[MacPorts] FAQ modified

[MacPorts]

Page "FAQ" was changed by cal at macports.org
Diff URL: <https://trac.macports.org/wiki/FAQ?action=diff&version=137>
Revision 137
Comment: Extend #usrlocal by the possible use of trace mode
Changes:
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Index: FAQ
=========================================================================
--- FAQ (version: 136)
+++ FAQ (version: 137)
@@ -74,7 +74,7 @@
 (For the same reasons, fink uses `/sw` as its prefix.)
 
 === So with MacPorts under /opt/local I can use /usr/local freely? === #usrlocal
-                                                                                
+
 No, not entirely. Even with MacPorts living elsewhere, `/usr/local` can still interfere.
 Some software (especially the GNU auto* tools and gcc) looks into `/usr/local` for external headers, libraries, and binaries. Certain ports might (and do) fail to build because during their build something incompatible is found and picked up
 from `/usr/local`. Good ports avoid this by explicitly specifying `--with-libfoo=/opt/local/lib/` or explicitly disabling all such possible dependencies altogether with `--disable-foo` or `--without-bar` but not all ports are able to do that.
@@ -82,8 +82,14 @@
 If you ask on a MacPorts mailing list about a port failing to build because it chokes on something
 that it found and picked up in `/usr/local` you will probably be advised to temporarily rename
 `/usr/local` to make it disappear entirely for the duration of the build.
-Obviously, this wouldn't be possible to do if MacPorts itself lived under `/usr/local`.          
- 
+Obviously, this wouldn't be possible to do if MacPorts itself lived under `/usr/local`.
+
+Note that starting with 2.3.0, MacPorts can automatically hide `/usr/local` (and all other files a port does not depend on) from ports' build systems. This feature is called trace mode and is activated by providing the `-t` flag to `port`, e.g.
+{{{
+sudo port -t install <portname>
+}}}
+You'll notice a small performance penalty when using trace mode, and a very little number of ports will fail to build due to the changes in the environment (currently, only the `go` port is known to be affected). Warnings about sandbox violations are normal when using trace mode. If a port fails to build in trace mode, the problem is very likely a bug. You should file a ticket if none exists yet.
+
 === How do I remove or uninstall MacPorts? === #uninstall
 
 See [http://guide.macports.org/chunked/installing.macports.uninstalling.html the Guide].
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