Any string that ultimately comes from notmuch_message_file_get_header
is in UTF-8.
---
 lib/notmuch-private.h |    2 +-
 lib/notmuch.h         |    6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
index cc55bb9..eced03e 100644
--- a/lib/notmuch-private.h
+++ b/lib/notmuch-private.h
@@ -386,7 +386,7 @@ void
 notmuch_message_file_restrict_headersv (notmuch_message_file_t *message,
 					va_list va_headers);
 
-/* Get the value of the specified header from the message.
+/* Get the value of the specified header from the message as a UTF-8 string.
  *
  * The header name is case insensitive.
  *
diff --git a/lib/notmuch.h b/lib/notmuch.h
index 9fc34ef..998a4ae 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -767,7 +767,7 @@ notmuch_thread_get_messages (notmuch_thread_t *thread);
 int
 notmuch_thread_get_matched_messages (notmuch_thread_t *thread);
 
-/* Get the authors of 'thread'
+/* Get the authors of 'thread' as a UTF-8 string.
  *
  * The returned string is a comma-separated list of the names of the
  * authors of mail messages in the query results that belong to this
@@ -781,7 +781,7 @@ notmuch_thread_get_matched_messages (notmuch_thread_t *thread);
 const char *
 notmuch_thread_get_authors (notmuch_thread_t *thread);
 
-/* Get the subject of 'thread'
+/* Get the subject of 'thread' as a UTF-8 string.
  *
  * The subject is taken from the first message (according to the query
  * order---see notmuch_query_set_sort) in the query results that
@@ -1013,7 +1013,7 @@ notmuch_message_set_flag (notmuch_message_t *message,
 time_t
 notmuch_message_get_date  (notmuch_message_t *message);
 
-/* Get the value of the specified header from 'message'.
+/* Get the value of the specified header from 'message' as a UTF-8 string.
  *
  * Common headers are stored in the database when the message is
  * indexed and will be returned from the database.  Other headers will
-- 
1.7.10.4