diff --git a/packages/binaryen_dsl/binaryen_dsl.0.7/opam b/packages/binaryen_dsl/binaryen_dsl.0.7/opam
index dd6c798247e..424fc32007e 100644
--- a/packages/binaryen_dsl/binaryen_dsl.0.7/opam
+++ b/packages/binaryen_dsl/binaryen_dsl.0.7/opam
@@ -15,7 +15,7 @@ depends: [
   "ocaml"
   "core" {< "v0.15"}
   "dune" {>= "2.8"}
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "libbinaryen" {< "102"}
 ]
 build: [
diff --git a/packages/ctypes-foreign/ctypes-foreign.0.21.1/opam b/packages/ctypes-foreign/ctypes-foreign.0.21.1/opam
new file mode 100644
index 00000000000..4fa7a1314b3
--- /dev/null
+++ b/packages/ctypes-foreign/ctypes-foreign.0.21.1/opam
@@ -0,0 +1,49 @@
+opam-version: "2.0"
+synopsis: "Dynamic access to foreign C libraries using Ctypes"
+description: """
+
+This installs the `ctypes-foreign` interface which
+uses `libffi` to provide dynamic access to foreign libraries."""
+maintainer: ["Jeremy Yallop <yallop@gmail.com>"]
+authors: ["Jeremy Yallop"]
+license: "MIT"
+tags: ["org:mirage"]
+homepage: "https://github.com/yallop/ocaml-ctypes"
+doc: "https://ocamllabs.github.io/ocaml-ctypes/"
+bug-reports: "https://github.com/yallop/ocaml-ctypes/issues"
+depends: [
+  "dune" {>= "2.9"}
+  "ocaml" {>= "4.03.0"}
+  "integers" {with-test & >= "0.2.2"}
+  "ctypes" {= version}
+  "dune-configurator"
+  "conf-pkg-config"
+  "lwt" {with-test & >= "2.4.7"}
+  "ounit2" {with-test}
+  "conf-ncurses" {with-test}
+  "stdlib-shims" {with-test}
+  "conf-fts" {with-test & os != "win32"}
+  "conf-libffi" {>= "2.0.0"}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "--promote-install-files=false"
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+  ["dune" "install" "-p" name "--create-install-files" name]
+]
+dev-repo: "git+https://github.com/yallop/ocaml-ctypes.git"
+url {
+  src: "https://github.com/yallop/ocaml-ctypes/archive/refs/tags/0.21.1.tar.gz"
+  checksum: "md5=8b201d932741c5096854e5eb39139b90"
+}
diff --git a/packages/ctypes/ctypes.0.21.1/opam b/packages/ctypes/ctypes.0.21.1/opam
new file mode 100644
index 00000000000..30d89801ddd
--- /dev/null
+++ b/packages/ctypes/ctypes.0.21.1/opam
@@ -0,0 +1,58 @@
+opam-version: "2.0"
+synopsis: "Combinators for binding to C libraries without writing any C"
+description: """
+
+ctypes is a library for binding to C libraries using pure OCaml. The primary
+aim is to make writing C extensions as straightforward as possible.
+The core of ctypes is a set of combinators for describing the structure of C
+types -- numeric types, arrays, pointers, structs, unions and functions. You
+can use these combinators to describe the types of the functions that you want
+to call, then bind directly to those functions -- all without writing or
+generating any C!
+
+To install the optional `ctypes-foreign` interface (which uses `libffi` to
+provide dynamic access to foreign libraries), you will need to also install
+the `ctypes-foreign` package.
+
+    opam install ctypes-foreign
+
+This will make the `ctypes-foreign` ocamlfind subpackage available."""
