diff --git a/src/common/compat.c b/src/common/compat.c
index 24b44fb055..69eb0643d0 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -870,6 +870,9 @@ tor_lockfile_unlock(tor_lockfile_t *lockfile)
 /** @{ */
 /** Some old versions of Unix didn't define constants for these values,
  * and instead expect you to say 0, 1, or 2. */
+#ifndef SEEK_SET
+#define SEEK_SET 0
+#endif
 #ifndef SEEK_CUR
 #define SEEK_CUR 1
 #endif