You need to follow the advice suggested to you by the linker diagnostic:
warning: libworld.so, needed by libhello.so, not found (try using -rpath or -rpath-link)
when you build libhello.so: specifically, using -rpath to instruct the linker to inscribe in libhello.so the runpath ( = directory) where its dynamic dependency libworld.so can be found when libhello.so is linked with a program, such as your ppp, (or with another library).
When a library libfoo is required for the linkage of a program, whether as a direct or indirect dependency, the linker must be told:
- that
libfoo is to be input to the linkage. - a search directory in which it can find (the right)
libfoo, unless libfoo resides in one of the default search directories hard-coded into the linker.
The linker can get to know what libraries are needed and any non-default directories in which to search for them from two sources:
- The commandline passed to the linker. Specifically,
-l lib and -L dir options in that commandline. There are in turn two sub-sources from which it can get such options: - The programmer who explicitly writes them for the linker.
- The language frontend (
gcc, g++,gfortran...) that the programmer normally uses to delegate and simplify the invocation of the linker. The simplifying work of the frontend consists in augmenting the linkage options written by the programmer with many boilerplate options that are invariant for linkages performed via that language frontend on the host system it was built for. The boilerplate linkage options generated by a frontend (which are passed silently to the linker unless it is invoked in verbose mode) include boilerplate -l lib and -L dir options. For simplicity I will lump together all the search directories that are hard-coded in the linker and all the boilerplate search directories that are passed to it by a frontend and call them the default search directories for that frontend. These default search directories are always ones which the system dynamic linker is configured to search at runtime to load the shared libraries that a program needs.
- The dynamic section in any required shared library
libfoo that the linker has already discovered. This ELF section can specify (among other things): - Further shared libraries that are dependencies of
libfoo, by way of NEEDED libname entries. - Further (non-default) search directories for such dependencies, by way of
RUNPATH dirname entries. A NEEDED libname entry is inscribed in the dynamic section of a program or shared library when it is created by the linker for each shared library specified by -l name in the linkage options. A RUNPATH dirname entry is inscribed for each -rpath=dirname in the linkage options. Any -rpath=dirname linkage option will be one that is explicitly written by the programmer. A language frontend does not need to to emit -rpath options for the linker because any libraries that it passes to the linker are bound to reside in default search directories.
The dynamic section of an ELF binary is interrogated by both the static linker the and dynamic linker to discover (among other things) dynamic dependency information: what shared libraries are NEEDED and what RUNPATHs to search for them.
So if:
- your program
ppp depends on libhello.so and libhello.so depends in libworld.so, and - for some reason you don't want
libworld.so to reside in a default search directory, and - for some reason you also don't want to mention the
libworld.so dependency and its location in the linkage command of ppp
then your only choice is to supply this information to the linker via the dynamic section of libhello.so.
Here's how you do that from first principles.
Source files:
$ cat main.cpp #include <cstdlib> extern void hello(); int main() { hello(); exit(EXIT_SUCCESS); } $ cat hello.cpp #include <iostream> extern void world(); void hello() { std::cout << "Hello "; world(); } $ cat world.cpp #include <iostream> void world() { std::cout << "World\n"; }
Compile the sources:
$ g++ -c main.cpp $ g++ -c -fPIC hello.cpp world.cpp # Position Independent Code for shared libraries.
Build libworld.so:
$ g++ -shared -o libworld.so world.o
Build libhello.so:
$ g++ -shared -o libhello.so hello.o -L . -lworld -Wl,-rpath=$(pwd)
Here, we've told the linker that libhello needs libworld, to search for it in the current directory (-L .) and that the absolute name of the current directory is to be inscribed as a RUNPATH in the dynamic section of the output file (-Wl,-rpath=$(pwd)). In this example I'm just using the scrap directory I'm working in as a specimen non-default directory. Let's look at the top of that dynamic section:
$ readelf --dynamic libhello.so Dynamic section at offset 0x2de0 contains 26 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libworld.so] 0x0000000000000001 (NEEDED) Shared library: [libstdc++.so.6] 0x000000000000001d (RUNPATH) Library runpath: [/home/imk/develop/so/scrap] ...[cut]...
Note that besides libworld.so a dynamic dependency has also been inserted on libstdc++, because the linker has found symbols that are resolved by that library, which g++ passes to the linker by default. That library will be found in a default search directory: no extra RUNPATH needed.
Now let's move libhello.so into one of the linker's default search directories and leave libworld.so in its non-default location.
$ sudo mv libhello.so /usr/local/lib/
Not forgetting to refresh the linker cache to catch that update:
$ sudo ldconfig
Finally, link a program prog against libhello without mentioning libworld or its location:
$ g++ -o prog main.o -lhello
Here's the top of the dynamic section of prog:
$ readelf --dynamic prog Dynamic section at offset 0x2db0 contains 28 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libhello.so] 0x0000000000000001 (NEEDED) Shared library: [libc.so.6] ...[cut]...
This linkage succeeds because:
-l hello is sufficient for the static linker to find libhello.so in the default directory /usr/local/lib and resolve the reference to hello therein.- The dynamic section of
libhello.so informs the static linker that libworld.so is needed and can be searched for in the non-default directory /home/imk/develop/so/scrap, where indeed it is found. The static linker resolves the reference to world therein.
