How are applications supposed to find libraries in /opt/local/lib and /opt/homebrew/lib ?

[larskanis]

People are using applications like ruby installed into their home directory, which make use of libraries installed by homebrew. Since homebrew doesn't install libraries into some system default library search path, but into /opt/local/lib and /opt/homebrew/lib, they aren't found without further measurements. That leaded to issues like libvips/ruby-vips#144 in the past.

As a workaround Ruby-FFI implements a hardcoded list of search paths, which is used as a fallback, if otherwise library loading would fail. This list was extended to the above two directories in https://github.com/ffi/ffi/pull/882/files to enable loading of homebrew libraries.

Is there a better way to locate homebrew libraries other than hardcoding filesystem paths into depending applications/libraries?

Why are there no system library load mechanisms to make sure the homebrew libraries are found? On Linux there's /etc/ld.so.conf.d/*.conf which is used to extend paths used by the dynamic loader of the system.

Is the assumption correct at all, that homebrew libraries are expected to work when used by non-homebrew applications?

How do homebrew's own applications find depending libraries? Something like LD_LIBRARY_PATH on Linux?

I'm sorry, my knowledge about homebrew and MacOS is very limited.

[carlocab]

This is build-system dependent, unfortunately. Usually, setting LDFLAGS=-L/opt/homebrew/lib should work (as long as the library you're looking for is not from a keg-only formula).

[larskanis]

I found that some environment variables are set by homebrew to enable homebrew binaries. Unfortunately there's nothing like LD_LIBRARY_PATH that points to the ${HOMEBREW_PREFIX}/lib directory. And even if there would be such a variable it would be disregarded by system executables (like ruby) as described here.

How about looking up into ${HOMEBREW_PREFIX}/lib by ruby-ffi (instead of absolute path)? Can we expect to have the HOMEBREW_PREFIX environment variable defined by all users of homebrew? Or is this optional?

[carlocab]

This is optional. You can get this with brew --prefix. You’ll want to cache this value if you use it in several places.

[larskanis]

Thank you! I think this answers my question.

[carlocab]

Sure. As an aside, adding RPATH entries to a binary object that calls dlopen (no _) is still useful. For example, I can dlopen("libcurl.dylib") and find Homebrew's libcurl if I've added $(brew --prefix)/opt/curl/lib to my binary's RPATHs (as long as I haven't added /usr/lib as an RPATH as well -- but that's in the default library search path anyway).

Not really familiar with the details of dl_open, but it's probably still just dlopen underneath.

[larskanis]

Sorry, I meant the well known dlopen() call - the dl_open() with underscore is just a ruby-ffi internal alias.