+maintainer: ["Jeremy Yallop <yallop@gmail.com>"]
+authors: ["Jeremy Yallop"]
+license: "MIT"
+tags: ["org:mirage"]
+homepage: "https://github.com/yallop/ocaml-ctypes"
+doc: "https://yallop.github.io/ocaml-ctypes/"
+bug-reports: "https://github.com/yallop/ocaml-ctypes/issues"
+depends: [
+  "dune" {>= "2.9"}
+  "ocaml" {>= "4.03.0"}
+  "integers"
+  "dune-configurator"
+  "bigarray-compat"
+  "ounit2" {with-test}
+  "conf-fts" {with-test & os != "win32"}
+  "conf-pkg-config" {with-test}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "--promote-install-files=false"
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+  ["dune" "install" "-p" name "--create-install-files" name]
+]
+dev-repo: "git+https://github.com/yallop/ocaml-ctypes.git"
+url {
+  src: "https://github.com/yallop/ocaml-ctypes/archive/refs/tags/0.21.1.tar.gz"
+  checksum: "md5=8b201d932741c5096854e5eb39139b90"
+}
diff --git a/packages/eigen/eigen.0.1.0/opam b/packages/eigen/eigen.0.1.0/opam
index 55c7c5ba172..808ab66b96f 100644
--- a/packages/eigen/eigen.0.1.0/opam
+++ b/packages/eigen/eigen.0.1.0/opam
@@ -12,7 +12,7 @@ build: [
 ]
 depends: [
   "ocaml" {>= "4.02"}
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "dune" {>= "1.1.0"}
 ]
 available: arch = "x86_32" | arch = "x86_64"
diff --git a/packages/eigen/eigen.0.1.1/opam b/packages/eigen/eigen.0.1.1/opam
index 68f0b78853f..c7d0365276d 100644
--- a/packages/eigen/eigen.0.1.1/opam
+++ b/packages/eigen/eigen.0.1.1/opam
@@ -12,7 +12,7 @@ build: [
 ]
 depends: [
   "ocaml" {>= "4.02"}
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "dune" {>= "1.1.0"}
 ]
 available: arch = "x86_32" | arch = "x86_64"
diff --git a/packages/eigen/eigen.0.1.2/opam b/packages/eigen/eigen.0.1.2/opam
index 04733a01a3d..340a7c228ca 100644
--- a/packages/eigen/eigen.0.1.2/opam
+++ b/packages/eigen/eigen.0.1.2/opam
@@ -12,7 +12,7 @@ build: [
 ]
 depends: [
   "ocaml" {>= "4.02"}
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "dune" {>= "1.1.0"}
 ]
 available: arch = "x86_32" | arch = "x86_64"
diff --git a/packages/eigen/eigen.0.1.3/opam b/packages/eigen/eigen.0.1.3/opam
index bac246e0e69..49c3affc2bf 100644
--- a/packages/eigen/eigen.0.1.3/opam
+++ b/packages/eigen/eigen.0.1.3/opam
@@ -12,7 +12,7 @@ build: [
 ]
 depends: [
   "ocaml" {>= "4.02"}
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "dune" {>= "1.1.0"}
 ]
 available: arch = "x86_32" | arch = "x86_64"
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.3.2/opam b/packages/hacl-star-raw/hacl-star-raw.0.3.2/opam
index dddacaafa9a..87cb82f2c1a 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.3.2/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.3.2/opam
@@ -13,7 +13,7 @@ bug-reports: "https://github.com/hacl-star/hacl-star/issues"
 depends: [
   "ocaml" {>= "4.05.0"}
   "ocamlfind" {build}
-  "ctypes"
+  "ctypes" {< "0.21.0"}
   "ctypes-foreign"
   "conf-which" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.4.0/opam b/packages/hacl-star-raw/hacl-star-raw.0.4.0/opam
index 7529bc89da6..c7731f7ec87 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.4.0/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.4.0/opam
