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From a very long time, a lot of people wanted to get the CPU usage from the computer.
You've a lot of way to do this like calling registry like key or PerfCounter. But TaskManager doesn't call any of these ... If you're looking from the import table of TaskManager you can find:
Import Adress Table: 0x00001414
Import Name Table: 0x00013C2C
0x7C90E213 260 NtQueryVirtualMemory
0x7C90DDF9 209 NtOpenThread
0x7C90D586 103 NtClose
0x7C90E1AA 255 NtQuerySystemInformation
So there is no other solution to have this information to dig into undocumented NtQuerySystemInformation. With this nice warning at the beginning of the article:[ NtQuerySystemInformation is available for use in Windows 2000 and Windows XP. It may be altered or unavailable in subsequent versions. Applications should use the alternate functions listed in this topic.]
No other solution?
Well GetSystemTimes is a good function if you have the Requirements
Client Requires Windows XP SP1.
Server Requires Windows Server 2003.
Header Declared in Winbase.h; include Windows.h.
Library Link to Kernel32.lib.
DLL Requires Kernel32.dll.
How to use it?
Call this :
BOOL res = GetSystemTimes( &idleTime, &kernelTime, &userTime );
and voilà you have almost what you need.
Now you have to poll this function and make a little calculus.
usr = userTime - last_userTime;
ker = kernelTime - last_kernelTime;
idl = idleTime - last_idleTime;
System time is:
sys = kerl + usr
idleTime is the rest because "System Idle" process is taking 100 % of CPU.
So CPU is :
cpu = int( (sys - idl) *100 / sys );
It was a very long wait before Microsoft gave us this function. Now it's done but there is still one problem. For multiple processor system, you don't have the right information.
In the sample you can find the use of GetSystemTimes and GetProcessTimes and a little class to do everything.
CPU( void );
~CPU( void );
int GetUsage( int* pSystemUsage, TKTime* pUpTime );