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28 <table class="rfc2822 field-list" frame="void" rules="none">
29 <col class="field-name" />
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31 <tbody valign="top">
32 <tr class="field"><th class="field-name">GLEP:</th><td class="field-body">47</td>
33 </tr>
34 <tr class="field"><th class="field-name">Title:</th><td class="field-body">Creating 'safe' environment variables</td>
35 </tr>
36 <tr class="field"><th class="field-name">Version:</th><td class="field-body">1.2</td>
37 </tr>
38 <tr class="field"><th class="field-name">Last-Modified:</th><td class="field-body"><a class="reference" href="http://www.gentoo.org/cgi-bin/viewcvs/xml/htdocs/proj/en/glep/glep-0047.txt?cvsroot=gentoo">2006/02/11 21:43:14</a></td>
39 </tr>
40 <tr class="field"><th class="field-name">Author:</th><td class="field-body">Diego Pettenò, Fabian Groffen</td>
41 </tr>
42 <tr class="field"><th class="field-name">Status:</th><td class="field-body">Draft</td>
43 </tr>
44 <tr class="field"><th class="field-name">Type:</th><td class="field-body">Standards Track</td>
45 </tr>
46 <tr class="field"><th class="field-name">Content-Type:</th><td class="field-body"><a class="reference" href="glep-0012.html">text/x-rst</a></td>
47 </tr>
48 <tr class="field"><th class="field-name">Created:</th><td class="field-body">14-Oct-2005</td>
49 </tr>
50 <tr class="field"><th class="field-name">Post-History:</th><td class="field-body">09-Feb-2006</td>
51 </tr>
52 </tbody>
53 </table>
54 <hr />
55 <div class="contents topic" id="contents">
56 <p class="topic-title first"><a name="contents">Contents</a></p>
57 <ul class="simple">
58 <li><a class="reference" href="#credits" id="id5" name="id5">Credits</a></li>
59 <li><a class="reference" href="#abstract" id="id6" name="id6">Abstract</a></li>
60 <li><a class="reference" href="#motivation" id="id7" name="id7">Motivation</a></li>
61 <li><a class="reference" href="#rationale" id="id8" name="id8">Rationale</a></li>
62 <li><a class="reference" href="#backwards-compatibility" id="id9" name="id9">Backwards Compatibility</a></li>
63 <li><a class="reference" href="#specification" id="id10" name="id10">Specification</a><ul>
64 <li><a class="reference" href="#variable-assignment" id="id11" name="id11">Variable Assignment</a></li>
65 </ul>
66 </li>
67 <li><a class="reference" href="#references" id="id12" name="id12">References</a></li>
68 <li><a class="reference" href="#copyright" id="id13" name="id13">Copyright</a></li>
69 </ul>
70 </div>
71 <div class="section" id="credits">
72 <h1><a class="toc-backref" href="#id5" name="credits">Credits</a></h1>
73 <p>The text of this GLEP is a result of a discussion and input of the
74 following persons, in no particular order: Mike Frysinger, Diego
75 Pettenò, Fabian Groffen and Finn Thain.</p>
76 </div>
77 <div class="section" id="abstract">
78 <h1><a class="toc-backref" href="#id6" name="abstract">Abstract</a></h1>
79 <p>In order for ebuilds and eclasses to be able to make host specific
80 decisions, it is necessary to have a number of environmental variables
81 which allow for such decisions. This GLEP introduces some measures that
82 need to be made to make these decisions 'safe', by making sure the
83 variables the decisions are based on are 'safe'. A small overlap with
84 GLEP 22 <a class="footnote-reference" href="#id3" id="id1" name="id1">[1]</a> is being handled in this GLEP where the use of 2-tuple
85 keywords are being kept instead of 4-tuple keywords. Additionally, the
86 <tt class="literal"><span class="pre">ELIBC</span></tt>, <tt class="literal"><span class="pre">KERNEL</span></tt> and <tt class="literal"><span class="pre">ARCH</span></tt> get auto filled starting from
87 <tt class="literal"><span class="pre">CHOST</span></tt> and the 2-tuple keyword, instead of solely from they 4-tuple
88 keyword as proposed in GLEP 22.</p>
89 <p>The destiny of the <tt class="literal"><span class="pre">USERLAND</span></tt> variable is out of the scope of this
90 GLEP. Depending on its presence in the tree, it may be decided to set
91 this variable the same way we propose to set <tt class="literal"><span class="pre">ELIBC</span></tt>, <tt class="literal"><span class="pre">KERNEL</span></tt> and
92 <tt class="literal"><span class="pre">ARCH</span></tt>, or alternatively, e.g. via the profiles.</p>
93 </div>
94 <div class="section" id="motivation">
95 <h1><a class="toc-backref" href="#id7" name="motivation">Motivation</a></h1>
