如何找到 rustc --target=$TARGET 将链接到哪个 libc.so？

Question

I want to find the libc.so file that's being used in a Rust build so that I can query it with --version . (Some libcs expose their version information via C macros, so an alternative for them would be to use the cc crate in a build script. But others like musl don't.)

I can figure out which libstd-*.so file a rust binary or library will be linked against. When this libstd.so is linked against the host 's libc, then running ldd on it shows that libc.so . But when the host system is using glibc and the targeted environment is musl, this doesn't work ("Invalid ELF header"). Instead of ldd , I could instead use readelf -d or objdump -p on the libstd.so . But these only show the file name of the libc.so file it uses, not its full path. And that libc.so isn't at any of the directories in LD_LIBRARY_PATH . (I do know where it is on my own systems, but I'm trying to find it programmatically on arbitrary systems.)

Running ldconfig -p only gives me information about the libc for the host system.

It would be great if there were a rustc equivalent of gcc's and clang's -print-file-name=libc.so , so that I could do something like rustc --target=$TARGET --print-file-name=libc.so .

Other ideas about how I could get this information?

Answer 1

You can pass linker arguments to rustc like so:

rustc -C link-args=...

To find out which libc.so is used, I believe the following command should suffice:

rustc -C link-args=-Wl,-t ...

From man ld :

  -t
  --trace
       Print the names of the input files as ld processes them. ...

Update :

This didn't work: rustc "eats up" the output from the linker.

I was able to get the desired output indirectly:

echo 'fn main() { println!("")}' | rustc -C link-args=-Wl,-Map=map.out -o foo -
grep 'libc\.so' map.out

libc.so.6                     /usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-f25e49a311b0f577.rlib(std-f25e49a311b0f577.std.cy8lhng1-cgu.2.rcgu.o) (setuid@@GLIBC_2.2.5)
LOAD /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libc.so
LOAD /lib/x86_64-linux-gnu/libc.so.6