15.2. POSIX clocks

POSIX is a standard for implementing and representing time sources. In contrast to the hardware clock, which is selected by the kernel and implemented across the system; the POSIX clock can be selected by each application, without affecting other applications in the system.
  • CLOCK_REALTIME: it represents the time in the real world, also referred to as 'wall time' meaning the time as read from the clock on the wall. This clock is used to timestamp events, and when interfacing with the user. It can be modified by an user with the right privileges. However, user modification should be used with caution as it can lead to erroneous data if the clock has its value changed between two readings.
  • CLOCK_MONOTONIC: represents the time monotonically increased since the system boot. This clock cannot be set by any process, and is the preferred clock for calculating the time difference between events. The following examples in this section use CLOCK_MONOTONIC as the POSIX clock.


For more information on POSIX clocks see the following man page and book:
  • clock_gettime()
  • Linux System Programming by Robert Love
The function used to read a given POSIX clock is clock_gettime(), which is defined at <time.h>. The clock_gettime() command takes two parameters: the POSIX clock ID and a timespec structure which will be filled with the duration used to read the clock. The following example shows the function to measure the cost of reading the clock:

Example 15.2. Using clock_gettime() to measure the cost of reading POSIX clocks

#include <time.h>

	int rc;
	long i;
	struct timespec ts;

	for(i=0; i<10000000; i++) {
		rc = clock_gettime(CLOCK_MONOTONIC, &ts);
You can improve upon the example above, for example by using more strings to verify the return code of clock_gettime(), to verify the value of the rc variable, or to ensure the content of the ts structure is to be trusted. The clock_gettime() manpage provides more information to help you write more reliable applications.


Programs using the clock_gettime() function must be linked with the rt library by adding '-lrt' to the gcc command line.
cc clock_timing.c -o clock_timing -lrt


The functions such as clock_gettime() and gettimeofday() have a counterpart in the kernel, in the form of a system call. When the user process calls clock_gettime(), the corresponding C library (glibc) calls the sys_clock_gettime() system call which performs the requested operation and then returns the result to the user program.
However, this context switch from user application to kernel has a cost. Even though this cost is very low, if the operation is repeated thousands of times, the accumulated cost can have an impact on the overall performance of the application.
To avoid the context switch to the kernel, thus making it faster to read the clock, support for the CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE POSIX clocks was created in the form of a VDSO library function. The _COARSE variants are faster to read and have a precision (also known as resolution) of one millisecond (ms).