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Top Forums UNIX for Advanced & Expert Users The UNIX clock/timer interrupt Post 302958810 by Ultrix on Monday 26th of October 2015 02:50:33 PM
Old 10-26-2015
The UNIX clock/timer interrupt

The book The Design of the Unix Operating System speaks of a clock or timer that is used in scheduling. The clock generates interrupts that update priority information for processes and preempt processes in a round-robin fashion. It's apparent that this "clock" is not the same thing as the CPU clock, since is only generates 50 to 100 signals every second, as opposed to millions or billions of pulses a second. It's also apparent that it's not the same thing as the mechanism that keeps track of the date - that is to say, the number of seconds since midnight before January 1, 1970. So what exactly is it? Is it a pulse generator? Does it store a value that increments? Is it synced with the CPU clock somehow? How does it work? And how can it be "reset"?
 

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clock(3C)						   Standard C Library Functions 						 clock(3C)

NAME
clock - report CPU time used SYNOPSIS
#include <time.h> clock_t clock(void); DESCRIPTION
The clock() function returns the amount of CPU time (in microseconds) used since the first call to clock() in the calling process. The time reported is the sum of the user and system times of the calling process and its terminated child processes for which it has executed the wait(3C) function, the pclose(3C) function, or the system(3C) function. RETURN VALUES
Dividing the value returned by clock() by the constant CLOCKS_PER_SEC, defined in the <time.h> header, will give the time in seconds. If the process time used is not available or cannot be represented, clock returns the value (clock_t) -1. USAGE
The value returned by clock() is defined in microseconds for compatibility with systems that have CPU clocks with much higher resolution. Because of this, the value returned will wrap around after accumulating only 2147 seconds of CPU time (about 36 minutes). ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ SEE ALSO
times(2), popen(3C), system(3C), wait(3C), attributes(5), standards(5) SunOS 5.11 24 Jul 2002 clock(3C)
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