diff --git a/src/dir.c b/src/dir.c
index 963d94d..cb72edf 100644
--- a/src/dir.c
+++ b/src/dir.c
@@ -1,10 +1,20 @@
-/* POSIX directory-stream wrappers. */
+/*
+ * POSIX directory-stream wrappers.
+ *
+ * opendir(3), fdopendir(3), readdir(3), closedir(3), and rewinddir(3) expose
+ * three different failure conventions between them. These wrappers turn all
+ * three into error contexts and make end-of-directory an explicit AKERR_EOF.
+ */
 #include <akstdlib.h>
 
 #include <errno.h>
 
 #include "aksl_internal.h"
 
+/*
+ * opendir(3) returns NULL for failure. Clear *dest first so a failed open
+ * cannot leave the caller holding a stale directory stream.
+ */
 akerr_ErrorContext AKERR_NOIGNORE *aksl_opendir(const char *pathname, DIR **dest)
 {
     PREPARE_ERROR(e);
@@ -19,6 +29,10 @@ akerr_ErrorContext AKERR_NOIGNORE *aksl_opendir(const char *pathname, DIR **dest
     SUCCEED_RETURN(e);
 }
 
+/*
+ * fdopendir(3) takes ownership of fd only when it succeeds. On success the
+ * matching aksl_closedir call closes both the stream and its descriptor.
+ */
 akerr_ErrorContext AKERR_NOIGNORE *aksl_fdopendir(int fd, DIR **dest)
 {
     PREPARE_ERROR(e);
@@ -30,6 +44,10 @@ akerr_ErrorContext AKERR_NOIGNORE *aksl_fdopendir(int fd, DIR **dest)
     SUCCEED_RETURN(e);
 }
 
+/*
+ * readdir(3) owns and may reuse its returned storage. Copy the entry into the
+ * caller's destination, and use errno to distinguish failure from exhaustion.
+ */
 akerr_ErrorContext AKERR_NOIGNORE *aksl_readdir(DIR *dirp, struct dirent *dest)
 {
     struct dirent *entry = NULL;
@@ -50,6 +68,7 @@ akerr_ErrorContext AKERR_NOIGNORE *aksl_readdir(DIR *dirp, struct dirent *dest)
     SUCCEED_RETURN(e);
 }
 
+/* closedir(3) reports its failure directly and invalidates dirp on success. */
 akerr_ErrorContext AKERR_NOIGNORE *aksl_closedir(DIR *dirp)
 {
     PREPARE_ERROR(e);
@@ -60,6 +79,10 @@ akerr_ErrorContext AKERR_NOIGNORE *aksl_closedir(DIR *dirp)
     SUCCEED_RETURN(e);
 }
 
+/*
+ * rewinddir(3) has no failure return. Preserve that contract after rejecting
+ * a NULL stream, which would otherwise be undefined behaviour.
+ */
 akerr_ErrorContext AKERR_NOIGNORE *aksl_rewinddir(DIR *dirp)
 {
     PREPARE_ERROR(e);
diff --git a/tests/test_dir.c b/tests/test_dir.c
index b212512..cd3e18a 100644
--- a/tests/test_dir.c
+++ b/tests/test_dir.c
@@ -23,14 +23,21 @@ static int test_open_errors_and_nulls(void)
     DIR *dirp = (DIR *)1;
     struct dirent entry;
 
+    /* opendir propagates missing-path and non-directory failures. */
     AKSL_CHECK_STATUS(aksl_opendir("/nonexistent/aksl/dir", &dirp), ENOENT);
     AKSL_CHECK(dirp == NULL);
     AKSL_CHECK(aksl_temp_file(file, sizeof(file)) == 0);
     AKSL_CHECK_STATUS(aksl_opendir(file, &dirp), ENOTDIR);
+
+    /* Every pointer required by the wrapped operation rejects NULL. */
     AKSL_CHECK_STATUS(aksl_opendir(NULL, &dirp), AKERR_NULLPOINTER);
     AKSL_CHECK_STATUS(aksl_opendir(".", NULL), AKERR_NULLPOINTER);
+
+    /* fdopendir propagates an invalid descriptor and validates its out-param. */
     AKSL_CHECK_STATUS(aksl_fdopendir(-1, &dirp), EBADF);
     AKSL_CHECK_STATUS(aksl_fdopendir(0, NULL), AKERR_NULLPOINTER);
+
+    /* The remaining wrappers reject NULL streams and destinations. */
     AKSL_CHECK_STATUS(aksl_readdir(NULL, &entry), AKERR_NULLPOINTER);
     AKSL_CHECK_OK(aksl_opendir(".", &dirp));
     AKSL_CHECK_STATUS(aksl_readdir(dirp, NULL), AKERR_NULLPOINTER);