Discuss external linking and linking order

[petrochenkov]

So you get linking errors when using lapacke-sys without some other crate linking to liblapacke.

This was found when testing some linking order changes in rustc in rust-lang/rust#102832 (comment) (build failure - https://crater-reports.s3.amazonaws.com/pr-102832-1/try%230c94de7c1a8f4b82b92d562847c35f513f520c7a/gh/lopez86.rust-mlearn/log.txt).

The dependency on liblapacke should be declared using a #[link] attribute or a build script with cargo:rustc-link-lib.

[IvanUkhov]

This is correct. The crate is not doing any linkage. Please refer to the architecture.

[petrochenkov]

That architecture worked largely by luck so far.
You can see from the build failure log that the crate that uses symbols (lapacke-sys) doesn't declare a dependency on a library that defines those symbols (liblapacke.(so,a)). In general, rustc cannot build a correct linker command line in such situation.

[IvanUkhov]

There might be some misunderstanding on my side. The linking happens when one chooses a source. In the case of the Netlib source, for instance, the corresponding instructions are given here:

https://github.com/blas-lapack-rs/netlib-src/blob/master/build.rs

Are you referring to this or something else?

[tari]

It looks like you've got the same problem as llvm-sys, where the set of linked libraries cannot be known until build-time. It seems like we'd need Cargo to support emitting an equivalent of links in the manifest from build scripts to make this work correctly.

[IvanUkhov]

Yes, this is accurate. Thank you for looking into this!

[petrochenkov]

I'll also look in more detail how this crate and llvm-sys setup things later this week.

The linking happens when one chooses a source.

I guess the main source of misunderstanding may come from the fact that it's not enough to link to the necessary library somewhere on the command line, it needs to be linked after it's used due to the order-dependent way in which unix-style linkers work (and the order is inferred from "crate -> crate" and "crate -> native lib" dependencies in rustc using topological sorting).

[IvanUkhov]

Thank you. Let us add the following to the list:

• blas-sys,
• cblas-sys,
• lapack-sys, and
• lapacke-sys.