The GNU C Library - Basic CPU Time

Basic CPU Time Inquiry

To get the elapsed CPU time used by a process, 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), like this:

	#include <time.h>
	
	clock_t start, end;
	double elapsed;
	
	start = clock();
	... /* Do the work. */
	end = clock();
	elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;

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

In the GNU system, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value. But in other systems, both clock_t and the type of the macro CLOCKS_PER_SEC can be either integer or floating-point types. Casting processor time values to double , 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.

Macro int CLOCKS_PER_SEC

The value of this macro is the number of clock ticks per second measured by the clock function.

Macro int CLK_TCK

This is an obsolete name for CLOCKS_PER_SEC .

Data Type clock_t

This is the type of the value returned by the clock function. Values of type clock_t are in units of clock ticks.

Function clock_t clock (void)

This function returns the elapsed processor time. The base time is arbitrary but doesn't change within a single process. If the processor time is not available or cannot be represented, clock returns the value (clock_t)(-1) .