diff --git a/heed/Cargo.toml b/heed/Cargo.toml
index 7466fff8..caa500c2 100644
--- a/heed/Cargo.toml
+++ b/heed/Cargo.toml
@@ -47,6 +47,15 @@ sync-read-txn = []
 lmdb = ["lmdb-rkv-sys"]
 mdbx = ["mdbx-sys"]
 
+# Whether to tell LMDB to use POSIX semaphores during compilation
+# (instead of the default, which are 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.
+# There are tradeoffs for both POSIX and SysV semaphores; which you
+# should look into before enabling this feature. Also, see here:
+# https://github.com/LMDB/lmdb/blob/3947014aed7ffe39a79991fa7fb5b234da47ad1a/libraries/liblmdb/lmdb.h#L46-L69
+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"]