Ubuntu imagemagick C transparent background - c

Question

hello i'm trying to create a png with transparent background using the C api of imagemagick. I'm working on macosx and this is what i wrote:
UPDATED
i have this function that create me new image:
Image *new_image(Image *composed_image, ImageInfo *composed_image_info){
ExceptionInfo *exception;
MagickPixelPacket *background;
// make it transparent
background->red = QuantumRange;
background->blue = QuantumRange;
background->green = QuantumRange;
background->opacity = QuantumRange;
exception=AcquireExceptionInfo();
composed_image = NewMagickImage(composed_image_info, WIDTH, HEIGHT, background);
if (exception->severity != UndefinedException) CatchException(exception);
if (composed_image == (Image *) NULL) printf("Errore composed \n");
exception = DestroyExceptionInfo(exception);
WriteImage(composed_image_info, composed_image);
return composed_image;
}
then the main main:
int main(int argc, char **argv) {
int images_counter = argc-3;
Image *images[images_counter], *composed_image;
ImageInfo *composed_image_info;
ExceptionInfo *exception;
// create new blank image
composed_image_info = CloneImageInfo((ImageInfo *) NULL);
strcpy(composed_image_info->filename, argv[argc-2]);
composed_image = new_image(composed_image, composed_image_info);
strcpy(composed_image->filename, argv[argc-2]);
// get all the images
for(size_t i = 0; i < images_counter; ++i) images[i] = read_image(argv[i+1]);
// build all the images together
for(size_t i = 0; i < images_counter; ++i) build_image(images[i], composed_image);
// write the image
WriteImage(composed_image_info, composed_image);
// close and return
MagickCoreTerminus();
return(0);
}
on osx it works but when i try it on Ubuntu server the created image is with white background, am i doing something wrong?
thanks