Compiling C++ Code in a Conda Environment Using Conda-managed Headers and Libraries

Prerequisites

Common Build Variables

Directly Used by gcc/clang

These environment variables are automatically used by gcc/clang themselves.

• CPATH
  • Colon-separated list of directories to search for headers before built-in include paths.
  • Like passing multiple -I options.
• LIBRARY_PATH
  • Colon-separated list of directories to search for libraries (.so/.a) at link time.
  • Like passing multiple -L options.
  • Doesn't affect how the executable finds shared libraries when running.

For Build Systems (make, cmake, etc.)

These environment variables are not used by compilers unless your build tool (make, cmake, etc.) or script expands them.

• CC / CXX:
  • Which C/C++ compiler to use.
• CFLAGS / CXXFLAGS:
  • Extra flags for compiling C or C++ respectively, e.g., -O2 -g -Wall.
• LDFLAGS:
  • Extra flags when linking, such as -L/path/to/lib, -lfoo, -Wl,-rpath,/my/libs.

🚫 Never stuff include/library/link flags inside $CXX or $CC. Always set them as separate variables, or pass them directly on the command line to the compiler.

C++ Libraries in Conda Environments

When you install a C++ library from conda-forge, it's placed inside your current environment, not in a system-wide directory.

• Headers: $CONDA_PREFIX/include
• Libraries: $CONDA_PREFIX/lib

Exception: The C runtime (libc.so, libm.so, etc.) always comes from the system, not Conda.

Example

Create a Conda Environment for C++ Compilation

conda install -c conda-forge clang clangxx libcxx libcxxabi libcxx-devel lld llvm-tools

• libcxx, libcxxabi, libcxx-devel: LLVM's C++ standard library and headers.

Tell the Compiler Where to Look for Headers and Libraries

export CPATH="$CONDA_PREFIX/include"
export LIBRARY_PATH="$CONDA_PREFIX/lib"

This automatically adds Conda's include and lib directories to clang's search paths.

Set Common Build Variables for Build Systems

export CC="clang"
export CXX="clang++ -stdlib=libc++"
export LDFLAGS="-Wl,-rpath,$LIBRARY_PATH"
export AR="$CONDA_PREFIX/bin/llvm-ar"
export RANLIB="$CONDA_PREFIX/bin/llvm-ranlib"
export LD="$CONDA_PREFIX/bin/lld"

• LDFLAGS sets an rpath such that the generated executable or shared library will hard-code LIBRARY_PATH as a shared library search path.

Compile a Program

Let's say you have hello_world.cpp.

$CXX hello_world.cpp -o hello_world $LDFLAGS

Check Your Binary

You can check what dynamic libraries your binary depends on using ldd hello_world:

$ ldd hello_world
    linux-vdso.so.1 (0x00007ffd05ead000)
    libc++.so.1 => /home/youruser/miniconda3/envs/clangxx-env/lib/libc++.so.1 (0x000071f5b0e86000)
    libc++abi.so.1 => /home/youruser/miniconda3/envs/clangxx-env/lib/libc++abi.so.1 (0x000071f5b0e48000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x000071f5b0d44000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x000071f5b0a00000)
    ...

Good sign: For anything above libc, it should point inside your Conda environment (e.g., libc++.so.1), not the system.