diff --git a/src/bin/pg_basebackup/walmethods.h b/src/bin/pg_basebackup/walmethods.h
index 8d679da..a3316a6 100644
--- a/src/bin/pg_basebackup/walmethods.h
+++ b/src/bin/pg_basebackup/walmethods.h
@@ -19,18 +19,50 @@ typedef enum
 	CLOSE_NO_RENAME
 }	WalCloseMethod;

+/*
+ * A WalWriteMethod structure represents the different methods used
+ * to write the streaming WAL as it's received.
+ *
+ * All methods that have a failure path will set errno on failure.
+ */
 typedef struct WalWriteMethod WalWriteMethod;
 struct WalWriteMethod
 {
+	/* Open a target file. Returns Walfile, or NULL if open failed. */
 	Walfile(*open_for_write) (const char *pathname, const char *temp_suffix, size_t pad_to_size);
+
+	/*
+	 * Close an open Walfile, using one or more methods for handling
+	 * automatic unlinking etc. Returns 0 on success, other values
+	 * for error.
+	 */
 	int			(*close) (Walfile f, WalCloseMethod method);
+
+	/* Check if a file exist */
 	bool		(*existsfile) (const char *pathname);
+
+	/* Return the size of a file, or -1 on failure. */
 	ssize_t		(*get_file_size) (const char *pathname);

+	/*
+	 * Write count number of bytes to the file, and return the number
+	 * of bytes actually written or -1 for error.
+	 */
 	ssize_t		(*write) (Walfile f, const void *buf, size_t count);
+
+	/* Return the current position in a file or -1 on error */
 	off_t		(*get_current_pos) (Walfile f);
+
+	/*
+	 * fsync the contents of the specified file. Returns 0 on
+	 * success.
+	 */
 	int			(*sync) (Walfile f);
+
+	/* Clean up the Walmethod, closing any shared resources */
 	bool		(*finish) (void);
+
+	/* Return a text for the last error in this Walfile */
 	char	   *(*getlasterror) (void);
 };