program without libc (without std lib) leads to cryptic error

[phil-opp]

The code at http://doc.rust-lang.org/guide-unsafe.html#avoiding-the-standard-library requires libc. It isn't possible to remove this line or to replace it with the platform independent rlibc. The following error occurs (playpen: http://is.gd/GWl8YT):

error: linking with `cc` failed: exit code: 1
note: cc '-m64' '-L' '/usr/lib64/rustlib/x86_64-unknown-linux-gnu/lib' '-o' 'out' 'out.o' '-Wl,--whole-archive' '-lmorestack' '-Wl,--no-whole-archive' '-nodefaultlibs' '-Wl,--gc-sections' '-pie' '-Wl,--as-needed' '-Wl,-O1' '-L' '/home/rust/.rust' '-L' '/home/rust' '-Wl,--whole-archive' '-Wl,-Bstatic' '-Wl,--no-whole-archive' '-Wl,-Bdynamic' '-lcompiler-rt'
note: /usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.1/../../../../lib/Scrt1.o: In function `_start':
(.text+0x12): undefined reference to `__libc_csu_fini'
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.1/../../../../lib/Scrt1.o: In function `_start':
(.text+0x19): undefined reference to `__libc_csu_init'
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.1/../../../../lib/Scrt1.o: In function `_start':
(.text+0x25): undefined reference to `__libc_start_main'
collect2: error: ld returned 1 exit status

error: aborting due to previous error

I want to use libcore, that doesn't require libc (http://doc.rust-lang.org/core/). The libcore code example in the guide (http://doc.rust-lang.org/guide-unsafe.html#using-libcore) has no libc dependency, but the playpen link in the top right of the code example reinserts it and removing it leads to the same error as above (http://is.gd/sxVAr0).

[Virtlink]

I also encountered this error in Rust 0.13 nightly, and it prevents me from continuing.

[thestinger]

The rlibc library provides a low quality implementation of a few primitives required by LLVM for the freestanding use case. It is not going to replace the platform's standard C library in a hosted environment. This is a documentation issue, not a bug in the compiler or libraries.

It's not likely that Rust will support running in a hosted environment without the platform's usual entry point. It does support freestanding usage, but in that environment the programmer is responsible for providing the entry code.

[Virtlink]

So, if I understand correctly, extern crate libc removes the errors but the resulting binary won't be freestanding. If I want a freestanding binary, how would I go about providing the missing functions __libc_csu_fini, __libc_csu_init and __libc_start_main (and others)?

[thestinger]

Those functions are provided by the C runtime and it's not trivial to replicate all of the functionality yourself. You need to handle differences between Linux kernel versions and set up stuff like the vdso yourself. It would be a much better idea to use a C standard library implementation like musl... I can't see an advantage to reinventing this wheel.

[thestinger]

Anyway, you really don't want to be calling the raw system calls yourself. For example, the setuid system call really only changes the uid of the thread rather than the process and you need to set up a bunch of real-time signal magic to interrupt the other threads and set it globally. That's just a small example of the obstacles you're going to face if for some reason you decide to build everything from the ground up. It's certainly not necessary to get a binary without any external library dependencies.

[phil-opp]

If you want to write an OS Kernel or something like that compile your crate as a staticlib and link it manually with some entry code (that provides a stack and sets the stack limit in fs:XY). Not sure if there's a better way to do this but at least it works :)

[Virtlink]

Thanks, I indeed intended to try my hand at writing an operating system kernel, which is why I was looking at freestanding Rust. I'll look into staticlib.

[steveklabnik]

This doesn't seem to be an actual bug, then.

[carlpaten]

@steveklabnik maybe the real bug is how cryptic the error is?

[steveklabnik]

@LilRed this issue was opened a very long time ago, does the same thing happen today? I've been doing osdev and I haven't run into it.

[carlpaten]

apparently yes

[phil-opp]

Just a quick brain dump since I'm on mobile right now:

I think I ran into this error back then when I tried to build a no_std executable. The solution was to build it as a staticlib instead and link it manually.

I think the main problem is that an executable isn't well defined without an operating system and libc. But the docs were/are very sparse on this topic, so I think we should at least improve them.

The error message is so cryptic because it is an error from the linker. I'm not sure if it can be improved easily.