[cairo] Direct manipulation and write to PNG

[Carlos López González]

Hi!
I'm working on doing direct manipulation of a image surface data and
I'm obtaining weird results when writing to png.

I want to obtain a image like this:
http://imagebin.org/220100

but I get this:
http://imagebin.org/220098

To create the image I use the following code:

/////////////////////////////////////////////////
#include <cairo.h>

int main (int argc, char *argv[])
{
	cairo_surface_t *surface;
	surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 480, 270);
	cairo_surface_flush(surface);
	
	unsigned char* data;
	unsigned char*position;
	int w, h, stride;
	data=cairo_image_surface_get_data(surface);
	w=cairo_image_surface_get_width(surface);
	h=cairo_image_surface_get_height(surface);
	stride=cairo_image_surface_get_stride(surface);
	unsigned char a, r, g, b;
	int x, y;
	a=255;
	r=178;
	g=55;
	b=55;
	for(y=0; y<h; y++)
	{
		for(x=0; x<w*4; x+=4)
		{
			position= (unsigned char*)(((char*)data)+y*stride);
			position[x]=a;
			position[x+1]=r;
			position[x+2]=g;
			position[x+3]=b;
		}
	}
	cairo_surface_mark_dirty(surface);
	
	cairo_surface_write_to_png(surface, "image.png");
	
	cairo_surface_destroy(surface);
	
	return 0;
}
//////////////////////////////////////////////

I compiled it with:

$ gcc -o cairo4 `pkg-config --cflags --libs gtk+-2.0` cairo4.c

The png image file has the color somehow twisted and modified:
a=56
r=255
g=9
b=88

 I've re-checked that the written bytes are fine on the memory but I
don't get the right results on the png file.
Can anyone help me to let me know what I'm doing wrong? Or maybe it is
something related to png backend?

Thank you very much!!
-- 
Carlos
http://synfig.org