diff --git a/strbuf.h b/strbuf.h
index 7987405313..83c5c98530 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -377,6 +377,8 @@ extern ssize_t strbuf_read_once(struct strbuf *, int fd, size_t hint);
 /**
  * Read the contents of a file, specified by its path. The third argument
  * can be used to give a hint about the file size, to avoid reallocs.
+ * Return the number of bytes read or a negative value if some error
+ * occurred while opening or reading the file.
  */
 extern ssize_t strbuf_read_file(struct strbuf *sb, const char *path, size_t hint);