reading system time to get milliseconds or nanoseconds

the simplest way for millisec is
double x = clock();
code
time_in_seconds = (clock() - x)/CLOCKS_PER_SEC;

include ctime for these.

the following is a high resolution timer that uses the Intel 64 bit timer register.  Its very precise!!

*Intel/Visual C++ only!!!*
(because of the asm, need intel. because of __int64 need visual, you can hack that part for other compilers)

header: (jtimer.h)

#include<cstdio>

__int64 jtimer[5]; //up to 5 nested timers (call timer
//inside a call to the time, etc, like time whole
//program, then subroutines a,b,c ...)

#define mhz 1000000 //one million hertz
#define machine_speed 1000*mhz //your speed here!!
inline void start_time(int dx);
inline void elapsed_time(int dx);

_________________________________________________
cpp

#include"jtimer.h"

inline void start_time(int dx)
{
// cpu instructions
#define rdtsc __asm __emit 0Fh __asm __emit 031h
#define cpuid __asm __emit 0Fh __asm __emit 0A2h

__int64 ts = 0; //working variable

__asm push EAX
__asm push EDX
//cpuid  //other info
rdtsc    //read time stamp register

__asm mov dword ptr ts, EAX //low bits
__asm and EDX, 07fffffffh //63 bit int, sign removed
__asm mov dword ptr ts+4,EDX //high bits

__asm pop EDX
__asm pop EAX

#undef rdtsc
#undef cpuid
jtimer[dx] = ts;
}


inline void elapsed_time(int dx)
{
#define rdtsc __asm __emit 0Fh __asm __emit 031h
#define cpuid __asm __emit 0Fh __asm __emit 0A2h

__int64 ts = 0;

__asm push EAX
__asm push EDX
//cpuid  //other info
rdtsc    //read time stamp register

__asm mov dword ptr ts, EAX //low bits
__asm and EDX, 07fffffffh //63 bit int, sign removed
__asm mov dword ptr ts+4,EDX //high bits

__asm pop EDX
__asm pop EAX

#undef rdtsc
#undef cpuid
ts -=jtimer[dx];
printf("time is %f seconds\n", (double)(ts)/(double)(machine_speed));
}

GetSystemTime
The GetSystemTime function retrieves the current system date and time. The system time is expressed in Coordinated Universal Time (UTC).

VOID GetSystemTime(
  LPSYSTEMTIME lpSystemTime   // address of system time structure
);
 
Parameters
lpSystemTime
Pointer to a SYSTEMTIME structure to receive the current system date and time.

typedef struct _SYSTEMTIME {  // st
    WORD wYear;
    WORD wMonth;
    WORD wDayOfWeek;
    WORD wDay;
    WORD wHour;
    WORD wMinute;
    WORD wSecond;
    WORD wMilliseconds;
} SYSTEMTIME;

2 mitchguy
I expect that you need milliseconds for relative time.
Often I use GetTickCount().

DWORD dwStart = GetTickCount();

for..
...
// Stop if this has taken too long
if( GetTickCount() - dwStart >= TIMELIMIT ){
  break;
}

For best result you can use "High-resolution performance counter" as DanRollins suggest:
QueryPerformanceFrequency, QueryPerformanceCounter.

For Windows NT try to use "The Performance Data Helper (PDH) library":
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnperfmo/html/msdn_pdhlib.asp

In some cases, you can also use GetLocalTime( ),which can
support your local machine current time .

For super fine resolution under Windows, use the QueryPerformanceCounter/QueryPerformanceFrequency API calls.

LARGE_INTEGER before;
LARGE_INTEGER after;

QueryPerformanceCounter (&before);
// Some code
QueryPerformanceCounter (&after);

// Calculate the diference.
LARGE_INTEGER diff = after-before;
LARGE_INTEGER freq;

QueryPerformanceFrecuency (&freq);
// The frequency is on seconds thus divide by 1000
freq = freq / 1000;

LARGE_INTEGER milliseconds = diff*freq;

Hope this helps

The QueryPerformanceCounter seems to work the best of all
of the suggested methods. My goal was to get the frame rate of my openGL display loop to run at 6ofps.

Most of the previous solutions seem to be for Windows systems. How would I go about measuring Milliseconds (needed for fixed frame rates for animations, seconds intime.h are too slow) in C++ cross-platform (Linux and Windows). Glut doesn't seem to carry any time capability so I assume I will have to use #defines to detect the OS and run seperate code sections?

Hello - does the QueryPerformanceCounter code work on a Windows 2000 system?

Under the Help for MSDN, it mentions Windows 95 and Windows NT - just want to make sure it applies to Windows 2000.

To use the above code in Visual C++, I just need to include "Windows.h"   -  is this correct?

Thanks,
--Sharon