diff --git a/src/bn/mod.rs b/src/bn/mod.rs
index 1982de45..e1ec1360 100644
--- a/src/bn/mod.rs
+++ b/src/bn/mod.rs
@@ -5,6 +5,7 @@ use std::ptr;
 use ssl::error::SslError;
 
 #[allow(dead_code)]
+#[repr(C)]
 struct BIGNUM {
     d: *mut c_void,
     top: c_int,
diff --git a/src/crypto/hash.rs b/src/crypto/hash.rs
index 045956ba..2696e77b 100644
--- a/src/crypto/hash.rs
+++ b/src/crypto/hash.rs
@@ -14,6 +14,7 @@ pub enum HashType {
 
 #[allow(dead_code)]
 #[allow(non_camel_case_types)]
+#[repr(C)]
 pub struct EVP_MD_CTX {
     digest: *mut EVP_MD,
     engine: *mut libc::c_void,
@@ -24,9 +25,11 @@ pub struct EVP_MD_CTX {
 }
 
 #[allow(non_camel_case_types)]
+#[repr(C)]
 pub struct EVP_MD;
 
 #[allow(non_camel_case_types)]
+#[repr(C)]
 pub struct EVP_PKEY_CTX;
 
 #[link(name = "crypto")]
diff --git a/src/crypto/hmac.rs b/src/crypto/hmac.rs
index 16be0f29..b2df4ea0 100644
--- a/src/crypto/hmac.rs
+++ b/src/crypto/hmac.rs
@@ -19,6 +19,7 @@ use crypto::hash;
 
 #[allow(dead_code)]
 #[allow(non_camel_case_types)]
+#[repr(C)]
 pub struct HMAC_CTX {
     md: *mut hash::EVP_MD,
     md_ctx: hash::EVP_MD_CTX,
@@ -37,6 +38,7 @@ extern {
 }
 
 #[allow(non_camel_case_types)]
+#[repr(C)]
 struct ENGINE;
 
 pub struct HMAC {