How may I tell dub where to find a C library for linking?

[Pab De Nápoli]

On Monday, 10 December 2018 at 05:24:16 UTC, evilrat wrote:
> On Monday, 10 December 2018 at 02:59:21 UTC, Pablo De Nápoli 
> wrote:
>>
>> On my system (Debian GNU/Linux 9, 64 bits) the library is in 
>> the directory /usr/lib/llvm-6.0/lib/
>>
>> $ ls -l /usr/lib/llvm-6.0/lib/libLLVM.so
>> lrwxrwxrwx 1 root root 14 oct 24 19:44 
>> /usr/lib/llvm-6.0/lib/libLLVM.so -> libLLVM-6.0.so
>>
>> But how am I supposed to tell dub this, so it passes this 
>> information to the linker??
>>
>> I guess I should set "libs" in dubs.json
>>
>
> IIRC your guess is correct. "libs" is passed directly to linker 
> and is the only way to do it, so you just do smth like this
>
>    "libs" : ["-L/usr/lib/llvm-6.0/lib/", "libLLVM-6.0"]
>
> note that this -L will be extended for linker as -L-L...
> (also note that you can do platform specific way with 
> "libs-windows", "libs-posix"? to pick proper paths per platform)
>
> Though I'm not a linux pro and don't remember if linking to .so 
> is same as linking import library, if this is not the case you 
> just need to set RPATH or whatever it is so it can find dynamic 
> lib.

Many thanks but that didn't work,as I got the message

$ dub build
Performing "debug" build using /usr/bin/dmd for x86_64.
llvm-d 2.4.1: target for configuration "native-target" is up to 
date.
fibonacci ~master: building configuration "link-single"...
Linking...
/usr/bin/ld: cannot find -l-L/usr/lib/llvm-6.0/lib/
/usr/bin/ld: cannot find -llibLLVM-6.0
collect2: error: ld returned 1 exit status
Error: linker exited with status 1
/usr/bin/dmd failed with exit code 1.

The problem is that GNU ld expects the -L for specifying the 
search path, not -l

What actually did work was:

1) Setting the LD_LIBRARY_PATH environment variable with

export LD_LIBRARY_PATH=/usr/lib/llvm-6.0/lib/

and using

  "libs" : ["LLVM-6.0"]

in dub.json. (this is somewhat nasty, it would be nice to keep 
all the information together in dub.json)

Moreover, the information about the path for linking with LLVM 
can be obtained from the shell script llvconfig as

$llvm-config-6.0 --libs --ldflags
-L/usr/lib/llvm-6.0/lib
-lLLVM-6.0

It would be better not to hard-code this information in dub.json 
but I don't know if it possible to invoke a shell script with 
something like
$(llvm-config-6.0 --libs --ldflags) (as you would do in a 
Makefile)

This method used to be employed by many other libraries besides 
llvm (but many are migrating to pkgconfig).

Many thanks!)