[aur-general] How to pkg-config (Was: TU application: Daurnimator)

[Eli Schwartz]

On 12/11/18 6:19 PM, Daurnimator wrote:
>> If lua does not officially compile a C++ version, it is the job of
>> Debian to both provide their own pkg-config files, and modify lua to
>> build using C++.
>
> Lua supports either compiling with longjmp or C++ based exceptions.
> See "Error-recovery functions" near
> https://www.lua.org/source/5.3/ldo.c.html#errorstatus

Okay, that's fine. So how does this work, then, and why does it need a
separate pkg-config file? Is it just a matter of renaming liblua.a to
liblua-c++.a to prevent filename clashes and thereby permit parallel
installation? So... why is this any different from renaming the
versioned libraries?

Once again: for anyone wishing to provide a different version of the lua
library, interpreter, and/or pkg-config file should presumably modify
the same names in all three as a set. This does not detract from the
logic of supporting pkg-config via a pkg-config file meant to describe
the project itself.

> So some extra context may be required here. Lua needs to be used by
> two different types of code:
> 1. code that embeds lua
> 2. lua libraries written in C
> 
> For most unix systems (this is *not* true on windows):
> 
> Things in category 1 include the `lua` executable itself, as well as
> applications like mame, powerdns, vlc, etc.
> If they link against lua statically, then they should expose lua C
> symbols via e.g. -Wl,--export-dynamic
> If they link against lua dynamically then they are fine.

I don't get how that argues against pkg-config...

That sounds like the job of the application to use, that they know they
absolutely need to expose all symbols just in case and don't want to
mess with linker scripts for upstream symbols.

What's the cost of doing that in your embedded application that links
against liblua and finds a system-provided dynamic lib? The failure to
optimize your symbol table, when you didn't expect to be able to dynlink
at all?

But let's imagine instead, lua devs who support static only, and wish to
support pkg-config discovery.

This is lua.pc, no versions

Name: Lua
Version: 5.3.5
Description: An Extensible Extension Language
Cflags: -I${includedir}
Libs: -L${libdir} -Wl,--export-dynamic -llua -lm

Applications which desire to embed lua can link using:

CFLAGS += $(shell pkg-config --cflags lua)
LDLIBS += $(shell pkg-config --libs lua)

Applications which wish to build modules for lua can skip the LDLIBS
since they don't link to lua, and coincidentally also don't need to
--export-dynamic.

Downstream distros which patch lua to build shared libraries are also
responsible to optimize their ecosystem symbol tables by patching out
-Wl,--export-dynamic

Downstream distros that provide both dynamic and static libs can instead
move it to Libs.private, and pkg-config will only emit it when using:

pkg-config --static --libs lua

...

In all cases, the pkg-config file by design should work as-is, with the
assumption that it describes the project which is compiled using make &&
sudo make install. It's a madness game to even pretend to also support
the modifications which people make to the build system.

> Things in category 2 include libraries such as lua-filesystem. They
> expose an entrypoint of luaopen_mylibname, and use the lua C api.
> These should *not* link against liblua, and need to rely on symbols
> provided by the host application.
> 
> However note that both categories of code need to find the lua headers.

That's fine, it makes it even more convenient to reserve --libs for the
exclusive use of -Wl,--export-dynamic

-- 
Eli Schwartz
Bug Wrangler and Trusted User

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