便利库中的符号未导出到可执行文件中
我有一个程序 myprogram，它与一个静态便利库链接，将其称为 libconvenience.a，其中包含一个函数 func() >。 func() 函数在 myprogram 中没有被调用；它需要能够从插件库 plugin.so 中调用。
符号 func() 未在 myprogram 中动态导出。如果我跑步，
nm myprogram | grep func
我什么也得不到。然而，libconvenience.a 中并没有缺少它：
nm libconvenience/libconvenience.a | grep 函数
00000000T函数
我正在使用 automake，但是如果我在命令行上手动执行最后一个链接步骤，它也不起作用：
gcc -Wl,--export-dynamic -o myprogram *.o libconvenience/libconvenience.a `pkg-config --libs somelibraries`
但是，如果我像这样链接程序，则跳过便利库的使用并链接直接进入 libconvenience.a 的目标文件，func() 按其应有的方式显示在 myprogram 的符号中：
gcc -Wl,--export-dynamic -o myprogram *.o libconvenience/*.o `pkg-config --libs somelibraries`
如果我在 myprogram 中的某处添加对 func() 的虚拟调用，然后 func() 也会显示在 myprogram 中' s 符号。但我认为 --export-dynamic 应该导出所有符号，无论它们是否在程序中使用！
我在 Fedora 14 上使用 automake 1.11.1 和 gcc 4.5.1。我还使用 Libtool 2.2.10 构建 plugin.so （但不是便利库。）
我没有忘记将 -Wl,--export-dynamic 放入 myprogram_LDFLAGS 中，我也没有忘记将包含 func() 的源代码放入 中libconvenience_a_SOURCES （一些谷歌搜索表明这些是此问题的常见原因。）
有人可以帮助我理解这里发生了什么吗？
I have a program, myprogram, which is linked with a static convenience library, call it libconvenience.a, which contains a function, func(). The function func() isn't called anywhere in myprogram; it needs to be able to be called from a plugin library, plugin.so.
The symbol func() is not getting exported dynamically in myprogram. If I run
nm myprogram | grep func
I get nothing. However, it isn't missing from libconvenience.a:
nm libconvenience/libconvenience.a | grep func
00000000 T func
I am using automake, but if I do the last linking step by hand on the command line instead, it doesn't work either:
gcc -Wl,--export-dynamic -o myprogram *.o libconvenience/libconvenience.a `pkg-config --libs somelibraries`
However, if I link the program like this, skipping the use of a convenience library and linking the object files that would have gone into libconvenience.a directly, func() shows up in myprogram's symbols as it should:
gcc -Wl,--export-dynamic -o myprogram *.o libconvenience/*.o `pkg-config --libs somelibraries`
If I add a dummy call to func() somewhere in myprogram, then func() also shows up in myprogram's symbols. But I thought that --export-dynamic was supposed to export all symbols regardless of whether they were used in the program or not!
I am using automake 1.11.1 and gcc 4.5.1 on Fedora 14. I am also using Libtool 2.2.10 to build plugin.so (but not the convenience library.)
I didn't forget to put -Wl,--export-dynamic in myprogram_LDFLAGS, nor did I forget to put the source that contains func() in libconvenience_a_SOURCES (some Googling suggests that these are common causes of this problem.)
Can somebody help me understand what is going on here?
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我设法解决了它。约翰·卡尔科特 (John Calcote) 的优秀 Autotools 书中的这条注释为我指明了正确的方向：
为了抵消这种行为，可以对 libtool 使用
--whole-archive标志。但是，这会导致所有系统库中的所有符号也被拉入，从而导致大量双符号定义错误。因此，在链接器命令行上，--whole-archive需要位于libconvenience.a之前，并且后面需要紧跟--no-whole- archive以便其他库不会被这样对待。这有点困难，因为 automake 和 libtool 并不能真正保证您的标志在命令行上保持相同的顺序，但是Makefile.am中的这一行做到了这一点：I managed to solve it. It was this note from John Calcote's excellent Autotools book that pointed me in the right direction:
To counteract this behavior, one can use the
--whole-archiveflag to libtool. However, this causes all the symbols from all the system libraries to be pulled in also, causing lots of double symbol definition errors. So--whole-archiveneeds to be right beforelibconvenience.aon the linker command line, and it needs to be followed by--no-whole-archiveso that the other libraries aren't treated that way. This is a bit difficult since automake and libtool don't really guarantee keeping your flags in the same order on the command line, but this line inMakefile.amdid the trick:如果您需要 func 位于 plugin.so 中，则应尽可能尝试在那里找到它。便利库就是为了方便地链接到可执行文件或库作为中间步骤。
If you need func to be in plugin.so, you should try and locate it there if possible. Convenience libraries are meant to be just that -- a convenience to link to an executable or lib as an intermediate step.