I have a shared library that I wish to link an executable against using GCC. The shared library has a nonstandard name not of the form libNAME.so, so I can not use the usual -l option. (It happens to also be a Python extension, and so has no 'lib' prefix.)
I am able to pass the path to the library file directly to the link command line, but this causes the library path to be hardcoded into the executable.
For example:
g++ -o build/bin/myapp build/bin/_mylib.so
Is there a way to link to this library without causing the path to be hardcoded into the executable?
There is the ":" prefix that allows you to give different names to your libraries.
If you use
g++ -o build/bin/myapp -l:_mylib.so other_source_files
should search your path for the _mylib.so.
If you can copy the shared library to the working directory when g++ is invoked then this should work:
g++ -o build/bin/myapp _mylib.so other_source_files
If you are on Unix or Linux I think you can create a symbolic link to the library in the directory you want the library.
For example:
ln -s build/bin/_mylib.so build/bin/lib_mylib.so
You could then use -l_mylib
http://en.wikipedia.org/wiki/Symbolic_link
Related
I'm trying to compile a shared library I wrote in C++ to use a specific version of another shared library in the current directory, however it seems to be ignoring that and it uses the (older and incompatible) .so file in my LD_LIBRARY_PATH at runtime. How would I go about overriding the .so file it uses to use my own? I also need to retain the older version for another use on the same system.
Here's my command I'm using to compile: clang++ /data/openpilot/selfdrive/df/libs/libSNPE.so -lsymphony-cpu -lsymphonypower -I/data/openpilot/phonelibs/snpe/include -std=c++14 -lstdc++ -fPIC -o d_f.so dynamic_follow.cc -shared
/data/openpilot/selfdrive/df/libs/libSNPE.so being the library I want to use.
I also tried to use the -l flag before my library file, however it returns cannot find -l/data/openpilot/selfdrive/df/libs/libSNPE.so
Confirmed to still use the library in LD_LIBRARY_PATH with this command as well: clang++ -Wl,-rpath,/data/openpilot/selfdrive/df/libs -L/data/openpilot/selfdrive/df/libs -lSNPE -lsymphony-cpu -lsymphonypower -I/data/openpilot/phonelibs/snpe/include -std=c++14 -stdlib=libc++ -fPIC -o d_f.so dynamic_follow.cc -shared
The -L flag tells where to look for libraries at link time, while LD_LIBRARY_PATH tells where to look for libraries at run-time. So whatever path you set at link-time, this will be ignored when running the executable.
You need to have LD_LIBRARY_PATH include the directory of your dynamic library at run-time for your executable to find it. So you may run your executable like this:
LD_LIBRARY_PATH=/data/openpilot/selfdrive/df/libs:"$LD_LIBRARY_PATH" ./your-exec
The below compiles:
g++ -L../../lib -o my_prog my_prog.cpp -ltest1 -ltest2
where ../../lib contains symlinks to libtest1.so and libtest2.so
But I am getting an error when I run the program: "error while loading shared libraries: libtest1.so: cannot open shared object file: No such file or directory" and am not sure if symlinking is the culprit.
Option -L is for linker ld to find .a and .so during linking.
Option -Wl,-rpath= is for the dynamic linker ld.so to find .so when the application is run. You need to use -Wl,-rpath= when a required shared library is not in (standard system) directories specified in /etc/ld.so.conf.
Use $ORIGIN dynamic linker variable to make rpath relative to the path of your executable:
g++ -L../../lib -Wl,-rpath='${ORIGIN}/../../lib' -o my_prog my_prog.cpp -ltest1 -ltest2
Be careful to make sure ${ORIGIN} is not expanded by the shell or your makefile (this is why it is in single quotes).
${ORIGIN}:
$ORIGIN and rpath
ld.so understands the string $ORIGIN (or equivalently ${ORIGIN}) in an rpath specification (DT_RPATH or DT_RUNPATH) to mean the directory containing the application executable. Thus, an application located in somedir/app could be compiled with gcc -Wl,-rpath,'$ORIGIN/../lib' so that it finds an associated shared library in somedir/lib no matter where somedir is located in the directory hierarchy. This facilitates the creation of "turn-key" applications that do not need to be installed into special directories, but can instead be unpacked into any directory and still find their own shared libraries.
What happens at runtime is related to the rpath.
You may want (not really recommended, see this) to set your LD_LIBRARY_PATH appropriately before running your executable, or better yet you want to set the executable's rpath when linking it (e.g. by passing -Wl,--rpath $(realpath ../../lib/) to the g++ command doing the link.
Read Drepper's How to write shared libraries paper and the Program Library HowTo
I was using gpgpu-sim, a GPU simulator, to conduct researches. There are several .so files in my own folder:
And there are some alternatives .so in Nvidia's cudart lib folder:
And there are some .o files and need to be linked with libcudart.so, when I type in the command:
g++ -L "Path/to/MyFolder" -l cudart *.o
I hope the generated a.out would link to libcudart.so, but it just linked to a strange so file:
libcudart_gpgpu-sim_git-commit-6443f21d433f1b642003867e56fe1f54efae55e3_modified_0.so => not found
And when I typed this code:
g++ -L "Path/to/NvidiaFolder" -l cudart *.o
The program can sussessfully find libcudart.so.9 in my LD_LIBRARY_PATH folder,but it shows that the version can't match!:
./a.out: /path/to/myFolder/libcudart.so.9.0: version `libcudart.so.9.0'not found (required by ./a.out)
Can anybody tell me how ld works and how to solve those problems?
