#import "SampleApp.h"
#include <time.h>
#include <math.h>

@implementation SampleApp

- (void)acceleratedInX:(float)xx Y:(float)yy Z:(float)zz
{
   // Create Status feedback string
   NSString *xstring = [NSString stringWithFormat:
      @"X (roll, %4.1f%%): %f\nY (pitch %4.1f%%): %f\nZ (%4.1f%%) : %f",
      100.0 - (xx + 0.5) * 100.0, xx, 
      100.0 - (yy + 0.5) * 100.0, yy, 
      100.0 - (zz + 0.5) * 100.0, zz];
   [textView setText:xstring];

   // Revert Arrow and then rotate to new coords
   [xarrow setTransform:CGAffineTransformIdentity];
   float angle = atan2(yy, xx);
   angle *= 180.0/3.14159;
   [xarrow setRotationBy:angle];
}

- (void) applicationDidFinishLaunching: (id) unused
{
    struct CGRect rect = [UIHardware fullScreenApplicationContentRect];
    rect.origin.x = rect.origin.y = 0.0f;
    window = [[UIWindow alloc] initWithContentRect: rect];
    mainView = [[UIView alloc] initWithFrame: rect];

    textView = [[UITextView alloc] initWithFrame:
          CGRectMake(0.0, 0.0, 320.0, 480.0)];
    [textView setEditable:NO];
    [textView setTextSize:24];
    [mainView addSubview: textView];

    xarrow = [[UIImageView alloc] initWithFrame:
          CGRectMake(50.0, 320.0, 200.0, 50.0)];

    UIImage *img = [[UIImage imageAtPath:
         [[NSBundle mainBundle] 
             pathForResource:@"arrow"
             ofType:@"png" 
             inDirectory:@"/"]] retain];

    [xarrow setImage:img];
    [textView addSubview:xarrow];

    [window orderFront: self];
    [window makeKey: self];
    [window _setHidden: NO];
    [window setContentView: mainView];

}

@end


2、http://blog.medallia.com/2007/08/iphone_accelerometer_source_co.html
Here's some code to initialize the accelerometer to run at full speed. Pass the desired sample rate (in Hz) to the initialize function. Go wild!

#include <IOKit/IOKitLib.h>#include <CoreFoundation/CoreFoundation.h>typedef struct {} *IOHIDEventSystemRef;typedef struct {} *IOHIDEventRef;float IOHIDEventGetFloatValue(IOHIDEventRef ref, int param);void handleHIDEvent(int a, int b, int c, IOHIDEventRef ptr) {  int type = IOHIDEventGetType(ptr);  if (type == 12) {    float x,y,z;    x = IOHIDEventGetFloatValue(ptr, 0xc0000);    y = IOHIDEventGetFloatValue(ptr, 0xc0001);    z = IOHIDEventGetFloatValue(ptr, 0xc0002);    // do whatever you need to do with the gravity    ballSetAccel(x, y);  }}   #define expect(x) if(!x) { printf("failed: %s\n", #x);  return; }void initialize(int hz) {  mach_port_t master;  expect(0 == IOMasterPort(MACH_PORT_NULL, &master));int page = 0xff00, usage = 3;CFNumberRef nums[2];  CFStringRef keys[2];  keys[0] = CFStringCreateWithCString(0, "PrimaryUsagePage", 0);  keys[1] = CFStringCreateWithCString(0, "PrimaryUsage", 0);  nums[0] = CFNumberCreate(0, kCFNumberSInt32Type, &page);  nums[1] = CFNumberCreate(0, kCFNumberSInt32Type, &usage);  CFDictionaryRef dict = CFDictionaryCreate(0, (const void**)keys, (const void**)nums, 2, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);  expect(dict);IOHIDEventSystemRef sys = (IOHIDEventSystemRef) IOHIDEventSystemCreate(0);  expect(sys);CFArrayRef srvs = (CFArrayRef)IOHIDEventSystemCopyMatchingServices(sys, dict, 0, 0, 0);  expect(CFArrayGetCount(srvs)==1);io_registry_entry_t serv = (io_registry_entry_t)CFArrayGetValueAtIndex(srvs, 0);  expect(serv);CFStringRef cs = CFStringCreateWithCString(0, "ReportInterval", 0);  int rv = 1000000/hz;  CFNumberRef cn = CFNumberCreate(0, kCFNumberSInt32Type, &rv);int res = IOHIDServiceSetProperty(serv, cs, cn);  expect(res == 1);res = IOHIDEventSystemOpen(sys, handleHIDEvent, 0, 0);  expect(res != 0);}