96 <p>The Gentoo/Alt project is in an emerging state to get ready to serve a
97 plethora of 'alternative' configurations such as FreeBSD, NetBSD,
98 DragonflyBSD, GNU/kFreeBSD, Mac OS X, (Open)Darwin, (Open)Solaris and so
99 on. As such, the project is in need for a better grip on the actual
100 host being built on. This information on the host environment is
101 necessary to make proper (automated) decisions on settings that are
102 highly dependant on the build environment, such as platform or C-library
103 implementation.</p>
104 </div>
105 <div class="section" id="rationale">
106 <h1><a class="toc-backref" href="#id8" name="rationale">Rationale</a></h1>
107 <p>Gentoo's unique Portage system allows easy installation of applications
108 from source packages. Compiling sources is prone to many environmental
109 settings and availability of certain tools. Only recently the Gentoo
110 for FreeBSD project has started, as second Gentoo project that operates
111 on a foreign host operating system using foreign (non-GNU) C-libraries
112 and userland utilities. Such projects suffer from the current implicit
113 assumption made within Gentoo Portage's ebuilds that there is a single
114 type of operating system, C-libraries and system utilities. In order to
115 enable ebuilds -- and also eclasses -- to be aware of these
116 environmental differences, information regarding it should be supplied.
117 Since decisions based on this information can be vital, it is of high
118 importance that this information can be trusted and the values can be
119 considered 'safe' and correct.</p>
120 </div>
121 <div class="section" id="backwards-compatibility">
122 <h1><a class="toc-backref" href="#id9" name="backwards-compatibility">Backwards Compatibility</a></h1>
123 <p>The proposed keywording scheme in this GLEP is fully compatible with the
124 current situation of the portage tree, this in contrast to GLEP 22. The
125 variables provided by GLEP 22 can't be extracted from the new keyword,
126 but since GLEP 22-style keywords aren't in the tree at the moment, that
127 is not a problem. The same information can be extracted from the CHOST
128 variable, if necessary. No modifications to ebuilds will have to be
129 made.</p>
130 </div>
131 <div class="section" id="specification">
132 <h1><a class="toc-backref" href="#id10" name="specification">Specification</a></h1>
133 <p>Unlike GLEP 22 the currently used keyword scheme is not changed.
134 Instead of proposing a 4-tuple <a class="footnote-reference" href="#id4" id="id2" name="id2">[2]</a> keyword, a 2-tuple keyword is chosen
135 for archs that require them. Archs for which a 1-tuple keyword
136 suffices, can keep that keyword. Since this doesn't change anything to
137 the current situation in the tree, it is considered to be a big
138 advantage over the 4-tuple keyword from GLEP 22. This GLEP is an
139 official specification of the syntax of the keyword.</p>
140 <p>Keywords will consist out of two parts separated by a hyphen ('-'). The
141 part up to the first hyphen from the left of the keyword 2-tuple is the
142 architecture, such as ppc64, sparc and x86. Allowed characters for the
143 architecture name are in <tt class="literal"><span class="pre">a-z0-9</span></tt>. The remaining part on the right of
144 the first hyphen from the left indicates the operating system or
145 distribution, such as linux, macos, darwin, obsd, et-cetera. If the
146 right hand part is omitted, it implies the operating system/distribution
147 type is Gentoo GNU/Linux. In such case the hyphen is also omitted, and
148 the keyword consists of solely the architecture. The operating system
149 or distribution name can consist out of characters in <tt class="literal"><span class="pre">a-zA-Z0-9_+:-</span></tt>.
150 Please note that the hyphen is an allowed character, and therefore the
151 separation of the two fields in the keyword is only determinable by
152 scanning for the first hyphen character from the start of the keyword
153 string. Examples of keywords following this specification are
154 ppc-darwin and x86. This is fully compatible with the current use of
155 keywords in the tree.</p>
156 <p>The variables <tt class="literal"><span class="pre">ELIBC</span></tt>, <tt class="literal"><span class="pre">KERNEL</span></tt> and <tt class="literal"><span class="pre">ARCH</span></tt> are currently set in
157 the profiles when other than their defaults for a GNU/Linux system.
