Don't link to any threading lib as libc already exports weak symbols.
[dragonfly.git] / lib / libpthread / README
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a4bded30 1$DragonFly: src/lib/libpthread/README,v 1.2 2008/01/20 19:14:09 corecode Exp $
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3Proxy libpthread
4================
5
6We want to be able to switch between threading libraries at execution
7time. This problem is solved by this proxy libpthread. Its operation is
8as follows.
9
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10The operation at link time is as follows.
11libc.so anyways defines all pthread functions as weak symbols
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12except for pthread_create. libpthread.so now also defines a dummy
13pthread_create as weak symbol and ld is satisfied. At execution time
14later rtld will resolve all these references to the strong symbols in the
15right thread library.
16
17Allowing the user to switch the threading lib works like this: At link
18time ld reads the SONAME of libpthread.so, which is set to libpthread.so.0
19(or a higher major, if ABI changes). Usually libfoo.so is a symlink to
20libfoo.so.3 which also has set its SONAME to libfoo.so.3, so that if
21libfoo.so.4 is being installed, programs will still continue to use
22libfoo.so.3 and not follow the symlink libfoo.so to the newer -- and
23possibly incompatible -- libfoo.so.4. What we do is approximately the
24opposite: libpthread.so is no symlink, but nevertheless has its SONAME
25set to libpthread.so.0. Now, however, libpthread.so.0 is a symlink to the
26threading library of your choice. The linker will use the default
27threading lib which libpthread.so is linked to, but the runtime linker
28will instead follow the symlink.
29
30This (obviously) does not work for static linking.