@@ -14,7 +14,7 @@ bug-reports: "https://github.com/hacl-star/hacl-star/issues"
 depends: [
   "ocaml" { >= "4.08.0" }
   "ocamlfind" {build}
-  "ctypes" { >= "0.18.0" }
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "ctypes-foreign"
   "conf-which" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.4.1/opam b/packages/hacl-star-raw/hacl-star-raw.0.4.1/opam
index a7f9de3778d..f3159afe3aa 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.4.1/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.4.1/opam
@@ -14,7 +14,7 @@ bug-reports: "https://github.com/hacl-star/hacl-star/issues"
 depends: [
   "ocaml" { >= "4.08.0" }
   "ocamlfind" {build}
-  "ctypes" { >= "0.18.0" }
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "ctypes-foreign"
   "conf-which" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.4.2/opam b/packages/hacl-star-raw/hacl-star-raw.0.4.2/opam
index d07322844fb..fc1b66f49fa 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.4.2/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.4.2/opam
@@ -14,7 +14,7 @@ bug-reports: "https://github.com/hacl-star/hacl-star/issues"
 depends: [
   "ocaml" { >= "4.08.0" }
   "ocamlfind" {build}
-  "ctypes" { >= "0.18.0" }
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "ctypes-foreign"
   "conf-which" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.4.3/opam b/packages/hacl-star-raw/hacl-star-raw.0.4.3/opam
index 30bb6463cd7..d3135f57165 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.4.3/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.4.3/opam
@@ -14,7 +14,7 @@ bug-reports: "https://github.com/hacl-star/hacl-star/issues"
 depends: [
   "ocaml" { >= "4.08.0" }
   "ocamlfind" {build}
-  "ctypes" { >= "0.18.0" }
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "ctypes-foreign"
   "conf-which" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.4.4/opam b/packages/hacl-star-raw/hacl-star-raw.0.4.4/opam
index c27cd5f5f37..b900f51be6c 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.4.4/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.4.4/opam
@@ -14,7 +14,7 @@ bug-reports: "https://github.com/hacl-star/hacl-star/issues"
 depends: [
   "ocaml" { >= "4.08.0" }
   "ocamlfind" {build}
-  "ctypes" { >= "0.18.0" }
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
 ]
 conflicts: ["ocaml-option-bytecode-only"]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.4.5/opam b/packages/hacl-star-raw/hacl-star-raw.0.4.5/opam
index 1c6064850c6..aed5e40202c 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.4.5/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.4.5/opam
@@ -14,7 +14,7 @@ bug-reports: "https://github.com/hacl-star/hacl-star/issues"
 depends: [
   "ocaml" { >= "4.08.0" }
   "ocamlfind" {build}
-  "ctypes" { >= "0.18.0" }
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
 ]
 conflicts: ["ocaml-option-bytecode-only"]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.5.0/opam b/packages/hacl-star-raw/hacl-star-raw.0.5.0/opam
index 6880265d99c..6b916bb8506 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.5.0/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.5.0/opam
