diff --git a/Sources/ZIPFoundation/Archive+MemoryFile.swift b/Sources/ZIPFoundation/Archive+MemoryFile.swift
index 2d7060ec..246c8d00 100644
--- a/Sources/ZIPFoundation/Archive+MemoryFile.swift
+++ b/Sources/ZIPFoundation/Archive+MemoryFile.swift
@@ -35,7 +35,7 @@ extension Archive {
                 : funopen(cookie.toOpaque(), readStub, nil, seekStub, closeStub)!
             #else
             let stubs = cookie_io_functions_t(read: readStub, write: writeStub, seek: seekStub, close: closeStub)
-            let result = fopencookie(cookie.toOpaque(), mode, stubs)!
+            let result = fopencookie(cookie.toOpaque(), mode.posixMode, stubs)!
             #endif
             return result
         }
diff --git a/Sources/ZIPFoundation/Archive.swift b/Sources/ZIPFoundation/Archive.swift
index 89b71c11..e3839456 100644
--- a/Sources/ZIPFoundation/Archive.swift
+++ b/Sources/ZIPFoundation/Archive.swift
@@ -104,6 +104,14 @@ public final class Archive: Sequence {
         case update
 
         var isWritable: Bool { self != .read }
+
+        var posixMode: String {
+            switch self {
+            case .read: return "rb"
+            case .create: return "wb+"
+            case .update: return "rb+"
+            }
+        }
     }
 
     /// The version of an `Archive`