added c++-ffi example

Author: BartMassey

README.md

@@ -49,3 +49,4 @@ in a controlled repository now.
 * `macro-nargs.rs`: example of recursive macro
 * `macro-tt.rs`: macro token tree example
 * `nullptr.rs`: null pointer example
+* `c++-ffi/`: example of FFI between C, C++ and Rust

c++-ffi/Makefile

@@ -0,0 +1,24 @@
+CC = clang
+CXX = clang++
+RUSTC = rustc -C panic="abort"
+
+OBJS = add1-c.o add1-rs.a demo.o
+
+TARGETS = demo-c demo-rs
+
+all: $(TARGETS)
+
+demo-c: demo.o add1-c.o
+	$(CXX) -o demo-c demo.o add1-c.o
+
+demo-rs: demo.o add1-rs.a
+	$(CXX) -o demo-rs demo.o add1-rs.a -lpthread -ldl
+
+add1-c.o:
+	$(CC) -c -o add1-c.o add1.c
+
+add1-rs.a: add1.rs
+	$(RUSTC) -o add1-rs.a add1.rs
+
+clean:
+	-rm -f $(TARGETS) $(OBJS)

c++-ffi/README.md

@@ -0,0 +1,26 @@
+# c++-ffi: Providing a C function to C++ code from Rust or C.
+
+This directory contains
+
+* A C++ driver `demo.cc`
+
+* A function called by the driver and written in C in `add1.c`
+
+* A function called by the driver and written in Rust in
+  `add1.rs`
+
+* A `Makefile` that makes the whole thing compile
+
+This has only been tested on Debian Linux with `clang` /
+`clang++`: other platforms and compilers might work. Who
+knows?
+
+The `demo-rs` executable is quite large: 4MB before stripping
+debug symbols, 1MB after. The executable brings in big
+pieces of `std` for various reasons. The good news is that
+this scales reasonably — adding a bunch of extra Rust isn't
+going to change it substantially.
+
+The build is controlled by `make`. One could add a fancy
+`build.rs` and build it all with Cargo; doesn't seem worth
+it here.

c++-ffi/add1.c

@@ -0,0 +1,8 @@
+#include <stdio.h>
+
+#include <inttypes.h>
+int64_t add1(int64_t *x, char * msg) {
+    *x += 1;
+    printf("%s %ld\n", msg, *x);
+    return *x;
+}

c++-ffi/add1.cc

@@ -0,0 +1,12 @@
+#include <iostream>
+
+using namespace std;
+
+extern "C" {
+    #include <inttypes.h>
+    int64_t add1(int64_t *x, char * msg) {
+        *x += 1;
+        cout << msg << " " << *x << endl;
+        return *x;
+    }
+}

c++-ffi/add1.rs

@@ -0,0 +1,18 @@
+#![crate_type = "staticlib"]
+
+use std::ffi::CStr;
+use std::os::raw::c_char;
+
+#[no_mangle]
+/// Add 1 to the number referenced by `x`, print the result of the add along with `msg`, return
+/// the result.
+///
+/// # Safety
+///
+/// `msg` must point to a valid C string, and `x` must point to a valid C `uint64_t`.
+pub unsafe extern "system" fn add1(x: &mut i64, msg: *const c_char) -> i64 {
+    let msg = CStr::from_ptr(msg);
+    *x += 1;
+    println!("{} {}", msg.to_string_lossy(), *x);
+    *x
+}

c++-ffi/demo.cc

@@ -0,0 +1,15 @@
+#include <iostream>
+
+extern "C" {
+    #include <inttypes.h>
+    extern int64_t add1(int64_t *, char *);
+}
+
+using namespace std;
+
+int main() {
+    int64_t x = 3;
+    int64_t y = add1(&x, (char *) "working with");
+    cout << "got " << x << " " << y << endl;
+    return 0;
+}