@@ -14,7 +14,7 @@ depends: [
   "ocaml" {>= "4.08.0"}
   "dune" {>= "1.2"}
   "ocamlfind" {build}
-  "ctypes" {>= "0.18.0"}
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
   "conf-cmake" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.6.0/opam b/packages/hacl-star-raw/hacl-star-raw.0.6.0/opam
index 8f418096fa9..11c9f709a20 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.6.0/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.6.0/opam
@@ -14,7 +14,7 @@ depends: [
   "ocaml" {>= "4.08.0"}
   "dune" {>= "1.2"}
   "ocamlfind" {build}
-  "ctypes" {>= "0.18.0"}
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
   "conf-cmake" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.6.1/opam b/packages/hacl-star-raw/hacl-star-raw.0.6.1/opam
index 0d7fa2bfc7a..c186671fb8f 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.6.1/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.6.1/opam
@@ -14,7 +14,7 @@ depends: [
   "ocaml" {>= "4.08.0"}
   "dune" {>= "1.2"}
   "ocamlfind" {build}
-  "ctypes" {>= "0.18.0"}
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
   "conf-cmake" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.6.2/opam b/packages/hacl-star-raw/hacl-star-raw.0.6.2/opam
index aae6d5b21fa..590b99e462c 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.6.2/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.6.2/opam
@@ -14,7 +14,7 @@ depends: [
   "ocaml" {>= "4.08.0"}
   "dune" {>= "1.2"}
   "ocamlfind" {build}
-  "ctypes" {>= "0.18.0"}
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
   "conf-cmake" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.7.0/opam b/packages/hacl-star-raw/hacl-star-raw.0.7.0/opam
index 196be1b8072..fba58e64a93 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.7.0/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.7.0/opam
@@ -14,7 +14,7 @@ depends: [
   "ocaml" {>= "4.08.0"}
   "dune" {>= "1.2"}
   "ocamlfind" {build}
-  "ctypes" {>= "0.18.0"}
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
   "conf-cmake" {build}
 ]
diff --git a/packages/hacl-star-raw/hacl-star-raw.0.7.1/opam b/packages/hacl-star-raw/hacl-star-raw.0.7.1/opam
index d5bc80f281a..80f48448772 100644
--- a/packages/hacl-star-raw/hacl-star-raw.0.7.1/opam
+++ b/packages/hacl-star-raw/hacl-star-raw.0.7.1/opam
@@ -14,7 +14,7 @@ depends: [
   "ocaml" {>= "4.08.0"}
   "dune" {>= "1.2"}
   "ocamlfind" {build}
-  "ctypes" {>= "0.18.0"}
+  "ctypes" {>= "0.18.0" & < "0.21.0"}
   "conf-which" {build}
   "conf-cmake" {build}
 ]
diff --git a/packages/libudev/libudev.0.2.1/opam b/packages/libudev/libudev.0.2.1/opam
index 7087419d0ee..17da9d9e2dd 100644
--- a/packages/libudev/libudev.0.2.1/opam
+++ b/packages/libudev/libudev.0.2.1/opam
@@ -11,7 +11,7 @@ depends: [
   "ocamlbuild" {build}
   "topkg" {build & >= "0.9.0"}
   "conf-libudev"
-  "ctypes" {>= "0.4.1"}
+  "ctypes" {>= "0.4.1" & < "0.21.0"}
   "ctypes-foreign"
   "stdint"
 ]
diff --git a/packages/lz4/lz4.1.1.1/opam b/packages/lz4/lz4.1.1.1/opam
index adfa82821a2..04e4f529338 100644
--- a/packages/lz4/lz4.1.1.1/opam
+++ b/packages/lz4/lz4.1.1.1/opam
@@ -31,7 +31,7 @@ depends: [
