Library search paths wrong in musl cross builds

[bossmc]

While investigating #47493 I tried compiling the linked example code (https://gist.github.com/steveklabnik/5abd59a8fe7e5abda3db58bd8c208fbb) against the x86_64-unknown-linux-musl and noticed something weird - glibc was linked into my binary (unsuccessfully but still...):

Link command:

error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-nostdlib" "-Wl,--eh-frame-hdr" "-Wl,-(" "-m64" "/home/andy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-musl/lib/crt1.o" "/home/andy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-musl/lib/crti.o" "-L" "/home/andy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-musl/lib" "test.test0.rcgu.o" "test.test1.rcgu.o" "test.test2.rcgu.o" "-o" "test" "-Wl,--gc-sections" "-Wl,-z,relro,-z,now" "-nodefaultlibs" "-L" "/home/andy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-musl/lib" "-l" "c" "-Wl,-Bstatic" "/home/andy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-musl/lib/libcore-9d4b7130b0117f1c.rlib" "-static" "-Wl,-Bdynamic" "/home/andy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-musl/lib/crtn.o" "-Wl,-)"

Selected error:

/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/libc.a(iofclose.o): In function `_IO_new_fclose':
          (.text+0x256): undefined reference to `_Unwind_Resume'

Note the x86_64-linux-gnu in there!

The rust code (as is standard) merely says #[link(name="c")], which is translated into "-l" "c" in the link command, but the cc (which on my system is a glibc one) is searching in the standard glibc locations rather than the muslibc ones (which would be -L/usr/lib/x86_64-linux-musl according to musl-gcc -v <the rest of the above command>). Not sure how to guess the correct location to search, one obvious solution might be to use musl-gcc in place of cc when linking for the ...-musl targets, but that's not necessarily correct on all systems (e.g. on Alpine where cc would be correct as that system is musl-native).

[sfackler]

If you're cross compiling, you're generally going to have to override the linker, to musl-gcc in this case. This is configured in .cargo/config: https://doc.rust-lang.org/cargo/reference/config.html#configuration-keys.

[bossmc]

Ah, thank you, that's perfect. There's a bit of a lack of information about how to use the -musl targets (or how to cross compile), there used to be https://doc.rust-lang.org/1.12.1/book/advanced-linking.html but this seems to have been removed in newer editions.