diff --git a/src/opts.rs b/src/opts.rs
index acac9a5083..88f4eb74b7 100644
--- a/src/opts.rs
+++ b/src/opts.rs
@@ -234,94 +234,106 @@ where
 }
 
 /// Get the maximum mmap window size
-pub fn get_mwindow_size() -> Result<libc::size_t, Error> {
+///
+/// # Safety
+/// This function is reading a C global without synchronization, so it is not
+/// thread safe, and should only be called before any thread is spawned.
+pub unsafe fn get_mwindow_size() -> Result<libc::size_t, Error> {
     crate::init();
 
     let mut size = 0;
 
-    unsafe {
-        try_call!(raw::git_libgit2_opts(
-            raw::GIT_OPT_GET_MWINDOW_SIZE as libc::c_int,
-            &mut size
-        ));
-    }
+    try_call!(raw::git_libgit2_opts(
+        raw::GIT_OPT_GET_MWINDOW_SIZE as libc::c_int,
+        &mut size
+    ));
 
     Ok(size)
 }
 
 /// Set the maximum mmap window size
-pub fn set_mwindow_size(size: libc::size_t) -> Result<(), Error> {
+///
+/// # Safety
+/// This function is modifying a C global without synchronization, so it is not
+/// thread safe, and should only be called before any thread is spawned.
+pub unsafe fn set_mwindow_size(size: libc::size_t) -> Result<(), Error> {
     crate::init();
 
-    unsafe {
-        try_call!(raw::git_libgit2_opts(
-            raw::GIT_OPT_SET_MWINDOW_SIZE as libc::c_int,
-            size
-        ));
-    }
+    try_call!(raw::git_libgit2_opts(
+        raw::GIT_OPT_SET_MWINDOW_SIZE as libc::c_int,
+        size
+    ));
 
     Ok(())
 }
 
 /// Get the maximum memory that will be mapped in total by the library
-pub fn get_mwindow_mapped_limit() -> Result<libc::size_t, Error> {
+///
+/// # Safety
+/// This function is reading a C global without synchronization, so it is not
+/// thread safe, and should only be called before any thread is spawned.
+pub unsafe fn get_mwindow_mapped_limit() -> Result<libc::size_t, Error> {
     crate::init();
 
     let mut limit = 0;
 
-    unsafe {
-        try_call!(raw::git_libgit2_opts(
-            raw::GIT_OPT_GET_MWINDOW_MAPPED_LIMIT as libc::c_int,
-            &mut limit
-        ));
-    }
+    try_call!(raw::git_libgit2_opts(
+        raw::GIT_OPT_GET_MWINDOW_MAPPED_LIMIT as libc::c_int,
+        &mut limit
+    ));
 
     Ok(limit)
 }
 
 /// Set the maximum amount of memory that can be mapped at any time
 /// by the library.
-pub fn set_mwindow_mapped_limit(limit: libc::size_t) -> Result<(), Error> {
+///
+/// # Safety
+/// This function is modifying a C global without synchronization, so it is not
+/// thread safe, and should only be called before any thread is spawned.
+pub unsafe fn set_mwindow_mapped_limit(limit: libc::size_t) -> Result<(), Error> {
     crate::init();
 
-    unsafe {
-        try_call!(raw::git_libgit2_opts(
-            raw::GIT_OPT_SET_MWINDOW_MAPPED_LIMIT as libc::c_int,
-            limit
-        ));
-    }
+    try_call!(raw::git_libgit2_opts(
+        raw::GIT_OPT_SET_MWINDOW_MAPPED_LIMIT as libc::c_int,
+        limit
+    ));
 
     Ok(())
 }
 
 /// Get the maximum number of files that will be mapped at any time by the
 /// library.
-pub fn get_mwindow_file_limit() -> Result<libc::size_t, Error> {
+///
+/// # Safety
+/// This function is reading a C global without synchronization, so it is not
+/// thread safe, and should only be called before any thread is spawned.
+pub unsafe fn get_mwindow_file_limit() -> Result<libc::size_t, Error> {
     crate::init();
 
     let mut limit = 0;
 
-    unsafe {
-        try_call!(raw::git_libgit2_opts(
-            raw::GIT_OPT_GET_MWINDOW_FILE_LIMIT as libc::c_int,
-            &mut limit
-        ));
-    }
+    try_call!(raw::git_libgit2_opts(
+        raw::GIT_OPT_GET_MWINDOW_FILE_LIMIT as libc::c_int,
+        &mut limit
+    ));
 
     Ok(limit)
 }
 
 /// Set the maximum number of files that can be mapped at any time
 /// by the library. The default (0) is unlimited.
-pub fn set_mwindow_file_limit(limit: libc::size_t) -> Result<(), Error> {
+///
+/// # Safety
+/// This function is modifying a C global without synchronization, so it is not
+/// thread safe, and should only be called before any thread is spawned.
+pub unsafe fn set_mwindow_file_limit(limit: libc::size_t) -> Result<(), Error> {
     crate::init();
 
-    unsafe {
-        try_call!(raw::git_libgit2_opts(
-            raw::GIT_OPT_SET_MWINDOW_FILE_LIMIT as libc::c_int,
-            limit
-        ));
-    }
+    try_call!(raw::git_libgit2_opts(
+        raw::GIT_OPT_SET_MWINDOW_FILE_LIMIT as libc::c_int,
+        limit
+    ));
 
     Ok(())
 }
@@ -337,19 +349,25 @@ mod test {
 
     #[test]
     fn mwindow_size() {
-        assert!(set_mwindow_size(1024).is_ok());
-        assert!(get_mwindow_size().unwrap() == 1024);
+        unsafe {
+            assert!(set_mwindow_size(1024).is_ok());
+            assert!(get_mwindow_size().unwrap() == 1024);
+        }
     }
 
     #[test]
     fn mwindow_mapped_limit() {
-        assert!(set_mwindow_mapped_limit(1024).is_ok());
-        assert!(get_mwindow_mapped_limit().unwrap() == 1024);
+        unsafe {
+            assert!(set_mwindow_mapped_limit(1024).is_ok());
+            assert!(get_mwindow_mapped_limit().unwrap() == 1024);
+        }
     }
 
     #[test]
     fn mwindow_file_limit() {
-        assert!(set_mwindow_file_limit(1024).is_ok());
-        assert!(get_mwindow_file_limit().unwrap() == 1024);
+        unsafe {
+            assert!(set_mwindow_file_limit(1024).is_ok());
+            assert!(get_mwindow_file_limit().unwrap() == 1024);
+        }
     }
 }