아래에 소스가 공개되어 있네요.

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#include <windows.h>
#include <conio.h>
#include <stdio.h>

#define SystemBasicInformation 0
#define SystemPerformanceInformation 2
#define SystemTimeInformation 3

#define Li2Double(x) ((double)((x).HighPart) * 4.294967296E9 +
(double)((x).LowPart))

typedef struct
{
    DWORD dwUnknown1;
    ULONG uKeMaximumIncrement;
    ULONG uPageSize;
    ULONG uMmNumberOfPhysicalPages;
    ULONG uMmLowestPhysicalPage;
    ULONG uMmHighestPhysicalPage;
    ULONG uAllocationGranularity;
    PVOID pLowestUserAddress;
    PVOID pMmHighestUserAddress;
    ULONG uKeActiveProcessors;
    BYTE bKeNumberProcessors;
    BYTE bUnknown2;
    WORD wUnknown3;
} SYSTEM_BASIC_INFORMATION;

typedef struct
{
    LARGE_INTEGER liIdleTime;
    DWORD dwSpare[76];
} SYSTEM_PERFORMANCE_INFORMATION;

typedef struct
{
    LARGE_INTEGER liKeBootTime;
    LARGE_INTEGER liKeSystemTime;
    LARGE_INTEGER liExpTimeZoneBias;
    ULONG uCurrentTimeZoneId;
    DWORD dwReserved;
} SYSTEM_TIME_INFORMATION;


// ntdll!NtQuerySystemInformation (NT specific!)
//
// The function copies the system information of the
// specified type into a buffer
//
// NTSYSAPI
// NTSTATUS
// NTAPI
// NtQuerySystemInformation(
// IN UINT SystemInformationClass, // information type
// OUT PVOID SystemInformation, // pointer to buffer
// IN ULONG SystemInformationLength, // buffer size in bytes
// OUT PULONG ReturnLength OPTIONAL // pointer to a 32-bit
// // variable that receives
// // the number of bytes
// // written to the buffer
// );
typedef LONG (WINAPI *PROCNTQSI)(UINT,PVOID,ULONG,PULONG);

PROCNTQSI NtQuerySystemInformation;


void main(void)
{
    SYSTEM_PERFORMANCE_INFORMATION SysPerfInfo;
    SYSTEM_TIME_INFORMATION SysTimeInfo;
    SYSTEM_BASIC_INFORMATION SysBaseInfo;
    double dbIdleTime;
    double dbSystemTime;
    LONG status;
    LARGE_INTEGER liOldIdleTime = {0,0};
    LARGE_INTEGER liOldSystemTime = {0,0};

    NtQuerySystemInformation = (PROCNTQSI)GetProcAddress(
                                          GetModuleHandle("ntdll"),
                                         "NtQuerySystemInformation"
                                         );

    if (!NtQuerySystemInformation)
        return;

    // get number of processors in the system
    status =
NtQuerySystemInformation(SystemBasicInformation,&SysBaseInfo,sizeof(SysBaseI
nfo),NULL);
    if (status != NO_ERROR)
        return;

printf("\nCPU Usage (press any key to exit): ");
    while(!_kbhit())
    {
        // get new system time
     status =
NtQuerySystemInformation(SystemTimeInformation,&SysTimeInfo,sizeof(SysTimeIn
fo),0);
        if (status!=NO_ERROR)
            return;

        // get new CPU's idle time
        status =
NtQuerySystemInformation(SystemPerformanceInformation,&SysPerfInfo,sizeof(Sy
sPerfInfo),NULL);
        if (status != NO_ERROR)
            return;

        // if it's a first call - skip it
       if (liOldIdleTime.QuadPart != 0)
       {
            // CurrentValue = NewValue - OldValue
            dbIdleTime = Li2Double(SysPerfInfo.liIdleTime) -
Li2Double(liOldIdleTime);
            dbSystemTime = Li2Double(SysTimeInfo.liKeSystemTime) -
Li2Double(liOldSystemTime);

            // CurrentCpuIdle = IdleTime / SystemTime
            dbIdleTime = dbIdleTime / dbSystemTime;

            // CurrentCpuUsage% = 100 - (CurrentCpuIdle * 100) /
NumberOfProcessors
            dbIdleTime = 100.0 - dbIdleTime * 100.0 /
(double)SysBaseInfo.bKeNumberProcessors + 0.5;

            printf("\b\b\b\b%3d%%",(UINT)dbIdleTime);
       }

        // store new CPU's idle and system time
        liOldIdleTime = SysPerfInfo.liIdleTime;
        liOldSystemTime = SysTimeInfo.liKeSystemTime;

        // wait one second
        Sleep(1000);
    }
    printf("\n");
}

처버 개발자 님이 쓰신 글 :
: 윈도우2000에서 작업관리자를 보면 현재 cpu사용률을 그래프로 그리잖아요.
: 그것을 구현하고 싶은데
: 현재 cpu사용률 구하려면 어떻게 해야하나요?
: 아시는 분 있으면 답변 바랍니다.