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<a name="1204515"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">9.4 Time arithmetic</font>
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<a name="1204516"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Arithmetic on timer values should always be performed using special modulo operators. These routines perform no overflow checking and so allow for timer values `wrapping round' to the most negative integer on the next tick after the most positive integer.</font>
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<a name="1204518"> </a><font size=2 face=Courier><strong>clock_t time_plus(const clock_t time1, const clock_t time2);<br>clock_t time_minus(const clock_t time1, const clock_t time2);<br>int time_after(const clock_t time1, const clock_t time2);<br></strong></font>
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<a name="1204521"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><font size=2 face=Courier><strong>time_plus</strong></font> adds two timer values together and returns the sum allowing for any wrap-around. For example, if a number of ticks is added to the current time using <font size=2 face=Courier><strong>time_plus</strong></font> then the result is the time after that many ticks.</font>
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<a name="1204522"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><font size=2 face=Courier><strong>time_minus</strong></font> subtracts the second value from the first and returns the difference allowing for any wrap-around. For example, if one time is subtracted from another using <font size=2 face=Courier><strong>time_minus</strong></font> then the result is the number of ticks between the two times. If the result is positive then the first time is after the second. If the result is negative then the first time is before the second.</font>
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<a name="1204523"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><font size=2 face=Courier><strong>time_after</strong></font> determines whether the first time is after the second time. One time is considered to be after another if the one is not more than half a full timer cycle later than the other. Half a full cycle is 2<font face="Verdana, Arial, Helvetica, sans-serif"><sup>31</sup></font> ticks. The function returns the integer value one if the first time is after the second, otherwise it returns zero.</font>
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<a name="1204709"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Some of these concepts are shown in <a href="clock5.htm#1204733"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Figure⌨️ 快捷键说明
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