cgimagesourcecreatewithurl pdf
===

I'm trying to load thumb images in CGPDFDocument 
I wrote the following code in a function called "main" in class "ReaderThumbFetch"


    NSURL *thumbURL = [self thumbFileURL]; 
    
    CGImageRef imageRef = NULL;
    
	CGImageSourceRef loadRef = CGImageSourceCreateWithURL((CFURLRef)thumbURL, NULL);
    
	if (loadRef != NULL) // Load the existing thumb image
	{
		imageRef = CGImageSourceCreateImageAtIndex(loadRef, 0, NULL); // Load it
        
		CFRelease(loadRef); // Release CGImageSource reference
	}

and the implementation of "thumbFileURL" function is:

    - (NSURL *)thumbFileURL
    {
    #ifdef DEBUGX
	NSLog(@"%s", __FUNCTION__);
    #endif
    
    NSString *cachePath = [ReaderThumbCache thumbCachePathForGUID:request.guid]; // Thumb cache path
    
    NSString *fileName = [NSString stringWithFormat:@"%@.png", request.thumbName]; // Thumb file name
    
	return [NSURL fileURLWithPath:[cachePath stringByAppendingPathComponent:fileName]]; // File URL
    }

but when I run it gives me that:

    Undefined symbols for architecture i386:
    "_CGImageSourceCreateWithURL", referenced from:
     -[ReaderThumbFetch main] in ReaderThumbFetch.o

any help ?? 
Thanks in advanced.