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Using High-resolution Timers : Page 2

For multimedia streaming, games, and profiling, Standard C++'s time-meausurement facilities are limited. For high-resolution timers, look to the POSIX libraries


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Time and time() Again
Applications that need a high-resolution time measurement can use the quasi-portable gettimeofday() function defined by the POSIX standard and also available on most versions of Windows. gettimeofday() is declared in the <sys/time.h> header:

int gettimeofday (struct timeval *tp, void*);
It writes to tp the number of seconds that have elapsed since the epoch, i.e., 01/01/1970 00:00:00. Unlike time(), which uses a one-second granularity, gettimeofday() expresses the current timestamp in seconds and microseconds (millionths of a second) since the epoch. As with may other time-measuring functions, the actual resolution of gettimeofday() depends on the underlying hardware of your machine, so you shouldn't assume that every platform supporting gettimeofday() will necessarily have a microsecond granularity.

gettimeofday() writes the current timestamp into a timeval data structure:


struct timeval
{
 time_t tv_sec; //seconds
 suseconds_t tv_usec; //microseconds
};
The following code listing obtains the current timestamp using gettimeofday() and displays it onscreen:

#include <sys/time.h> 
#include <iostream>
int main()
{
 struct timeval tv;
 gettimeofday(&tv,0);
 std::cout<<"the current timestamp is: "
   <<tv.tv_sec<<"."<<tv.tv_usec<<std::endl;
}
A granularity of one millionth of a second sounds impressive, yet some applications require even higher. Timers with a resolution of a nanosecond are now available on real-time POSIX systems.



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