diff --git a/heed/Cargo.toml b/heed/Cargo.toml
index 19aabae2..32df3844 100644
--- a/heed/Cargo.toml
+++ b/heed/Cargo.toml
@@ -39,6 +39,12 @@ default = ["serde", "serde-bincode", "serde-json"]
 # a read-only transaction from multiple threads at the same time.
 sync-read-txn = []
 
+# Whether to tell LMDB to use POSIX semaphores during compilation
+# (instead of System V semaphores).
+# POSIX semaphores are required for Apple's App Sandbox on iOS & macOS,
+# and are possibly faster and more appropriate for single-process use.
+posix-sem = ["lmdb-rkv-sys/posix-sem"]
+
 # Enable the serde en/decoders for bincode or serde_json
 serde-bincode = ["heed-types/serde", "heed-types/bincode"]
 serde-json = ["heed-types/serde", "heed-types/serde_json"]