   "base-bigarray"
   "ocamlfind" {build}
   "ocamlbuild" {build & != "0.9.0"}
-  "ctypes" {>= "0.4.1"}
+  "ctypes" {>= "0.4.1" & < "0.21.0"}
   "ounit" {with-test}
   "conf-liblz4"
 ]
diff --git a/packages/lz4/lz4.1.2.0/opam b/packages/lz4/lz4.1.2.0/opam
index 067a1b553a8..e658a2bf0d9 100644
--- a/packages/lz4/lz4.1.2.0/opam
+++ b/packages/lz4/lz4.1.2.0/opam
@@ -17,7 +17,7 @@ depends: [
   "ocaml" {>= "4.02"}
   "conf-liblz4"
   "dune" { >= "2.0" }
-  "ctypes" {>= "0.4.1"}
+  "ctypes" {>= "0.4.1" & < "0.21.0"}
   "ounit2" {with-test}
   "odoc" {with-doc}
 ]
diff --git a/packages/mariadb/mariadb.1.1.5/opam b/packages/mariadb/mariadb.1.1.5/opam
index 63694670e6a..f67cb9aadb9 100644
--- a/packages/mariadb/mariadb.1.1.5/opam
+++ b/packages/mariadb/mariadb.1.1.5/opam
@@ -17,7 +17,7 @@ depends: [
   "ocaml" {>= "4.07.0" & < "5.0"}
   "ocamlfind" {build}
   "ocamlbuild" {build}
-  "ctypes" {>= "0.7.0"}
+  "ctypes" {>= "0.7.0" & < "0.21.0"}
   "conf-mariadb"
   "conf-gcc"
 ]
diff --git a/packages/mariadb/mariadb.1.1.6/opam b/packages/mariadb/mariadb.1.1.6/opam
index 9637004151b..e5a74103a5f 100644
--- a/packages/mariadb/mariadb.1.1.6/opam
+++ b/packages/mariadb/mariadb.1.1.6/opam
@@ -17,7 +17,7 @@ depends: [
   "ocaml" {>= "4.07.0" & < "5.0"}
   "ocamlfind" {build}
   "ocamlbuild" {build}
-  "ctypes" {>= "0.7.0"}
+  "ctypes" {>= "0.7.0" & < "0.21.0"}
   "conf-mariadb"
   "conf-gcc"
 ]
diff --git a/packages/memcpy/memcpy.0.2.1/opam b/packages/memcpy/memcpy.0.2.1/opam
index acfa76e96a1..494badbdcc1 100644
--- a/packages/memcpy/memcpy.0.2.1/opam
+++ b/packages/memcpy/memcpy.0.2.1/opam
@@ -12,7 +12,7 @@ install: [[make "install"]]
 remove: [["ocamlfind" "remove" "memcpy"]]
 depends: [
   "ocaml" {>= "4.01.0"}
-  "ctypes" {>= "0.12.0"}
+  "ctypes" {>= "0.12.0" & < "0.21.0"}
   "ounit" {with-test}
   "ocamlfind" {build}
   "ocamlbuild" {build}
diff --git a/packages/netlink/netlink.0.2.1/opam b/packages/netlink/netlink.0.2.1/opam
index 85c38dfeab9..fe60ef839dd 100644
--- a/packages/netlink/netlink.0.2.1/opam
+++ b/packages/netlink/netlink.0.2.1/opam
@@ -16,7 +16,7 @@ remove: [
 depends: [
   "ocaml" {< "5.0.0"}
   "ocamlfind"
-  "ctypes"
+  "ctypes" {< "0.21.0"}
   "ctypes-foreign"
   "ocamlbuild" {build}
 ]
diff --git a/packages/pci/pci.1.0.1/opam b/packages/pci/pci.1.0.1/opam
index 01b6528954f..76f272b673e 100644
--- a/packages/pci/pci.1.0.1/opam
+++ b/packages/pci/pci.1.0.1/opam
@@ -17,7 +17,7 @@ remove: [
 ]
 depends: [
   "ocaml" {>= "4.01.0" & < "5.0.0"}
-  "ctypes" {>= "0.4"}
+  "ctypes" {>= "0.4" & < "0.21.0"}
   "ocamlfind" {build}
 #  "ounit" {with-test}
   "ocamlbuild" {build}
diff --git a/packages/tgls/tgls.0.8.4/opam b/packages/tgls/tgls.0.8.4/opam
index 5a5ce5e14e4..d2d35c6df53 100644
--- a/packages/tgls/tgls.0.8.4/opam
+++ b/packages/tgls/tgls.0.8.4/opam
@@ -13,7 +13,7 @@ depends: [
   "ocamlbuild" {build}
   "topkg" {build}
   "base-bytes"
-  "ctypes" {>= "0.4.0"}
+  "ctypes" {>= "0.4.0" & < "0.21.0"}
   "ctypes-foreign"
   "tsdl" {with-test}
   "result" {with-test}
diff --git a/packages/tgls/tgls.0.8.5/opam b/packages/tgls/tgls.0.8.5/opam
index 42a99cf021c..ba6042913a6 100644
