#### 21.3.1 CPU Time Inquiry

To get a process' CPU time, you can use the `clock` function. This facility is declared in the header file `time.h`. In typical usage, you call the `clock` function at the beginning and end of the interval you want to time, subtract the values, and then divide by `CLOCKS_PER_SEC` (the number of clock ticks per second) to get processor time, like this:

         #include <time.h>

         clock_t start, end;
         double cpu_time_used;

         start = clock();
         ... /* Do the work. */
         end = clock();
         cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;

Do not use a single CPU time as an amount of time; it doesn't work that way. Either do a subtraction as shown above or query processor time directly. See Processor Time.

Different computers and operating systems vary wildly in how they keep track of CPU time. It's common for the internal processor clock to have a resolution somewhere between a hundredth and millionth of a second.

— Macro: int CLOCKS_PER_SEC

> The value of this macro is the number of clock ticks per second measured by the `clock` function. POSIX requires that this value be one million independent of the actual resolution.

— Data Type: clock_t

> This is the type of the value returned by the `clock` function. Values of type `clock_t` are numbers of clock ticks.

— Function: clock_t clock (void)

> This function returns the calling process' current CPU time. If the CPU time is not available or cannot be represented, `clock` returns the value `(clock_t)(-1)`.