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List:       boost-build
Subject:    [Boost-build] library_order failures
From:       Vladimir Prus <vladimir.prus () gmail ! com>
Date:       2016-02-04 8:00:13
Message-ID: n8v0eg$pef$1 () ger ! gmane ! org
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I've looked into failures of the library_order test, and arrived at this failing \
command line:

     $ g++ -o main main.o -Wl,--start-group libd.so liba.so libb.so libc.a \
-Wl,--end-group  libc.a(c.o): In function `c()':
     /tmp/tmpSiuzvi/c.cpp:2: undefined reference to `d()'

In this example, each of the libraries has a single function whose name matches \
library name, and there's a call chain of main->a->b->c->d.

I can make this work like so:

     $ g++ -o main main.o -Wl,--no-as-needed -Wl,--start-group libd.so liba.so \
libb.so libc.a -Wl,--end-group

What happens, apparently is that:

- ld now has --as-needed option as default, which makes the final executable not \
refer to shared  libraries that are included on the command line, but not needed.
- Whether the library is needed is determined by checking whether it provides any \
symbols that  are unresolved at this point in the command line, similarly to how \
                static linking works
- While start-group/end-group fixes order dependency for static libraries, it does \
not appear to  do so for shared libraries.

So, for the original command line, the linker sees libd.so, notes that it does not \
export any previously unresolved symbol, and skips that library completely. By the \
time it arrives at libc.a and sees unresolved reference to 'd', libd.so is already \
thrown away.

I can solve this in two ways:

- Just stick "-Wl,--no-as-needed" at the start of the command line
- Re-enable automatic sorting of libraries on the command line. That's how \
Boost.Build  worked before, until I decided that --start-group/--end-group is fast \
enough and  we don't need to bother.

Comments?

-- 
Vladimir Prus
http://vladimirprus.com

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