The thumb* target family (which have newlib libc) are not supported

[SimonSapin]

I have embedded hardware without an OS. I build Rust code for it with --target thumbv7em-none-eabi and this thumbv7em-none-eabi.json file:

{
    "arch": "arm",
    "cpu": "cortex-m4",
    "data-layout": "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64",
    "disable-redzone": true,
    "executables": true,
    "llvm-target": "thumbv7em-none-eabi",
    "morestack": false,
    "os": "none",
    "relocation-model": "static",
    "target-endian": "little",
    "target-pointer-width": "32",
    "no-compiler-rt": true,
    "pre-link-args": [
        "-mcpu=cortex-m4", "-mthumb",
        "-Tlayout.ld"
    ],
    "post-link-args": [
        "-lm", "-lgcc", "-lnosys"
    ]
}

If I add a Cargo dependency on the libc crate and build with this target, I get many build errors. The first and last are below. The entire thing is at https://gist.github.com/anonymous/780fe74b2ae2834fc9c738dbff60acc0

   Compiling libc v0.2.15
error[E0412]: type name `c_char` is undefined or not in scope
   --> /home/simon/tmp/cargo-home/registry/src/github.com-1ecc6299db9ec823/libc-0.2.15/src/lib.rs:153:35
    |
153 |     pub fn fopen(filename: *const c_char,
    |                                   ^^^^^^ undefined or not in scope
    |
    = help: no candidates by the name of `c_char` found in your project; maybe you misspelled the name or forgot to import an external crate?


// Removed: ~4000 more lines


error[E0204]: the trait `Copy` may not be implemented for this type
   --> /home/simon/tmp/cargo-home/registry/src/github.com-1ecc6299db9ec823/libc-0.2.15/src/macros.rs:44:9
    |
44  |         impl ::dox::Copy for $i {}
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ field `rlim_cur` does not implement `Copy`
    | 
   ::: /home/simon/tmp/cargo-home/registry/src/github.com-1ecc6299db9ec823/libc-0.2.15/src/lib.rs
    |
261 | cfg_if! {
    | - in this macro invocation

error: aborting due to 5 previous errors

error: Could not compile `libc`.

[SimonSapin]

While it is possible to make a "bare metal" program with nothing but Rust and a linker, the arm-none-eabi-gcc toolchain does have a libc called newlib.

[alexcrichton]

This is pretty standard for most new platforms, I think. I don't think liblibc will ever compile for an arbitrary new platform because it has to know what the actual definitions are. The theory is that the backing libc implementation will almost always have bindings, however. (although right now we don't have newlib bindings in tree)

[FenrirWolf]

Any chance of newlib support being added at some point?

[posborne]

Any chance of newlib support being added at some point?

The newlib API surface is going to be quite small compared to other libc implementations, but I'm guessing the answer would be yes so long as people are willing to contribute and it can be tested by CI. I have used newlib on embedded systems before and can see cases where having libc provide access would be useful.

For pure-rust embedded systems, you might want to check out rlibc which provides bare bones required functions: https://github.com/alexcrichton/rlibc

[kristate]

Trying to clean-up issues: is this still an issue @SimonSapin ? (Thanks for your work on Servo!)

[SimonSapin]

Yes.

The thumbv7m-none-eabi target is now supported by rustc, thought xargo is needed to build libcore. This issue can be reproduced by making a no_std crate, adding libc = {version = "*", default-features = false} to its dependencies, and running xargo build --target thumbv7m-none-eabi

   Compiling libc v0.2.37
error[E0412]: cannot find type `c_char` in this scope
   --> /home/simon/tmp/cargo-home/registry/src/github.com-1ecc6299db9ec823/libc-0.2.37/src/lib.rs:171:43
    |
171 |             pub fn fopen(filename: *const c_char,
    |                                           ^^^^^^ did you mean `c_schar`?

error[E0412]: cannot find type `c_char` in this scope
   --> /home/simon/tmp/cargo-home/registry/src/github.com-1ecc6299db9ec823/libc-0.2.37/src/lib.rs:172:39
    |
172 |                          mode: *const c_char) -> *mut FILE;
    |                                       ^^^^^^ did you mean `c_schar`?

[…]

error[E0412]: cannot find type `c_long` in this scope
   --> /home/simon/tmp/cargo-home/registry/src/github.com-1ecc6299db9ec823/libc-0.2.37/src/lib.rs:278:39
    |
278 |             pub fn labs(i: c_long) -> c_long;
    |                                       ^^^^^^ not found in this scope

error: aborting due to 76 previous errors

If you want more information on this error, try using "rustc --explain E0412"
error: Could not compile `libc`.

[FenrirWolf]

There are some newlib bindings in-tree for ARM targets now, which should get selected if you compile with the target-env set to newlib in some fashion (adding "env": "newlib" to a custom target spec would do the trick). But there would probably have to be some extra changes made to add thumb support and possibly to account for toolchain differences.

[SimonSapin]

A first step would be defining c_char and other primitive type aliases for those targets. Even if there isn’t bindings for newlib functions yet, this would already help with FFI. (std::os::raw is not available in core.)

[gbip]

Is there someone tackling this issue ? This is a blocking issue for an embedded project. If it is just defining type aliases I can take care of it 😄

[SimonSapin]

Since this issue was opened, some thumb* targets have been added to be built-in in the compiler: https://forge.rust-lang.org/platform-support.html. I think it makes sense to make the libc crate compile out of the box for those targets, and provide at least the c_* type aliases. @gbip, wanna make a PR?

[gbip]

Yes I will try to tackle this issue within the end of this week !
Can anyone give me a little insight on where should I look ?

[SimonSapin]

@FenrirWolf In this repository, newlib seems to be only mentioned under src/unix/ which is under #[cfg(unix)] which presumably should not apply to microcontrollers without a kernel / OS.

@gbip Steps to reproduce, in this repository:

rustup target add thumbv7em-none-eabi
cargo build --target thumbv7em-none-eabi --no-default-features

You should see a bunch of errors like:

error[E0412]: cannot find type `c_char` in this scope
   --> /home/simon/tmp/cargo-home/registry/src/github.com-1ecc6299db9ec823/libc-0.2.43/src/lib.rs:172:43
    |
172 |             pub fn fopen(filename: *const c_char,
    |                                           ^^^^^^ did you mean `c_schar`?

They are all caused by c_char, c_long, c_ulong, or wchar_t not being defined. For other platforms, these type aliases are defined in platform-specific files that are included from this part at the bottom of src/lib.rs:

libc/src/lib.rs

Lines 298 to 320 in 41944d5

	cfg_if! {
	if #[cfg(windows)] {
	mod windows;
	pub use windows::*;
	} else if #[cfg(target_os = "redox")] {
	mod redox;
	pub use redox::*;
	} else if #[cfg(target_os = "cloudabi")] {
	mod cloudabi;
	pub use cloudabi::*;
	} else if #[cfg(target_os = "fuchsia")] {
	mod fuchsia;
	pub use fuchsia::*;
	} else if #[cfg(target_os = "switch")] {
	mod switch;
	pub use switch::*;
	} else if #[cfg(unix)] {
	mod unix;
	pub use unix::*;
	} else {
	// Unknown target_family
	}
	}

https://doc.rust-lang.org/reference/attributes.html#conditional-compilation documents what’s available to cfg* macros. I think cfg(target_arch = "arm", target_os = "none") would be relevant here, but I’m not sure if there’s a single definition of all those aliases that would be correct for all such targets…