use tuple style struct in C binding

bindings/c/README.md

@@ -9,9 +9,9 @@
 A simple read and write example
 
 ```C
-#include "assert.h"
+#include <assert.h>
+#include <stdio.h>
 #include "opendal.h"
-#include "stdio.h"
 
 int main()
 {
@@ -37,7 +37,7 @@ int main()
     assert(read_bytes->len == 24);
 
     /* Lets print it out */
-    for (int i = 0; i < 24; ++i) {
+    for (size_t i = 0; i < 24; ++i) {
         printf("%c", read_bytes->data[i]);
     }
     printf("\n");

bindings/c/include/opendal.h

@@ -138,7 +138,7 @@ typedef struct opendal_entry {
    * The pointer to the opendal::Entry in the Rust code.
    * Only touch this on judging whether it is NULL.
    */
-  void *inner;
+  void *_0;
 } opendal_entry;
 
 /**

bindings/c/src/entry.rs

@@ -27,26 +27,24 @@ use ::opendal as core;
 /// @see opendal_list_entry_path()
 /// @see opendal_list_entry_name()
 #[repr(C)]
-pub struct opendal_entry {
+pub struct opendal_entry(
     /// The pointer to the opendal::Entry in the Rust code.
     /// Only touch this on judging whether it is NULL.
-    inner: *mut c_void,
-}
+    *mut c_void,
+);
 
 impl opendal_entry {
     fn deref(&self) -> &core::Entry {
         // Safety: the inner should never be null once constructed
         // The use-after-free is undefined behavior
-        unsafe { &*(self.inner as *mut core::Entry) }
+        unsafe { &*(self.0 as *mut core::Entry) }
     }
 }
 
 impl opendal_entry {
     /// Used to convert the Rust type into C type
     pub(crate) fn new(entry: core::Entry) -> Self {
-        Self {
-            inner: Box::into_raw(Box::new(entry)) as _,
-        }
+        Self(Box::into_raw(Box::new(entry)) as _)
     }
 
     /// \brief Path of entry.
@@ -79,7 +77,7 @@ impl opendal_entry {
     #[no_mangle]
     pub unsafe extern "C" fn opendal_entry_free(ptr: *mut opendal_entry) {
         if !ptr.is_null() {
-            drop(Box::from_raw((*ptr).inner as *mut core::Entry));
+            drop(Box::from_raw((*ptr).0 as *mut core::Entry));
             drop(Box::from_raw(ptr));
         }
     }