158 They can as such easily be overridden and defined by the user. To
159 prevent this from happening, the variables should be auto filled by
160 Portage itself, based on the <tt class="literal"><span class="pre">CHOST</span></tt> variable. While the <tt class="literal"><span class="pre">CHOST</span></tt>
161 variable can be as easy as the others set by the user, it still is
162 assumed to be 'safe'. This assumption is grounded in the fact that the
163 variable itself is being used in various other places with the same
164 intention, and that an invalid <tt class="literal"><span class="pre">CHOST</span></tt> will cause major malfunctioning
165 of the system. A user that changes the <tt class="literal"><span class="pre">CHOST</span></tt> into something that is
166 not valid for the system, is already warned that this might render the
167 system unusable. Concluding, the 'safeness' of the <tt class="literal"><span class="pre">CHOST</span></tt> variable
168 is based on externally assumed 'safeness', which's discussion falls
169 outside this GLEP.</p>
170 <p>Current USE-expansion of the variables is being maintained, as this
171 results in full backward compatibility. Since the variables themself
172 don't change in what they represent, but only how they are being
173 assigned, there should be no problem in maintaining them. Using
174 USE-expansion, conditional code can be written down in ebuilds, which is
175 not different from any existing methods at all:</p>
176 <pre class="literal-block">
177 ...
178 RDEPEND=&quot;elibc_FreeBSD? ( sys-libs/com_err )&quot;
179 ...
180 src_compile() {
181 ...
182 use elibc_FreeBSD &amp;&amp; myconf=&quot;${myconf} -Dlibc=/usr/lib/libc.a&quot;
183 ...
184 }
185 </pre>
186 <p>Alternatively, the variables <tt class="literal"><span class="pre">ELIBC</span></tt>, <tt class="literal"><span class="pre">KERNEL</span></tt> and <tt class="literal"><span class="pre">ARCH</span></tt>
187 are available in the ebuild evironment and they can be used instead of
188 invoking <tt class="literal"><span class="pre">xxx_Xxxx</span></tt> or in switch statements where they are actually
189 necessary.</p>
190 <p>A map file can be used to have the various <tt class="literal"><span class="pre">CHOST</span></tt> values being
191 translated to the correct values for the four variables. This change is
192 invisible for ebuilds and eclasses, but allows to rely on these
193 variables as they are based on a 'safe' value -- the <tt class="literal"><span class="pre">CHOST</span></tt> variable.
194 Ebuilds should not be sensitive to the keyword value, but use the
195 aforementioned four variables instead. They allow specific tests for
196 properties. If this is undesirable, the full <tt class="literal"><span class="pre">CHOST</span></tt> variable can be
197 used to match a complete operating system.</p>
198 <div class="section" id="variable-assignment">
199 <h2><a class="toc-backref" href="#id11" name="variable-assignment">Variable Assignment</a></h2>
200 <p>The <tt class="literal"><span class="pre">ELIBC</span></tt>, <tt class="literal"><span class="pre">KERNEL</span></tt>, <tt class="literal"><span class="pre">ARCH</span></tt> variables are filled from a profile
201 file. The file can be overlaid, such that the following entries in the
202 map file (on the left of the arrow) will result in the assigned
203 variables on the right hand side of the arrow:</p>
204 <pre class="literal-block">
205 *-*-linux-* -&gt; KERNEL=&quot;linux&quot;
206 *-*-*-gnu -&gt; ELIBC=&quot;glibc&quot;
207 *-*-kfreebsd-gnu -&gt; KERNEL=&quot;FreeBSD&quot; ELIBC=&quot;glibc&quot;
208 *-*-freebsd* -&gt; KERNEL=&quot;FreeBSD&quot; ELIBC=&quot;FreeBSD&quot;
209 *-*-darwin* -&gt; KERNEL=&quot;Darwin&quot; ELIBC=&quot;Darwin&quot;
210 *-*-netbsd* -&gt; KERNEL=&quot;NetBSD&quot; ELIBC=&quot;NetBSD&quot;
211 *-*-solaris* -&gt; KERNEL=&quot;Solaris&quot; ELIBC=&quot;Solaris&quot;
212 </pre>
213 <p>A way to achieve this is proposed by Mike Frysinger, which
214 suggests to have a env-map file, for instance filled with:</p>
215 <pre class="literal-block">
216 % cat env-map
217 *-linux-* KERNEL=linux
218 *-gnu ELIBC=glibc
219 x86_64-* ARCH=amd64
220 </pre>
221 <p>then the following bash script can be used to set the four variables to
222 their correct values:</p>
223 <pre class="literal-block">
224 % cat readmap
225 #!/bin/bash
226
227 CBUILD=${CBUILD:-${CHOST=${CHOST:-$1}}}
228 [[ -z ${CHOST} ]] &amp;&amp; echo need chost
229
230 unset KERNEL ELIBC ARCH
231
232 while read LINE ; do
233 set -- ${LINE}
234 targ=$1
235 shift
236 [[ ${CBUILD} == ${targ} ]] &amp;&amp; eval $&#64;
237 done &lt; env-map
238
239 echo ARCH=${ARCH} KERNEL=${KERNEL} ELIBC=${ELIBC}
240 </pre>
241 <p>Given the example env-map file, this script would result in:</p>
242 <pre class="literal-block">
243 % ./readmap x86_64-pc-linux-gnu
244 ARCH=amd64 KERNEL=linux ELIBC=glibc
245 </pre>
246 <p>The entries in the <tt class="literal"><span class="pre">env-map</span></tt> file will be evaluated in a forward
247 linear full scan. A side-effect of this exhaustive search is that the
248 variables can be re-assigned if multiple entries match the given
249 <tt class="literal"><span class="pre">CHOST</span></tt>. Because of this, the order of the entries does matter.
