Add a function to free slice

Author: yutannihilation

src/myrustlib/api.h

@@ -13,7 +13,10 @@ typedef struct
 
 char * string_from_rust();
 char * string_from_rust2(const char*);
+
 Slice rev_slice(Slice);
+void free_slice(Slice);
+
 int32_t random_number();
 void run_threads();
 

src/myrustlib/src/lib.rs

@@ -14,4 +14,5 @@ pub use hello::string_from_rust2;
 pub use random::random_number;
 pub use mythreads::run_threads;
 pub use slice::rev_slice;
+pub use slice::free_slice;
 pub use slice::Slice;

src/myrustlib/src/slice.rs

@@ -8,7 +8,7 @@ pub struct Slice {
 }
 
 #[no_mangle]
-pub extern fn rev_slice(s: Slice) -> Slice {
+pub extern "C" fn rev_slice(s: Slice) -> Slice {
     // convert from Slice to Rust slice
     let s = unsafe { std::slice::from_raw_parts_mut(s.data, s.len as _) };
 
@@ -24,3 +24,14 @@ pub extern fn rev_slice(s: Slice) -> Slice {
         len: len as _,
     }
 }
+
+// c.f. https://users.rust-lang.org/t/how-to-return-byte-array-from-rust-function-to-ffi-c/18136/4
+#[no_mangle]
+pub extern "C" fn free_slice(s: Slice) {
+    // convert to Rust slice
+    let s = unsafe { std::slice::from_raw_parts_mut(s.data, s.len as _) };
+    let s = s.as_mut_ptr();
+    unsafe {
+        Box::from_raw(s);
+    }
+}

src/wrapper.c

@@ -5,6 +5,9 @@
 // Import C headers for rust API
 #include "myrustlib/api.h"
 
+// Need to include to use memcpy()
+#include <string.h>
+
 // Actual Wrappers
 SEXP hello_wrapper(){
   return Rf_ScalarString(Rf_mkCharCE(string_from_rust(), CE_UTF8));
@@ -20,11 +23,14 @@ SEXP rev_wrapper(SEXP x){
   Slice s_rev = rev_slice(s);
 
   SEXP out = PROTECT(Rf_allocVector(REALSXP, s_rev.len));
-  for (int i = 0; i < s_rev.len; i++) {
-    SET_REAL_ELT(out, i, s_rev.data[i]);
-  }
+  memcpy(REAL(out), s_rev.data, s.len * sizeof(double));
+  // AS soon as the Rust-allocated memory become no longer in use, free it.
+  // Since Rust's allocator is different from C's allocator, this needs to be
+  // freed on C's side.
+  free_slice(s_rev);
   UNPROTECT(1);
 
+
   return out;
 }