From 08dccd05a79d2cf2bdd4b6cd8ccaa8baa02b0a54 Mon Sep 17 00:00:00 2001
From: Martin Pitt <martin@piware.de>
Date: Fri, 27 Aug 2021 08:43:12 +0200
Subject: [PATCH] Initial libmount function call from Rust

Follow "A little C with your Rust" [1].

Set up cargo and main.rs skeleton and call one of the simplest possible
functions from libmount: mnt_get_fstab_path().

Use bindgen [1] to translate libmount's header file into a Rust
interface. This requires a few hacks:

 - bindgen does not accept the original header right away, as it can't
   find stdarg.h. Avoid this by dropping the (unnecessary) stdio.h
   include.
 - bindgen does not know about `size_t`. Hack around that with a typedef
   for now.
 - Suppress warnings about constant/identifier naming.

[1] https://rust-lang.github.io/rust-bindgen/command-line-usage.html
[2] https://docs.rust-embedded.org/book/interoperability/c-with-rust.html
---
 .gitignore                     |  1 +
 call-c-from-rust/.cargo/config |  2 ++
 call-c-from-rust/Cargo.toml    |  8 ++++++++
 call-c-from-rust/Makefile      | 16 ++++++++++++++--
 call-c-from-rust/src/main.rs   | 10 ++++++++++
 5 files changed, 35 insertions(+), 2 deletions(-)
 create mode 100644 call-c-from-rust/.cargo/config
 create mode 100644 call-c-from-rust/Cargo.toml
 create mode 100644 call-c-from-rust/src/main.rs

diff --git a/.gitignore b/.gitignore
index 5c03031..7ecae2e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,5 +3,6 @@
 Cargo.lock
 target
 
+libmount.rs
 c-langinfo
 c-mounts
diff --git a/call-c-from-rust/.cargo/config b/call-c-from-rust/.cargo/config
new file mode 100644
index 0000000..53fb2de
--- /dev/null
+++ b/call-c-from-rust/.cargo/config
@@ -0,0 +1,2 @@
+[build]
+rustflags = "-C link-args=-lmount"
diff --git a/call-c-from-rust/Cargo.toml b/call-c-from-rust/Cargo.toml
new file mode 100644
index 0000000..535c6b9
--- /dev/null
+++ b/call-c-from-rust/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "learning-c-from-rust"
+version = "0.1.0"
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/call-c-from-rust/Makefile b/call-c-from-rust/Makefile
index 583e2f2..cbeefdd 100644
--- a/call-c-from-rust/Makefile
+++ b/call-c-from-rust/Makefile
@@ -1,7 +1,7 @@
 CFLAGS += $(shell pkg-config --cflags mount)
 LIBMOUNT = $(shell pkg-config --libs mount)
 
-all: c-mounts c-langinfo
+all: c-mounts c-langinfo target/debug/learning-c-from-rust
 
 c-mounts: c-mounts.o
 	$(CC) -Wall -o $@ $^ $(LIBMOUNT)
@@ -9,13 +9,25 @@ c-mounts: c-mounts.o
 c-langinfo: c-langinfo.o
 	$(CC) -Wall -o $@ $^
 
+# HACK: does not find stdarg.h (through stdio.h), missing size_t
+# also avoid hundreds of "should have an upper camel case name" warnings
+src/libmount.rs:
+	sed 's/#include.*stdio.h.*/typedef unsigned long size_t;/' /usr/include/libmount/libmount.h > libmount-hack.h
+	bindgen libmount-hack.h -- -I/usr/include/linux/ > $@.tmp
+	(echo '#![allow(non_camel_case_types,non_upper_case_globals,dead_code)]'; cat $@.tmp) > $@
+	rm libmount-hack.h $@.tmp
+
+target/debug/learning-c-from-rust: src/libmount.rs src/main.rs
+	cargo build
+
 clean:
-	rm -f c-mounts c-langinfo *.o
+	rm -f c-mounts c-langinfo *.o src/libmount.rs
 
 run: all
 	LANG=en_US.UTF-8 ./c-langinfo
 	LANG=en_GB.UTF-8 ./c-langinfo
 	LANG=en_IE.UTF-8 ./c-langinfo
 	./c-mounts
+	target/debug/learning-c-from-rust
 
 .PHONY: clean run
diff --git a/call-c-from-rust/src/main.rs b/call-c-from-rust/src/main.rs
new file mode 100644
index 0000000..b453d81
--- /dev/null
+++ b/call-c-from-rust/src/main.rs
@@ -0,0 +1,10 @@
+mod libmount;
+
+use std::ffi::CStr;
+
+fn main() {
+    let fstab_path_cstr: &CStr = unsafe { CStr::from_ptr (libmount::mnt_get_fstab_path()) };
+    println!("fstab path C-String: {:?}", fstab_path_cstr);
+    let fstab_path = fstab_path_cstr.to_str().unwrap(); // may cause UTF-8 decoding error
+    println!("fstab path str: {}", fstab_path);
+}
-- 
2.39.2