Do permanent linking to other shared libraries - linux

currently I'm creating a shared library "libmylib.so" using libtool with statements like this:
libtool --mode=compile g++ -Wall -fPIC -shared $(CFLAGS) $(LFLAGS) $(LSTATIC)
libtool --mode=link g++ -shared -export-symbols-regex beam_ -rpath /usr/lib -lotherlib
The library created with this depends on an other (non-static) library "libotherlib.so", that's why I'm using the statement "-lotherlib".
Now when I build an executable that links against libmylib.so something strange happens: I have to link against libotherlib.so again, means I have to add a statement "-lotherlib" for this executable too.
So it seems former linking of libotherlib.so when building libmylib.so does not to be persistent. But how can I change this? How can I build libmylib.so so that it implicitely knows about its dependency to shared library libotherlib.so?
Thanks!

The answer seems to be the same like for this question: Link a static library to a shared one during build?
Big difference: when shared objects are specified for linking instead of static ones they are not included but the generated shared library remembers its dependnecy to these so's.

Related

Linking static and dynamic libraries on LINUX

I am trying to build an executable from different libraries using the below statement
g++ -fPIC -O -DUSING_PCH -D_REENTRANT -I"/app1/home1/quickfast/boost_1_61_0/include/boost-1_61" -I"/app1/home1/quickfast/boost_1_61_0/." -I"../../src" -I"../src/Examples" -I"/app1/home1/quickfast/boost_1_61_0/include" -L"." -L"/app1/home1/quickfast/boost_1_61_0/stage/lib" -L/app1/home1/quickfast/quickfast-master/lib -l"QuickFAST" -l"boost_thread-gcc44-mt-1_61" -l"boost_system-gcc44-mt-1_61" -l"boost_filesystem-gcc44-mt-1_61" -l"boost_unit_test_framework-gcc44-mt-1_61" -static -ldl -lpthread -o "../../bin/testSequences"
The above compilation statement includes lot of libraries, which are availabe as dynamic libaries(.so file) as well as static libraries (.a file). For example library boost_system-gcc44-mt-1_61 is present in the system as libboost_system-gcc44-mt-1_61.so as well as libboost_system-gcc44-mt-1_61.a . What I want is that while compilation only dynamic libraries (.so files) are picked up and static libraries are ignored. Can someone please guide.
You are telling it to pick up static libraries by using the -static link option. If you remove it, it'll probably work as you expect.

g++ link an .a file and its dependencies into a static .so

I have a libSomelib.a that can be linked to an executable by the following command:
g++ -L. -lsomeLib -lpcrecpp -lpcre -lpthread main.cpp -o main
But how could I link a shared object from it, that contains all depentencies?
I want to achieve the following with my new someLib.so:
g++ -L. -lsomeLib main.cpp -o main
I have tried the following:
g++ -shared -L. -lsomeLib -lpcrecpp -lpcre -lpthread -o libSomelib_static.so
This gives me an .so file with no symbols.
PS: I'm completely beginer of compilers.
There are a few issues at play here:
Linkers only use object files from an archive that resolve unresolved symbols. This is why the order of archives in the command line is important. Correct order is object files, followed by static libraries, followed by shared libraries. E.g. g++ -o main -pthread main.cpp -L. -lsomeLib -lpcrecpp -lpcre.
When you build the shared library that archive does not resolve any symbols, hence the linker ignores it completely. See 1.
Object files for a shared library must be compiled as position independent code (-fPIC compiler flag). Archives are normally built without this flag.
Use -pthread flag both when compiling and linking multi-threaded applications, not -lpthread.

Mixing static libraries and shared libraries

I have a project where I have one static library libhelper.a and another with my actual shared object library, libtestlib.so. My goal is to link libhelper.a into libtestlib.so. Is that possible on Linux/BSD? When I tried and created a test program I got the following errors:
./prog1:/usr/local/lib/libtestlib.so.1.0: undefined symbol ''
My guess is that this is occurring because libhelper.a was not compiled with -fPIC while libtestlib.so was. What is the proper way to build programs that use shared libraries that also have dependancies on static libraries?
Thanks!
My goal is to link libhelper.a into libtestlib.so. Is that possible on Linux?
Sure. This should do:
gcc -shared -fPIC -o libtestlib.so $(OBJS) \
-Wl,--whole-archive -lhelper -Wl,--no-whole-archive
libhelper.a was not compiled with -fPIC
It's best to rebuild libhelper.a with -fPIC. If that's not possible, above command will still work on Linux/ix86, but not on e.g. Linux/x86_64.
What is the proper way to build programs that use shared libraries that also have dependancies on static libraries?
If you include libhelper.a into libtestlib.so as above, then simple:
gcc main.c -ltestlib
is all you need. If you insist on linking with libhelper.a, then you must tell the end-user that he must link with e.g.
gcc main.c -ltestlib -lhelper
There is no way to specify that libtestlib.so depends on libhelper.a.

How can I force linking with a static library when a shared library of same name is present

Suppose I have a file main.cpp which uses sin() function which is defined in libmath. Also suppose that we have both libmath.a and libmath.so available in the same directory. Now if I issue the command g++ -o main main.cpp -lmath the default behaviour of Linux is to link to the shared library libmath.so. I want to know is there a way to force the program to link with the static library libmath.a without deleting or moving the shared library?
You'll need to pass the -static to the linker, but only for particular libraries you want. e.g.:
g++ -o main main.cpp -Wl,-Bstatic -lmath -Wl,-Bdynamic
If your linker supports -l:<filename> you may use:
g++ -o main main.cpp -l:libmath.a
Use this function:
g++ -o main main.cpp /path_to/libmath.a

Trying to statically link Boost

I am working in Linux, Eclipse CDT, g++, with Boost library. Having existing program which uses Boost thread, I try to link it statically instead of dynamically. /usr/local/lib directory contains the following files:
libbost_thread.a
libbost_thread.so
libbost_thread.1.41.0
Dynamic linking works:
g++ -o"MyProgram" ./main.o -lboost_thread
Static linking:
g++ -static -o"MyProgram" ./main.o -lboost_thread
produces huge number of messages like:
undefined reference to `pthread_mutex_init'
How can I link statically to the Boost library?
For pthread_mutex_init, you want to compile/link with -pthread option:
g++ -static -pthread -o"MyProgram" ./main.o -lboost_thread
The problem is that functions like pthread_mutex_init are in a separate library. Dynamic libraries can include the metadata for the fact that it needs the separate library (so libboost_thread.so includes the fact that it needs libpthread).
But static libraries don't have that information. So you need to provide the reference to any necessary libraries when you link statically.
As for using -pthread instead of -lpthread, it's slightly preferable because it not only links the necessary library, but provides any other options that should be used (such a -D_REENTRANT to the compiler).
Try adding -lpthread to your invocation.
On Linux a dynamic library may automatically depend on other dynamic libraries so that when you link it, you get the other libraries for free. When linking statically, there is no such system and you have to specify the other libraries manually.

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