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| <h3 class="section">21.3 Processor And CPU Time</h3> |
| |
| <p>If you're trying to optimize your program or measure its efficiency, |
| it's very useful to know how much processor time it uses. For that, |
| calendar time and elapsed times are useless because a process may spend |
| time waiting for I/O or for other processes to use the CPU. However, |
| you can get the information with the functions in this section. |
| |
| <p>CPU time (see <a href="Time-Basics.html#Time-Basics">Time Basics</a>) is represented by the data type |
| <code>clock_t</code>, which is a number of <dfn>clock ticks</dfn>. It gives the |
| total amount of time a process has actively used a CPU since some |
| arbitrary event. On the GNU system, that event is the creation of the |
| process. While arbitrary in general, the event is always the same event |
| for any particular process, so you can always measure how much time on |
| the CPU a particular computation takes by examining the process' CPU |
| time before and after the computation. |
| <a name="index-CPU-time-2603"></a><a name="index-clock-ticks-2604"></a><a name="index-ticks_002c-clock-2605"></a> |
| In the GNU system, <code>clock_t</code> is equivalent to <code>long int</code> and |
| <code>CLOCKS_PER_SEC</code> is an integer value. But in other systems, both |
| <code>clock_t</code> and the macro <code>CLOCKS_PER_SEC</code> can be either integer |
| or floating-point types. Casting CPU time values to <code>double</code>, as |
| in the example above, makes sure that operations such as arithmetic and |
| printing work properly and consistently no matter what the underlying |
| representation is. |
| |
| <p>Note that the clock can wrap around. On a 32bit system with |
| <code>CLOCKS_PER_SEC</code> set to one million this function will return the |
| same value approximately every 72 minutes. |
| |
| <p>For additional functions to examine a process' use of processor time, |
| and to control it, see <a href="Resource-Usage-And-Limitation.html#Resource-Usage-And-Limitation">Resource Usage And Limitation</a>. |
| |
| <ul class="menu"> |
| <li><a accesskey="1" href="CPU-Time.html#CPU-Time">CPU Time</a>: The <code>clock</code> function. |
| <li><a accesskey="2" href="Processor-Time.html#Processor-Time">Processor Time</a>: The <code>times</code> function. |
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