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90 lines
4.5 KiB
Markdown
90 lines
4.5 KiB
Markdown
# About LD_LIBRARY_PATH and library paths
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## Background
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Libraries are hard. Libraries can be needed for user code, but also to run compilers.
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For CE we use a lot of different compilers and environments that need to be separated from the OS's installation, and
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that makes things more complicated than just building with your standard OS's compiler.
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Including header files or equivalent is usually the easy part. If there are binaries involved, things get complicated.
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We have a couple of separate stages where we use and mix different techniques to be able to produce the right assembly
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or executables.
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- Compilation without linking
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- The `LD_LIBRARY_PATH` environment variable is used here to enable the compiler to find the `.so` files that they
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need to run.
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- If you're running a local installation, this is usually your own systems' `LD_LIBRARY_PATH` plus extra things that
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CE adds through properties.
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- On godbolt.org we always start with an empty `LD_LIBRARY_PATH` and add what is set in the properties.
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- Building an executable or binary
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- We use `-Wl,-rpath=` (or equivalent `rpathFlag`) to force library paths into the executable so that they will always
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find the same `.so` files no matter where they are run. Usually this also includes lib64 and lib folders that the
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compiler offers for standard libraries that the toolchain offers.
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- Library paths supplied through `-Wl,-rpath=` for shared libraries will be able to dynamically link to the right
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architecture's `.so` even if multiple paths are given that contain the same `.so` file.
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- We use `-L` (or equivalent `libpathFlag`) to enable the compiler to find both static (`.a`) and shared (`.so`)
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libraries.
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- We always add '.' as a path as well because that's where we put libraries that are downloaded from our Conan server.
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- We use `-l` (or equivalent `linkFlag`) to say we want to statically or dynamically link to a named library binary
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(the compiler and linker decide if it's gonna be static or dynamic).
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- Running the executable
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- We use `LD_LIBRARY_PATH` just in case these are dependencies inherited from the compiler - and for the libraries
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that are used (also just in case).
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## Specific properties that are used in certain situations
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- Compiler .ldPath
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- is used for `LD_LIBRARY_PATH` to support running the compiler
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- is used for linking (`-Wl,-rpath=` and/or `-L`) during building binaries
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- is used for `LD_LIBRARY_PATH` to enable the users's executable to find `.so` files
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- Compiler .libPath
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- is used for linking (`-Wl,-rpath=` and/or `-L`) during building binaries
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- is used for `LD_LIBRARY_PATH` to enable the users's executable to find `.so` files
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- Library .libPath
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- is used for linking (`-Wl,-rpath=` and/or `-L`) during building binaries
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- is used for `LD_LIBRARY_PATH` to enable the users's executable to find `.so` files (just in case)
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## Example
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Say we have the following things in a `c++.local.properties` file:
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```INI
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compilers=mycl
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compiler.mycl.exe=/home/ubuntu/mycl/bin
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compiler.mycl.ldPath=/home/ubuntu/mycl/lib/lib64
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compiler.mycl.libPath=/home/ubuntu/mycl/lib/lib64:/home/ubuntu/mycl/lib/lib32
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compiler.mycl.options=--gcc-toolchain=/home/ubuntu/gcc10
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compiler.mycl.includeFlag=-I
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libs=mylib
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libs.mylib.name=My library
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libs.mylib.path=/home/ubuntu/mylib/include
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libs.mylib.libpath=/home/ubuntu/mylib/lib
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libs.mylib.staticliblink=mylib
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```
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This will result in the following situations if we want to compile some code with both the mycl compiler and the mylib
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library:
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- Compilation without linking
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- `LD_LIBRARY_PATH` is set to `/home/ubuntu/mycl/lib/lib64`
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- `-I/home/ubuntu/mylib/include` is added to the compilation arguments
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- Building an executable or binary
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- `LD_LIBRARY_PATH` is set to `/home/ubuntu/mycl/lib/lib64`
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- The following are added to the compilation arguments
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- `-I/home/ubuntu/mylib/include` (library include path)
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- `-Wl,-rpath=/home/ubuntu/mycl/lib/lib64` (compiler library paths)
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- `-Wl,-rpath=/home/ubuntu/mycl/lib/lib32`
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- `-Wl,-rpath=.` (conan library path)
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- `-L.`
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- `-Wl,-rpath=/home/ubuntu/gcc10/lib/lib` (gcc toolchain library paths)
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- `-Wl,-rpath=/home/ubuntu/gcc10/lib/lib32`
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- `-Wl,-rpath=/home/ubuntu/gcc10/lib/lib64`
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- `-Wl,-rpath=/home/ubuntu/mylib/lib` (mylib library path - just in case there are `.so` files used)
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- `-L/home/ubuntu/mylib/lib` (mylib library path used to find `libmylib.a`)
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- `-lmylib` (mylib library name)
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- Running the executable
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- `LD_LIBRARY_PATH` is set to `/home/ubuntu/mycl/lib/lib64:/home/ubuntu/mycl/lib/lib32:/home/ubuntu/mylib/lib`
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