diff --git a/crates/core/src/document/djvulibre_sys.rs b/crates/core/src/document/djvulibre_sys.rs
index d7cca2a7..efbb00a2 100644
--- a/crates/core/src/document/djvulibre_sys.rs
+++ b/crates/core/src/document/djvulibre_sys.rs
@@ -42,7 +42,7 @@ pub type RenderMode = libc::c_uint;
 pub type FormatStyle = libc::c_uint;
 
 #[link(name="djvulibre")]
-extern {
+extern "C" {
     pub fn ddjvu_context_create(name: *const libc::c_char) -> *mut ExoContext;
     pub fn ddjvu_context_release(ctx: *mut ExoContext);
     pub fn ddjvu_cache_set_size(ctx: *mut ExoContext, size: libc::c_ulong);
diff --git a/crates/core/src/document/mupdf_sys.rs b/crates/core/src/document/mupdf_sys.rs
index 2bcac493..6a312019 100644
--- a/crates/core/src/document/mupdf_sys.rs
+++ b/crates/core/src/document/mupdf_sys.rs
@@ -42,7 +42,7 @@ pub enum FzImage {}
 #[link(name="mupdf")]
 #[link(name="mupdf_wrapper", kind="static")]
 
-extern {
+extern "C" {
     pub fn fz_new_context_imp(alloc_ctx: *const FzAllocContext, locks_ctx: *const FzLocksContext, cache_size: libc::size_t, version: *const libc::c_char) -> *mut FzContext;
     pub fn fz_drop_context(ctx: *mut FzContext);
     pub fn fz_register_document_handlers(ctx: *mut FzContext);
diff --git a/crates/core/src/font/freetype_sys.rs b/crates/core/src/font/freetype_sys.rs
index b1e3f25f..8a92a968 100644
--- a/crates/core/src/font/freetype_sys.rs
+++ b/crates/core/src/font/freetype_sys.rs
@@ -23,7 +23,7 @@ pub type FtPos = libc::c_long;
 pub type FtFixed = libc::c_long;
 pub type FtGlyphFormat = libc::c_uint;
 pub type GlyphBBoxMode = libc::c_uint;
-pub type FtGenericFinalizer = extern fn(*mut libc::c_void);
+pub type FtGenericFinalizer = extern "C" fn(*mut libc::c_void);
 
 pub enum FtLibrary {}
 pub enum FtCharMap {}
@@ -36,7 +36,7 @@ pub enum FtMemory {}
 pub enum FtStream {}
 
 #[link(name="freetype")]
-extern {
+extern "C" {
     pub fn FT_Init_FreeType(lib: *mut *mut FtLibrary) -> FtError;
     pub fn FT_Done_FreeType(lib: *mut FtLibrary) -> FtError;
     pub fn FT_New_Face(lib: *mut FtLibrary, path: *const libc::c_char, idx: libc::c_long, face: *mut *mut FtFace) -> FtError;
diff --git a/crates/core/src/font/harfbuzz_sys.rs b/crates/core/src/font/harfbuzz_sys.rs
index 52ed1e08..dda3e329 100644
--- a/crates/core/src/font/harfbuzz_sys.rs
+++ b/crates/core/src/font/harfbuzz_sys.rs
@@ -20,7 +20,7 @@ pub enum HbBuffer {}
 pub enum HbFont {}
 
 #[link(name="harfbuzz")]
-extern {
+extern "C" {
     pub fn hb_ft_font_create(face: *mut FtFace, destroy: *const libc::c_void) -> *mut HbFont;
     pub fn hb_ft_font_changed(font: *mut HbFont);
     pub fn hb_font_destroy(font: *mut HbFont);
diff --git a/crates/core/src/font/mod.rs b/crates/core/src/font/mod.rs
index 45eca13d..be410df7 100644
--- a/crates/core/src/font/mod.rs
+++ b/crates/core/src/font/mod.rs
@@ -100,7 +100,7 @@ pub const MD_SIZE: Style = Style {
 
 #[cfg(any(not(target_os = "linux"), target_arch = "arm"))]
 #[link(name="mupdf")]
-extern {
+extern "C" {
     // Based on the outputs of:
     // arm-linux-gnueabihf-readelf -Ws ./libs/libmupdf.so | grep '\b_binary_' | \
     // grep -v '_size$' | awk '{print $8, strtonum($3)-1}' | sort -u
@@ -255,7 +255,7 @@ extern {
 
 #[cfg(all(target_os = "linux", not(target_arch = "arm")))]
 #[link(name="mupdf")]
-extern {
+extern "C" {
     pub static _binary_resources_fonts_droid_DroidSansFallback_ttf_start: [libc::c_uchar; 3556308];
     pub static _binary_resources_fonts_noto_NotoEmoji_Regular_ttf_start: [libc::c_uchar; 418804];
     pub static _binary_resources_fonts_noto_NotoMusic_Regular_otf_start: [libc::c_uchar; 60812];