From 1db8abb7fc62cd9e2ab417d167fbfebf5b5bc56c Mon Sep 17 00:00:00 2001
From: Adrien Beraud <adrien.beraud@savoirfairelinux.com>
Date: Mon, 26 Oct 2026 15:24:19 -0400
Subject: [PATCH] c: align hash structs with native types

---
 BUILD.md        |  4 ++++
 c/opendht.cpp   |  5 +++++
 c/opendht_c.h   | 13 +++++++++++--
 rust/src/ffi.rs |  4 ++--
 4 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/BUILD.md b/BUILD.md
index da33f292..e207b8ce 100644
--- a/BUILD.md
+++ b/BUILD.md
@@ -122,6 +122,10 @@ Enable `OPENDHT_C=ON` to build the C ABI. With `BUILD_TESTING=ON`, CTest runs
 enabled. Rust regression and callback-lifetime tests run with Cargo; see
 [the Rust binding guide](rust/README.md) for build and library-path settings.
 
+`dht_infohash` and `dht_pkid` have `uint32_t` alignment, matching the native hash
+types, while retaining their 20-byte and 32-byte sizes. Rebuild consumers against
+the updated header; foreign-language declarations must match this alignment.
+
 C handles returned by constructors, imports, and reference functions are owned
 and require their matching delete/unref function. Data views, string getters,
 query fields, and callback arguments are borrowed: copy them or take a reference
diff --git a/c/opendht.cpp b/c/opendht.cpp
index e1c49e4b..390091a3 100644
--- a/c/opendht.cpp
+++ b/c/opendht.cpp
@@ -12,6 +12,11 @@ using PrivkeySp = std::shared_ptr<dht::crypto::PrivateKey>;
 using PubkeySp = std::shared_ptr<const dht::crypto::PublicKey>;
 using CertSp = std::shared_ptr<dht::crypto::Certificate>;
 
+static_assert(sizeof(dht_infohash) == sizeof(dht::InfoHash));
+static_assert(alignof(dht_infohash) == alignof(dht::InfoHash));
+static_assert(sizeof(dht_pkid) == sizeof(dht::PkId));
+static_assert(alignof(dht_pkid) == alignof(dht::PkId));
+
 struct CompletionGuard
 {
     CompletionGuard(dht_done_cb callback, void* data)
diff --git a/c/opendht_c.h b/c/opendht_c.h
index dcdc00ef..68ab868c 100644
--- a/c/opendht_c.h
+++ b/c/opendht_c.h
@@ -39,9 +39,17 @@ OPENDHT_C_PUBLIC dht_data_view dht_blob_get_data(const dht_blob* data);
 OPENDHT_C_PUBLIC void dht_blob_delete(dht_blob* data);
 
 // dht::InfoHash
+#ifdef __cplusplus
+#define OPENDHT_C_ALIGN_UINT32 alignas(uint32_t)
+#elif defined(_MSC_VER)
+#define OPENDHT_C_ALIGN_UINT32 __declspec(align(4))
+#else
+#define OPENDHT_C_ALIGN_UINT32 _Alignas(uint32_t)
+#endif
+
 struct OPENDHT_C_PUBLIC dht_infohash
 {
-    uint8_t d[HASH_LEN];
+    OPENDHT_C_ALIGN_UINT32 uint8_t d[HASH_LEN];
 };
 typedef struct dht_infohash dht_infohash;
 OPENDHT_C_PUBLIC void dht_infohash_zero(dht_infohash* h);
@@ -56,9 +64,10 @@ OPENDHT_C_PUBLIC bool dht_infohash_is_zero(const dht_infohash* h);
 // dht::PkId
 struct OPENDHT_C_PUBLIC dht_pkid
 {
-    uint8_t d[32];
+    OPENDHT_C_ALIGN_UINT32 uint8_t d[32];
 };
 typedef struct dht_pkid dht_pkid;
+#undef OPENDHT_C_ALIGN_UINT32
 OPENDHT_C_PUBLIC const char* dht_pkid_print(const dht_pkid* h);
 
 // dht::crypto::PublicKey
diff --git a/rust/src/ffi.rs b/rust/src/ffi.rs
index 2630b021..edd53838 100644
--- a/rust/src/ffi.rs
+++ b/rust/src/ffi.rs
@@ -26,13 +26,13 @@ pub struct Blob {
     _thread_bound: std::marker::PhantomData<*mut ()>,
 }
 
-#[repr(C)]
+#[repr(C, align(4))]
 #[derive(PartialEq)]
 pub struct InfoHash {
     pub d: [u8; HASH_LEN],
 }
 
-#[repr(C)]
+#[repr(C, align(4))]
 pub struct PkId {
     pub d: [u8; PKID_LEN],
 }