diff --git a/proxmox-protocol/Cargo.toml b/proxmox-protocol/Cargo.toml
index 1d8361e0..8ae08a86 100644
--- a/proxmox-protocol/Cargo.toml
+++ b/proxmox-protocol/Cargo.toml
@@ -14,4 +14,5 @@ crate-type = ['lib', 'cdylib']
 chrono = "0.4"
 endian_trait = "0.6"
 failure = "0.1"
+libc = "0.2"
 openssl = "0.10"
diff --git a/proxmox-protocol/src/c_chunker.rs b/proxmox-protocol/src/c_chunker.rs
index ba7835d0..c24730eb 100644
--- a/proxmox-protocol/src/c_chunker.rs
+++ b/proxmox-protocol/src/c_chunker.rs
@@ -2,11 +2,13 @@
 
 use std::os::raw::c_void;
 
+use libc::size_t;
+
 use crate::Chunker;
 
 /// Creates a new chunker instance.
 #[no_mangle]
-pub extern "C" fn proxmox_chunker_new(chunk_size_avg: u64) -> *mut Chunker {
+pub extern "C" fn proxmox_chunker_new(chunk_size_avg: size_t) -> *mut Chunker {
     Box::leak(Box::new(Chunker::new(chunk_size_avg as usize)))
 }
 
@@ -26,18 +28,20 @@ pub extern "C" fn proxmox_chunker_done(me: *mut Chunker) {
 pub extern "C" fn proxmox_chunker_scan(
     me: *mut Chunker,
     data: *const c_void,
-    size: u64,
-) -> u64 {
+    size: size_t,
+) -> size_t {
     let me = unsafe { &mut *me };
-    me.scan(unsafe {
-        std::slice::from_raw_parts(data as *const u8, size as usize)
-    }) as u64
+    me.scan(unsafe { std::slice::from_raw_parts(data as *const u8, size as usize) }) as size_t
 }
 
 /// Compute a chunk digest. This is mostly a convenience method to avoid having to lookup the right
 /// digest method for your language of choice.
 #[no_mangle]
-pub extern "C" fn proxmox_chunk_digest(data: *const c_void, size: u64, out_digest: *mut [u8; 32]) {
+pub extern "C" fn proxmox_chunk_digest(
+    data: *const c_void,
+    size: size_t,
+    out_digest: *mut [u8; 32],
+) {
     let digest = crate::FixedChunk::from_data(unsafe {
         std::slice::from_raw_parts(data as *const u8, size as usize)
     });
diff --git a/proxmox-protocol/src/c_client.rs b/proxmox-protocol/src/c_client.rs
index a8d936ae..a3144bb9 100644
--- a/proxmox-protocol/src/c_client.rs
+++ b/proxmox-protocol/src/c_client.rs
@@ -4,9 +4,10 @@
 
 use std::ffi::{CStr, CString};
 use std::io;
-use std::os::raw::{c_char, c_int, c_ulong, c_void};
+use std::os::raw::{c_char, c_int, c_void};
 
 use failure::{bail, format_err, Error};
+use libc::size_t;
 
 /// Read callback. The first parameter is the `opaque` parameter passed to `proxmox_backup_new`,
 /// the rest are the usual read function parameters. This should return the number of bytes
@@ -328,7 +329,7 @@ pub extern "C" fn proxmox_backup_create(
     backup_id: *const c_char,
     time_epoch: i64,
     file_name: *const c_char,
-    chunk_size: c_ulong,
+    chunk_size: size_t,
     file_size: i64,
     is_new: bool,
 ) -> c_int {
@@ -391,7 +392,7 @@ pub extern "C" fn proxmox_backup_dynamic_data(
 pub extern "C" fn proxmox_backup_fixed_data(
     me: *mut CClient,
     stream: c_int,
-    index: c_ulong,
+    index: size_t,
     digest: *const [u8; 32],
 ) -> c_int {
     let me = unsafe { &mut *me };
diff --git a/proxmox-protocol/src/lib.rs b/proxmox-protocol/src/lib.rs
index acfa8a86..2e571d4a 100644
--- a/proxmox-protocol/src/lib.rs
+++ b/proxmox-protocol/src/lib.rs
@@ -16,5 +16,5 @@ pub use client::*;
 mod types;
 pub use types::*;
 
-pub mod c_client;
 pub mod c_chunker;
+pub mod c_client;