ntp源码解读20160326

/*
 * NTP uses two fixed point formats.  The first (l_fp) is the "long"
 * format and is 64 bits long with the decimal between bits 31 and 32.
 * This is used for time stamps in the NTP packet header (in network
 * byte order) and for internal computations of offsets (in local host
 * byte order). We use the same structure for both signed and unsigned
 * values, which is a big hack but saves rewriting all the operators
 * twice. Just to confuse this, we also sometimes just carry the
 * fractional part in calculations, in both signed and unsigned forms.
 * Anyway, an l_fp looks like:
 *ntp使用了两个定点的格式。这第一个l_fp是long型的在31和31位之间是小数点。在ntp包头里面代表时间戳，这时当然是网络字节序。在计算内部偏移的时候，当然使用本机字节序。
 *我们使用了同一个结构来表示有符号值和无符号值，这里有点技巧，就是免了重写这个结构体。我们有时也需要在计算的时候带着小数部分，也是有符号和无符号两种类型。
 *    0              1              2              3
 *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *   |                   Integral Part                 |
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *   |                   Fractional Part                 |
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 */
typedef struct {
    union {
        u_int32 Xl_ui;
        int32 Xl_i;
    } Ul_i;
    union {
        u_int32 Xl_uf;
        int32 Xl_f;
    } Ul_f;
} l_fp;

#define l_ui    Ul_i.Xl_ui        /* unsigned integral part */
#define    l_i    Ul_i.Xl_i        /* signed integral part */
#define    l_uf    Ul_f.Xl_uf        /* unsigned fractional part */
#define    l_f    Ul_f.Xl_f        /* signed fractional part */

/*
 * Fractional precision (of an l_fp) is actually the number of
 * bits in a long.
 */
#define    FRACTION_PREC    (32)


/*
 * The second fixed point format is 32 bits, with the decimal between
 * bits 15 and 16.  There is a signed version (s_fp) and an unsigned
 * version (u_fp).  This is used to represent synchronizing distance
 * and synchronizing dispersion in the NTP packet header (again, in
 * network byte order) and internally to hold both distance and
 * dispersion values (in local byte order).  In network byte order
 * it looks like:
 *第二种固定点形式是32位的，小数点在15和16点之间，有一个有符号的和一个无符号的版本。这代表同步距离和同步偏差在ntp的包头，这当然是网络字节序。内部保持着距离和偏差，
 *以本地字节序。
 *    0              1              2              3
 *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *   |          Integer Part         |       Fraction Part         |
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 */

typedef int32 s_fp;
typedef u_int32 u_fp;