--- a/packages/tgls/tgls.0.8.5/opam
+++ b/packages/tgls/tgls.0.8.5/opam
@@ -13,7 +13,7 @@ depends: [
   "ocamlbuild" {build}
   "topkg" {build}
   "base-bytes"
-  "ctypes" {>= "0.4.0"}
+  "ctypes" {>= "0.4.0" & < "0.21.0"}
   "ctypes-foreign"
   "tsdl" {with-test}
   "result" {with-test}
diff --git a/packages/tgls/tgls.0.8.6/opam b/packages/tgls/tgls.0.8.6/opam
index 063cd2389e2..b9bc17f1b14 100644
--- a/packages/tgls/tgls.0.8.6/opam
+++ b/packages/tgls/tgls.0.8.6/opam
@@ -23,7 +23,7 @@ depends: [
   "ocamlfind" {build}
   "ocamlbuild" {build}
   "topkg" {build & >= "1.0.3"}
-  "ctypes" {>= "0.4.0"}
+  "ctypes" {>= "0.4.0" & < "0.21.0"}
   "ctypes-foreign"
   "xmlm" {dev}
 ]
diff --git a/packages/tsdl/tsdl.0.9.7/opam b/packages/tsdl/tsdl.0.9.7/opam
index 887bb7a9e0b..3049a26fc29 100644
--- a/packages/tsdl/tsdl.0.9.7/opam
+++ b/packages/tsdl/tsdl.0.9.7/opam
@@ -14,7 +14,7 @@ depends: [
   "ocamlbuild" {build}
   "topkg" {build}
   "conf-sdl2"
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "ctypes-foreign" ]
 build: [[
   "ocaml" "pkg/pkg.ml" "build"
diff --git a/packages/tsdl/tsdl.0.9.8/opam b/packages/tsdl/tsdl.0.9.8/opam
index cc8a3ce028c..6e8895752f5 100644
--- a/packages/tsdl/tsdl.0.9.8/opam
+++ b/packages/tsdl/tsdl.0.9.8/opam
@@ -14,7 +14,7 @@ depends: [
   "ocamlbuild" {build}
   "topkg" {build & >= "1.0.1"}
   "conf-sdl2"
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "ctypes-foreign" ]
 build: [[ "ocaml" "pkg/pkg.ml" "build" "--pinned" "%{pinned}%" ]]
 
diff --git a/packages/tsdl/tsdl.0.9.9/opam b/packages/tsdl/tsdl.0.9.9/opam
index 238ab18f001..4e7fcc8da81 100644
--- a/packages/tsdl/tsdl.0.9.9/opam
+++ b/packages/tsdl/tsdl.0.9.9/opam
@@ -34,7 +34,7 @@ depends: [
   "ocamlbuild" {build}
   "topkg" {build & >= "1.0.3"}
   "conf-sdl2"
-  "ctypes" {>= "0.14.0"}
+  "ctypes" {>= "0.14.0" & < "0.21.0"}
   "ctypes-foreign"
 ]
 build: ["ocaml" "pkg/pkg.ml" "build" "--dev-pkg" "%{dev}%"]
diff --git a/packages/unix-errno/unix-errno.0.6.0/opam b/packages/unix-errno/unix-errno.0.6.0/opam
index c9722f76ed6..53f59683962 100644
--- a/packages/unix-errno/unix-errno.0.6.0/opam
+++ b/packages/unix-errno/unix-errno.0.6.0/opam
@@ -22,7 +22,7 @@ depends: [
   "base-bytes"
   "integers"
   "result"
-  "ctypes" {>= "0.12.0"}
+  "ctypes" {>= "0.12.0" & < "0.21.0"}
 ]
 depopts: ["base-unix"]
 build: [
diff --git a/packages/unix-errno/unix-errno.0.6.1/opam b/packages/unix-errno/unix-errno.0.6.1/opam
index 146b9958694..01354061a02 100644
--- a/packages/unix-errno/unix-errno.0.6.1/opam
+++ b/packages/unix-errno/unix-errno.0.6.1/opam
@@ -21,7 +21,7 @@ depends: [
   "alcotest" {with-test}
   "base-bytes"
   "result"
-  "ctypes" {>= "0.12.0"}
+  "ctypes" {>= "0.12.0" & < "0.21.0"}
   "integers"
 ]
 depopts: ["base-unix"]
diff --git a/packages/zstd/zstd.0.3/opam b/packages/zstd/zstd.0.3/opam
index 8355433f55f..6dd6e5fb4ba 100644
--- a/packages/zstd/zstd.0.3/opam
+++ b/packages/zstd/zstd.0.3/opam
@@ -17,7 +17,7 @@ build: [
 depends: [
   "ocaml"
   "dune" {>= "2.0"}
-  "ctypes"
+  "ctypes" {< "0.21.0"}
   "integers"
   ("extlib" {with-test} | "extlib-compat" {with-test})
   "base-unix" {with-test}