Implemented context create/destroy in rust

src/ffi.rs

@@ -16,7 +16,7 @@
 //! # FFI bindings
 //! Direct bindings to the underlying C library functions. These should
 //! not be needed for most users.
-use core::{mem, hash};
+use core::{mem, hash, ptr, slice};
 use types::*;
 
 /// Flag for context to enable no precomputation
@@ -260,6 +260,49 @@ extern "C" {
 }
 
 
+#[cfg(feature = "std")]
+#[no_mangle]
+/// A reimplementation of the C function `secp256k1_context_create` in rust.
+///
+/// This function allocates memory, the pointer should be deallocated using `secp256k1_context_destroy`
+/// A failure to do so will result in a memory leak.
+///
+/// This will create a secp256k1 raw context.
+// Returns: a newly created context object.
+//  In:      flags: which parts of the context to initialize.
+pub unsafe extern "C" fn secp256k1_context_create(flags: c_uint) -> *mut Context {
+    assert!(mem::align_of::<usize>() >= mem::align_of::<u8>());
+
+    let size: usize =  secp256k1_context_preallocated_size(flags)/mem::size_of::<usize>();
+
+    let buf = vec![0usize; size + 1].into_boxed_slice();
+    let ptr = Box::into_raw(buf) as *mut usize;
+    ptr::copy_nonoverlapping(&size, ptr, 1);
+    let ptr: *mut usize = ptr.offset(1);
+
+    secp256k1_context_preallocated_create(ptr as *mut c_void, flags)
+}
+
+#[cfg(feature = "std")]
+#[no_mangle]
+/// A reimplementation of the C function `secp256k1_context_destroy` in rust.
+///
+/// This function destroys and deallcates the context created by `secp256k1_context_create`.
+///
+/// The pointer shouldn't be used after passing to this function, consider it as passing it to `free()`.
+///
+pub unsafe extern "C" fn secp256k1_context_destroy(ctx: *mut Context) {
+    assert_eq!(ctx as usize % mem::align_of::<usize>(), 0);
+    secp256k1_context_preallocated_destroy(ctx);
+    let ctx: *mut usize = ctx as *mut usize;
+
+    let size_ptr: *mut usize = ctx.offset(-1);
+    let size: usize = ptr::read(size_ptr);
+    let slice: &mut [usize] = slice::from_raw_parts_mut(size_ptr , size+1);
+    let _ = Box::from_raw(slice as *mut [usize]);
+}
+
+
 #[no_mangle]
 /// **This function is an override for the C function, this is the an edited version of the original description:**
 ///
@@ -302,7 +345,7 @@ pub unsafe extern "C" fn secp256k1_default_illegal_callback_fn(message: *const c
 /// See also secp256k1_default_illegal_callback_fn.
 ///
 pub unsafe extern "C" fn secp256k1_default_error_callback_fn(message: *const c_char, _data: *mut c_void) {
-    use core::{str, slice};
+    use core::str;
     let msg_slice = slice::from_raw_parts(message as *const u8, strlen(message));
     let msg = str::from_utf8_unchecked(msg_slice);
     panic!("[libsecp256k1] internal consistency check failed {}", msg);