I finally find out the reason.
if you use this code to link objects to generate a shared library:
g++ -shared -Wl,-soname,libNAME_A.so -o libNAME_B.so
then, if some on is trying to link NAME_B.so using:
g++ <INPUT> -lNAME_B -o <OUTPUT>
the output will finally look for libNAME_A.so.
refer to g++ man page:
-Wl,option
Pass option as an option to the linker. If option contains commas,
it is split into multiple options at the commas. You can use this
syntax to pass an argument to the option. For example,
-Wl,-Map,output.map passes -Map output.map to the linker. When
using the GNU linker, you can also get the same effect with
-Wl,-Map=output.map.
and for ld man page:
-soname=name
When creating an ELF shared object, set the internal DT_SONAME
field to the specified name. When an executable is linked with a
shared object which has a DT_SONAME field, then when the executable
is run the dynamic linker will attempt to load the shared object
specified by the DT_SONAME field rather than the using the file
name given to the linker.
There is nothing to do with CUDA here, it's just a linking and runtime environment setup problem.
The ld linker searches for objects and library archives following the order specified by -L option parameters, and only after into default system directories. The linker will link the object code that first match this search.
At runtime, if you linked against dynamic libraries (.so files) you will need to properly define the LD_LIBRARY_PATH environment variable with a list of paths to look for dynamic libraries, separated by colon (:).
So if you link to your objects using libraries from your local path (assuming you are looking for libcudart.so):
g++ -o myprogram *.o -L "/Path/to/myFolder" -lcudart
you need to set LD_LIBRARY_PATH as follows before running your program:
export LD_LIBRARY_PATH="/Path/to/myFolder:$LD_LIBRARY_PATH"
./myprogram
I hope this help and clarify your understanding. Frankly I don't understand the origin of your libcudart_gpgpu-sim_git-commit match
I have the following problem:
I have two separate c++ projects, and want to use certain functions from one of them in the other. I am compiling using g++, on Linux.
I decided to do this by creating a shared library out of the project from which to use the functions. I added -fPIC to the compiler flags in the Makefile and then created a shared library like this:
g++ -shared -Wl,-soname,libmyproject.so.1 -o libmyproject.so a.o b.o c.o -lc
Then I simply copied the .so file and the header files into the (parent) directory of the new project and added the following to its Makefile:
To LIBS:
-L/../external_proj/libmyproject.so
To CXXFLAGS:
-I/../external_proj
Then I #include the appropriate header file in the destination project code and try to call certain functions from the original project. However, when I compile the destination project I get an error "undefined reference" for the imported function.
My question is then: is there something I'm missing in this setup? Is there perhaps something that needs to be added to the headers in the original project in order to export the functions I want to use?
I should note this is the first time I have attempted to use a shared library in this way. Thanks.
The -L option only specifies the directory where the linker will search for libraries to link with. Then you will need to use the -l option to specify the base name of the shared library (without the "lib" prefix and the ".so" suffix).
But even that will unlikely to be enough. The runtime loader needs to find the shared library, when you attempt to try to execute it. -L and -l will be sufficient to successfully link, but the runtime loader only searches /usr/lib(64)?, and a few other places by default. It does NOT search the current directory, and the ELF binary only records the names of the shared libraries that must be loaded, and not their full pathnames. You have to explicitly record any extra directories to search for any shared libraries, which is the -rpath option.
To finish the job you will also need to pass -rpath to the linker, but g++ does not support this option directory, you will have to use -W to do that.
The full set of options you will likely need are:
-L/../external_proj -lmyproject -Wl,-rpath -Wl,`cd ../external_proj && pwd`
See gcc documentation for more information on the -W option.
Absolute pathnames should be used with -rpath, hence the need to obtain the full pathname to the directory where the shared library is.
The -L flag is to add a path to search libraries in. The -l (lower-case L) is for linking with a library in the search path.
Or you can skip the flags and link with the library directly, almost like you do now (but without the -L option).
If you use the -l option, then remember that for a file libname.so you use only name as the library name. As in -lname. The linker will search for the correct files with the added prefix and suffix.
And lastly an important note about the paths used when linking: If you use -L and -l to link with a shared library, it's only the linker which will find the library. The OS runtime-loader will not be able to see the path used and will not find the library, if it's in a non-standard location. For that you must also set the runtime-path using the special linker option -rpath.
Unfortunately the GCC frontend program g++ doesn't recognize that option, you have to use -Wl to tell g++ to pass on an option to the actual linker. As in -Wl,-rpath,/path/to/libraries.
To summarize, here are the different variants you can use:
Link directly with the library: g++ your_source.cpp ../external_proj/libmyproject.so
Use the -L and -l options: g++ your_source.cpp -L../external_proj -lmyproject
To set the runtime linker path: g++ your_source.cpp -L../external_proj -lmyproject -Wl,-rpath,../external_proj
I am currently trying to link a library using MinGW with the following command:
g++ main.cpp -l"C:\Users\Trent\Desktop\glfw3"
This command does not work, however the following does:
g++ main.cpp -lglfw3
Because of this, I think the compiler is probably searching for glfw3.dll in C:\Users\Trent\Desktop\C:\Users\Trent\Desktop which obviously doesn't exist. Is there a way to tell G++ to search for a library using an absolute path rather than a relative one?
P.S. The main.cpp file contains no code, I am simply just trying to link a DLL before I actually write anything.
For gcc family -l is option to specify library name, it searches names in system folders (defined in environment), you can add folders to lookup list by the -L option, just like VTT commented.