250 Because the <tt class="literal"><span class="pre">env-map</span></tt> file size is assumed not to exceed the block
251 size of the file system, the performance penalty of a full scan versus
252 'first-hit-stop technique' is assumed to be minimal.</p>
253 <p>It should be noted, however, that the above bash script is a proof of
254 concept implementation. Since Portage is largerly written in Python, it
255 will be more efficient to write an equivalent of this code in Python
256 also. Coding wise, this is considered to be a non-issue, but the format
257 of the <tt class="literal"><span class="pre">env-map</span></tt> file, and especially its wildcard characters, might
258 not be the best match with Python. For this purpose, the format
259 specification of the <tt class="literal"><span class="pre">env-map</span></tt> file is deferred to the Python
260 implementation, and only the requirements are given here.</p>
261 <p>The <tt class="literal"><span class="pre">env-map</span></tt> file should be capable of encoding a <tt class="literal"><span class="pre">key</span></tt>, <tt class="literal"><span class="pre">value</span></tt>
262 pair, where <tt class="literal"><span class="pre">key</span></tt> is a (regular) expression that matches a
263 chost-string, and <tt class="literal"><span class="pre">value</span></tt> contains at least one, distinct variable
264 assignment for the variables <tt class="literal"><span class="pre">ARCH</span></tt>, <tt class="literal"><span class="pre">KERNEL</span></tt> and <tt class="literal"><span class="pre">ELIBC</span></tt>. The
265 interpreter of the <tt class="literal"><span class="pre">env-map</span></tt> file must scan the file linearly and
266 continue trying to match the <tt class="literal"><span class="pre">key</span></tt>s and assign variables if
267 appropriate until the end of file.</p>
268 <p>Since Portage will use the <tt class="literal"><span class="pre">env-map</span></tt> file, the location of the file is
269 beyond the scope of this GLEP and up to the Portage implementors.</p>
270 </div>
271 </div>
272 <div class="section" id="references">
273 <h1><a class="toc-backref" href="#id12" name="references">References</a></h1>
274 <table class="footnote" frame="void" id="id3" rules="none">
275 <colgroup><col class="label" /><col /></colgroup>
276 <tbody valign="top">
277 <tr><td class="label"><a class="fn-backref" href="#id1" name="id3">[1]</a></td><td>GLEP 22, New &quot;keyword&quot; system to incorporate various
278 userlands/kernels/archs, Goodyear,
279 (<a class="reference" href="http://glep.gentoo.org/glep-0022.html">http://glep.gentoo.org/glep-0022.html</a>)</td></tr>
280 </tbody>
281 </table>
282 <table class="footnote" frame="void" id="id4" rules="none">
283 <colgroup><col class="label" /><col /></colgroup>
284 <tbody valign="top">
285 <tr><td class="label"><a class="fn-backref" href="#id2" name="id4">[2]</a></td><td>For the purpose of readability, we will refer to 1, 2 and
286 4-tuples, even though tuple in itself suggest a field consisting of
287 two values. For clarity: a 1-tuple describes a single value field,
288 while a 4-tuple describes a field consisting out of four values.</td></tr>
289 </tbody>
290 </table>
291 </div>
292 <div class="section" id="copyright">
293 <h1><a class="toc-backref" href="#id13" name="copyright">Copyright</a></h1>
294 <p>This document has been placed in the public domain.</p>
295 </div>
296 </div>
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