Success means that the static linker (besides succeeding with all the static symbol resolution) has determined that the dynamic linker will be able to resolve all dynamic symbols recursively required by the program when the dynamic linker loads it: The NEEDED and RUNPATH entries in the dynamic sections of prog and libhello.so provide the library and search directory information that the dynamic linker will need to do that - as we can see:
$ ./prog Hello World
But note that success does not show you an answer to this question:
for a dependency relationship like main ----> libhello.so -----> libworld.so, how can I ensure that during compilation, the executable [you mean the linker] only checks for the existence of the direct dependency libhello.so and does not recursively check for the existence of the indirect dependency libworld.so?
That shortcut is not taken by the linkage above and and cannot be taken in a linkage that determines the dynamic linker will be able to resolve all symbols at runtime. To determine that, the static linker must recursively search for and find libraries that resolve all symbol references and make sure that the dynamic linker will be able to the same at runtime (via NEEDED and RUNPATH entries). If you want the static linker to link a program without doing that, so that you can really omit indirect dynamic dependency information the linkage, then you must use unusual options to coerce the linker to tolerate undefined symbol references. And unless you then manually pre-load shared libraries at runtime that resolve all the symbols the program will seg-fault.
How you do specify an -rpath with CMake.
There's more than one way. The most obvious is to specify target_link_options for libhello.so in its CMakeLists.txt:
... target_link_directories(${PROJECT_NAME} PRIVATE <non-default-dir-of-libworld> ... target_link_options(${PROJECT_NAME} PRIVATE "-Wl,-rpath=<non-default-dir-of-libworld")
A more CMake-ish way is to set the BUILD_RPATH property for the libhello target:
... set_property(TARGET ${PROJECT_NAME} APPEND PROPERTY BUILD_RPATH <non-default-dir-of-libworld>) ... target_link_directories(${PROJECT_NAME} PRIVATE <non-default-dir-of-libworld>)
But do you really want to do that? (Optional reading).
When you say:
In the CMakeLists.txt file used to compile libhello.so, the target_link_libraries command has already linked the libworld.so library.
it suggests you were thinking: My libhello build already links libworld. So that dependency info is already baked into libhello, so why should I have to repeat it to the linker again when I link a program against libhello?
You've now learned, however, that an essential component of that dependency info - namely, the non-default search-directory for libworld - is not baked into libhello, unless you specify it with -rpath. The part that is baked into libhello.sois the dynamic section entry NEEDED libworld.so, which is not enough for successfully linking a program against libhello, unless libworld.so is in a default search directory. And it is baked in because you linked libhello.so against libworld.so - which you have no need to do in order to build libhello. It is a shared library and, unlike a program, external references do not need to be resolved in the successful linkage of a shared library. External references need only be resolved for successful linkage of a program.
When we say, e.g. that libhello.so depends on libworld.so, what we strictly mean is that linking a program against libhello imposes upon the program an additional dependency on libworld. Not that the linkage of libhello.so requires libworld.so.
When building a shared library we do not as matter of course link it against it other shared libraries it refers to: in the absence of technical reasons for linking against those others, we just let the undefined symbols in the library remain unresolved, as they would be in static library. Equally, when we build a program our default expectation is that we need to link it against all the shared libraries that it actually requires, not just its direct dependencies. These conventions make the -l lib options for a linkage invariant whether we choose to link static or shared versions of the libraries; they defer commitments about exactly what library binaries to link with a program and where to find them to the point where they have to be made - that is, the program linkage - and put them under the control of the person responsible for getting a program to link and run as desired in a specific environment. That's how program builders generally prefer it. There is no virtue in just attempting to "hide" indirect dynamic dependencies in a program linkage and they can't be hidden from anyone who knows how to do:
$ ldd prog linux-vdso.so.1 (0x00007ffc7e914000) libhello.so => /usr/local/lib/libhello.so (0x000079d4a57c8000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x000079d4a5400000) libworld.so => /home/imk/develop/so/scrap/libworld.so (0x000079d4a57c3000) libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x000079d4a5000000) /lib64/ld-linux-x86-64.so.2 (0x000079d4a57e7000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x000079d4a56da000) libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x000079d4a56ab000)
Nothing in your post suggests itself as a technical motive for linking libhello against libworld except possibly:
I do not want to place libworld.so in the default library search path.
The -rpath option answers that requirement because it enables linking shared libraries that are not in the default search directories. But you can equally use -rpath for that purpose in the linkage of a program as well as the linkage of a shared library. Without linking libhello against libworld, a program can load libworld.so from a non-default search directory with linkages like the following:
$ g++ -shared -o libworld.so world.o $ g++ -shared -o libhello.so hello.o # Then move `libhello.so` to a default search directory. $ g++ -o prog -lhello -L . -lworld -Wl,-rpath=$(pwd)
This would be the ordinary approach. The dynamic dependency info in libhello.so is the same as in libworld.so:
$ readelf --dynamic /usr/local/lib/libhello.so Dynamic section at offset 0x2e00 contains 24 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libstdc++.so.6] ...[cut]...
And in prog it looks like:
$ readelf --dynamic prog Dynamic section at offset 0x2d90 contains 30 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libhello.so] 0x0000000000000001 (NEEDED) Shared library: [libworld.so] 0x0000000000000001 (NEEDED) Shared library: [libc.so.6] 0x000000000000001d (RUNPATH) Library runpath: [/home/imk/develop/so/scrap] ...[cut]...
In CMake, you would implement an -rpath for the ppp target in the same way as you'd do it for libhello.
-allow-shlib-undefined? (you can build with -v to check)