How to save picture to iPhone photo library?

The simplest way is:

UIImageWriteToSavedPhotosAlbum(myUIImage, nil, nil, nil);

For Swift, you can refer to Saving to the iOS photo library using swift

You can use this function:

UIImageWriteToSavedPhotosAlbum(UIImage *image, 
                               id completionTarget, 
                               SEL completionSelector, 
                               void *contextInfo);

You only need completionTarget, completionSelector and contextInfo if you want to be notified when the UIImage is done saving, otherwise you can pass in nil.

See the official documentation for UIImageWriteToSavedPhotosAlbum().

Deprecated in iOS 9.0.

There`s much more fast then UIImageWriteToSavedPhotosAlbum way to do it using iOS 4.0+ AssetsLibrary framework

    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

    [library writeImageToSavedPhotosAlbum:[image CGImage] orientation:(ALAssetOrientation)[image imageOrientation] completionBlock:^(NSURL *assetURL, NSError *error){
    if (error) {
    // TODO: error handling
    } else {
    // TODO: success handling
    }
}];
[library release];