diff --git a/mongoose.h b/mongoose.h
index 495d07178c..08a9bdffa2 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -988,7 +988,9 @@ enum { MG_FS_READ = 1, MG_FS_WRITE = 2, MG_FS_DIR = 4 };
 // stat(), write(), read() calls.
 struct mg_fs {
   int (*st)(const char *path, size_t *size, time_t *mtime);  // stat file
-  void (*ls)(const char *path, void (*fn)(const char *, void *), void *);
+  void (*ls)(const char *path, void (*fn)(const char *, void *),
+             void *);  // List directory entries: call fn(file_name, fn_data)
+                       // for each directory entry
   void *(*op)(const char *path, int flags);             // Open file
   void (*cl)(void *fd);                                 // Close file
   size_t (*rd)(void *fd, void *buf, size_t len);        // Read file
diff --git a/src/fs.h b/src/fs.h
index 100295296f..a5fa31d906 100644
--- a/src/fs.h
+++ b/src/fs.h
@@ -14,7 +14,9 @@ enum { MG_FS_READ = 1, MG_FS_WRITE = 2, MG_FS_DIR = 4 };
 // stat(), write(), read() calls.
 struct mg_fs {
   int (*st)(const char *path, size_t *size, time_t *mtime);  // stat file
-  void (*ls)(const char *path, void (*fn)(const char *, void *), void *);
+  void (*ls)(const char *path, void (*fn)(const char *, void *),
+             void *);  // List directory entries: call fn(file_name, fn_data)
+                       // for each directory entry
   void *(*op)(const char *path, int flags);             // Open file
   void (*cl)(void *fd);                                 // Close file
   size_t (*rd)(void *fd, void *buf, size_t len);        // Read file