr/include/expat_external.h
    tmp/libexpat.1-dev.data/usr/include/expat.h
    tmp/xmlwf.data/
    tmp/xmlwf.data/usr/
    tmp/xmlwf.data/usr/bin/
    tmp/xmlwf.data/usr/bin/xmlwf*
    tmp/xmlwf-doc.data/
    tmp/xmlwf-doc.data/usr/
    tmp/xmlwf-doc.data/usr/share/
    tmp/xmlwf-doc.data/usr/share/man/
    tmp/xmlwf-doc.data/usr/share/man/man1/
    tmp/xmlwf-doc.data/usr/share/man/man1/xmlwf.1

Cleaning Up Installed Files
---------------------------

There are few things we can do to improve our `build` makefile's `install`
target.

You may have noticed **oh-installfiles**(1) warn that something hasn't been
installed:

> oh-installfiles: Warning: Some files have not been installed into packages

With **find**(1), we can see that this is the `libexpat.la` file that GNU
libtool generated.

    $ find tmp/dest -type f -exec ls -Fd '{}' ';' | sed 's|^tmp/dest||'
    /usr/lib/amd64-linux-glibc/libexpat.la*

We don't need this, and we can simply delete it in the `install` target.

Next, note that some file permissions aren't entirely correct.  For example,
`libexpat.so.1.6.0` is executable, but almost all libraries need not be.

So we can call **oh-fixperms**(1) in our `install` target to automatically set
correct permissions for us.

Finally, note that the executable and linkable objects are not stripped: they
contain all of their symbols, including those only needed for debugging.

    $ file tmp/libexpat.1.data/usr/lib/amd64-linux-glibc/libexpat.so.1.6.0
    tmp/libexpat.1.data/usr/lib/amd64-linux-glibc/libexpat.so.1.6.0: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0x2d88e36feeb8245bfa2f63f2f0e9a9f8232f6d2c, not stripped
    $ file tmp/xmlwf.data/usr/bin/xmlwf
    tmp/xmlwf.data/usr/bin/xmlwf: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.26, BuildID[sha1]=0xdb5f686930b13b8a5e7519efb446a2da14de9856, not stripped

We can call **oh-strip**(1) in our `install` target to automatically strip
objects for us.

So our `build` makefile should now look like this:

    #!/usr/bin/make -f
    
    nop:
            @:
    
    build:
            oh-autoconfigure
            oh-autobuild
            touch $@
    
    install: build
            oh-autoinstall
            rm -f 'dest/usr/lib/$(OPK_HOST_ARCH)/libexpat.la'
            oh-fixperms
            oh-strip
            oh-installfiles

[spf-binpkg.pkg]: http://specs.proteanos.com/spf-2.0/overview.html#files-binpkg.pkg
[spf-fields-bin]: http://specs.proteanos.com/spf-2.0/fields.html#fields-src


Documentation and Finishing Touches
===================================

Source Package Documentation
----------------------------

SPF 2.0 [specifies][spf-docs] that one of the binary packages built from a
source package provides documentation files about the source package and is
depended upon by all of the other binary packages from the source package.

So we should pick one common binary package that should be a dependency of all
of our other binary packages.  `libexpat.1` is a good candidate for this, since
it is already a direct dependency of `libexpat.1-dev` and `xmlwf` and an
indirect dependency of `xmlwf-doc`.

Per SPF 2.0, we can mark `libexpat.1` as providing source package documentation
by making a `docs` file in its metadata directory.

    $ touch libexpat.1.pkg/docs

We should make all of our other binary packages directly depend on `libexpat.1`
version `2.1.0-1`.  For example, `xmlwf-doc.pkg/control` should now look like
this:

    Architecture: all 
    Platform: all 
    Depends: libexpat.1 (= 2.1.0-1), xmlwf
    Description: XML parser library - example application documentation files
     This package provides the manual page for xmlwf, an example application of
     Expat that determines if an XML document is well-formed.

Substitution Variables
----------------------

We've hardcoded the `libexpat.1` binary package version in many of our control
files.  What will we do when we make a new version of our source package?  We'll
have to change all of these values in all of these places.

[*Substitution variables*][spf-substvars] (*substvars* for short) make this
unnecessary.  We can just use the `Binary-Version` substitution variable in our
control files to refer to the version of our binary packages.  For example, our
`xmlwf-doc.pkg/control` file should now look like this:

    Architecture: all 
    Platform: all 
    Depends: libexpat.1 (= ${Binary-Version}), xmlwf
    Description: XML parser library - example application documentation files
     This package provides the manual page for xmlwf, an example application of
     Expat that determines if an XML document is well-formed.

But that's not all!  We can define our own variables as well.

Note that the descriptions of our `libexpat.1` and `libexpat.1-dev` packages
have a common paragraph.  We can put that in a file called `substvars`:

    Common-Description:
     Expat is an XML parser library written in C. It is a stream-oriented parser in
     which an application registers handlers for things the parser might find in the
     XML document (like start tags).

As noted by the SPF 2.0 specification, the leading newline character in the
value is fine:

> Values may be comprised of multiple lines, and empty lines at the beginning
> and end of each substitution variable value shall be removed.

We can now use this variable in our `control` files.  Here's
`libexpat.1.pkg/control`:

    Architecture: any
    Platform: all
    Description: XML parser library
     ${Common-Description}

And here's `libexpat.1-dev.pkg/control`:

    Architecture: any
    Platform: all
    Depends: libexpat.1 (= ${Binary-Version})
    Description: XML parser library - development files
     ${Common-Description}
     .
     This package provides development files for Expat.

Copyright and License Information
---------------------------------

We're almost done; we just have one more important thing to do.  We need to
document the copyright information for the upstream software and our own
packaging work.

