text: /*
   * xlogdefs.h
   *
   * Postgres write-ahead log manager record pointer and
   * timeline number definitions
   *
   * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * src/include/access/xlogdefs.h
   */
  #ifndef XLOG_DEFS_H
  #define XLOG_DEFS_H
  
  #include <fcntl.h>				/* need open() flags */
  
  /*
   * Pointer to a location in the XLOG.  These pointers are 64 bits wide,
   * because we don't want them ever to overflow.
   */
  typedef uint64 XLogRecPtr;
  
  /*
   * Zero is used indicate an invalid pointer. Bootstrap skips the first possible
   * WAL segment, initializing the first WAL page at XLOG_SEG_SIZE, so no XLOG
   * record can begin at zero.
   */
  #define InvalidXLogRecPtr	0
  #define XLogRecPtrIsInvalid(r)	((r) == InvalidXLogRecPtr)
  
  /*
   * XLogSegNo - physical log file sequence number.
   */
  typedef uint64 XLogSegNo;
  
  /*
   * TimeLineID (TLI) - identifies different database histories to prevent
   * confusion after restoring a prior state of a database installation.
   * TLI does not change in a normal stop/restart of the database (including
   * crash-and-recover cases); but we must assign a new TLI after doing
   * a recovery to a prior state, a/k/a point-in-time recovery.  This makes
   * the new WAL logfile sequence we generate distinguishable from the
   * sequence that was generated in the previous incarnation.
   */
  typedef uint32 TimeLineID;
  
  /*
   * Replication origin id - this is located in this file to avoid having to
   * include origin.h in a bunch of xlog related places.
   */
  typedef uint16 RepOriginId;
  
  /*
   *	Because O_DIRECT bypasses the kernel buffers, and because we never
   *	read those buffers except during crash recovery or if wal_level != minimal,
   *	it is a win to use it in all cases where we sync on each write().  We could
   *	allow O_DIRECT with fsync(), but it is unclear if fsync() could process
   *	writes not buffered in the kernel.  Also, O_DIRECT is never enough to force
   *	data to the drives, it merely tries to bypass the kernel cache, so we still
   *	need O_SYNC/O_DSYNC.
   */
  #ifdef O_DIRECT
  #define PG_O_DIRECT				O_DIRECT
  #else
  #define PG_O_DIRECT				0
  #endif
  
  /*
   * This chunk of hackery attempts to determine which file sync methods
   * are available on the current platform, and to choose an appropriate
   * default method.  We assume that fsync() is always available, and that
   * configure determined whether fdatasync() is.
   */
  #if defined(O_SYNC)
  #define OPEN_SYNC_FLAG		O_SYNC
  #elif defined(O_FSYNC)
  #define OPEN_SYNC_FLAG		O_FSYNC
  #endif
  
  #if defined(O_DSYNC)
  #if defined(OPEN_SYNC_FLAG)
  /* O_DSYNC is distinct? */
  #if O_DSYNC != OPEN_SYNC_FLAG
  #define OPEN_DATASYNC_FLAG		O_DSYNC
  #endif
  #else							/* !defined(OPEN_SYNC_FLAG) */
  /* Win32 only has O_DSYNC */
  #define OPEN_DATASYNC_FLAG		O_DSYNC
  #endif
  #endif
  
  #if defined(PLATFORM_DEFAULT_SYNC_METHOD)
  #define DEFAULT_SYNC_METHOD		PLATFORM_DEFAULT_SYNC_METHOD
  #elif defined(OPEN_DATASYNC_FLAG)
  #define DEFAULT_SYNC_METHOD		SYNC_METHOD_OPEN_DSYNC
  #elif defined(HAVE_FDATASYNC)
  #define DEFAULT_SYNC_METHOD		SYNC_METHOD_FDATASYNC
  #else
  #define DEFAULT_SYNC_METHOD		SYNC_METHOD_FSYNC
  #endif
  
  #endif   /* XLOG_DEFS_H */
  