This is done in the `copyright` file.  There is currently no standard format for
this file.

We need to collect the copyright and license information from the upstream
source code (usually in comments at the tops of source files).

There are some resources available to assist us with this.  First, we can look
at the work already done by package maintainers in the Debian Project.  Find the
[copyright file][deb-expat-copyright] for Debian's `expat` source package.

We see the following copyright information:

    Copyright (c) 1998, 1999, 2000 Thai Open Source Software Center Ltd
                                   and Clark Cooper
    Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006 Expat maintainers.

We also see that Expat can be dealt in under the terms of, unsurprisingly, the
Expat (a.k.a. "MIT") license.

Another resource we can use is the [**licensecheck**(1) tool][licensecheck],
maintained in Debian's `devscripts` package and originally based on a script
from the KDE SDK.  Recursively run **licensecheck**(1) to report copyright and
license information.

    $ licensecheck -r --copyright tmp/src/

We see that some source files have publication dates in their copyright notices
that are newer than those that Debian's copyright file lists:

    tmp/src/amiga/expat_lib.c: MIT/X11 (BSD like)
      [Copyright: 2001-2009 Expat maintainers / HOLDERS BE LIABLE FOR ANY]

So collect some representative copyright notices – e.g. from
`tmp/src/lib/xmlparse.c`, `tmp/src/examples/outline.c`,
`tmp/src/vms/expat_config.h`, and `tmp/src/amiga/expat_lib.c` – and add them to
the `copyright` file.

Then describe the license under which the software may be used.  `Expat` is a
"common license" included under `/usr/share/common-licenses/` in this
distribution, so you can refer to it there.

You should also document the location from which the source was obtained.

Finally, add your own copyright notice and license information.  You should
allow your work to be used under the terms of a license that is equivalent to or
compatible with the terms of the upstream software's copyright license.

Your resulting `copyright` file might look something like this:

    Upstream Source
    ===============
    
    Location: <http://sourceforge.net/projects/expat/files/expat/>
    
    Copyright (c) 1998, 1999, 2000 Thai Open Source Software Center Ltd
    Copyright 1999, Clark Cooper
    Copyright 2000, Clark Cooper
    Copyright (c) 2001-2009 Expat maintainers.
    
    These files may be reproduced, distributed, modified, and otherwise dealt in
    under the terms of the Expat License.
    
    On this system, a copy of the Expat License may be found at
    <file:///usr/share/common-licenses/Expat>.
    
    
    Distribution Packaging
    ======================
    
    Copyright (C) 2012 J. Random Hacker
    
    These files may be reproduced, distributed, modified, and otherwise dealt in
    under the terms of the Expat License.
    
    On this system, a copy of the Expat License may be found at
    <file:///usr/share/common-licenses/Expat>.

Building Everything
-------------------

Now we can build all of our source and binary packages and verify that
everything is correct.

**opkbuild**(1) maintains a cache file in the work area; because we've modified
the metadata in our packaging since the first time we ran **opkbuild**(1), this
cache file is out-of-date.  Also, we should make sure that the entire build
process still works.  So let's clean up the work area before going any further.

    $ rm -Rf tmp/

Now let's run **opkbuild**(1) again, this time completely building all of our
source and binary packages and cleaning up automatically when we're done.

    $ opkbuild

After that finishes, you should see the built packages in the parent directory.

    $ ls -1 ../*.opk
    ../libexpat.1_2.1.0-2_amd64-linux-glibc_all.opk
    ../libexpat.1-dev_2.1.0-2_amd64-linux-glibc_all.opk
    ../src-expat_2.1.0-2_src_all.opk
    ../xmlwf_2.1.0-2_amd64-linux-glibc_all.opk
    ../xmlwf-doc_2.1.0-2_all_all.opk

`src-expat` is a *source binary package* – a binary package installable with the
package manager that provides the files in our source package.  This binary
package is a convenient way to distribute our source package to others.

You can use the **tar**(1) command to verify that the control information and
data files in packages look correct.

    $ tar -xzO control.tar.gz \
    > <../libexpat.1_2.1.0-2_amd64-linux-glibc_all.opk | tar -xzO ./control
    Package: libexpat.1
    Source: expat
    Version: 2.1.0-2
    Architecture: amd64-linux-glibc
    Platform: all
    Maintainer: "J. Random Hacker" <jrandom@example.com>
    Installed-Size: 164
    Description: XML parser library
     Expat is an XML parser library written in C. It is a stream-oriented parser in
     which an application registers handlers for things the parser might find in the
     XML document (like start tags).
    Homepage: http://expat.sourceforge.net/
    $ tar -xzO data.tar.gz \
    > <../libexpat.1_2.1.0-2_amd64-linux-glibc_all.opk | tar -tz
    ./
    ./usr/
    ./usr/share/
    ./usr/share/doc/
    ./usr/share/doc/libexpat.1/
    ./usr/share/doc/libexpat.1/changelog.dist
    ./usr/share/doc/libexpat.1/copyright
    ./usr/lib/
    ./usr/lib/amd64-linux-glibc/
    ./usr/lib/amd64-linux-glibc/libexpat.so.1
    ./usr/lib/amd64-linux-glibc/libexpat.so.1.6.0

Congratulations!  You've made a source package that successfully builds four
binary packages!

[spf-docs]: http://specs.proteanos.com/spf-2.0/metadata.html#docs
[spf-substvars]: http://specs.proteanos.com/spf-2.0/substvars.html
[deb-expat-copyright]: http://packages.debian.org/changelogs/pool/main/e/expat/current/copyright
[licensecheck]: http://anonscm.debian.org/gitweb/?p=devscripts/devscripts.git;a=blob;f=scripts/licensecheck.pl;hb=HEAD