f you can dig down some way against teh wall, all the better as you can concrete the bottom in, and drill through the post carfully, then either use a couple of long rawlbolts, or some threaded rod resined into the wall.
they do make a concrete wall plate which is basically half a post in terms of thickness, so it easier to get against the wall
If you are fastening it to the wall, why not use a wooden post? It can be raised above the ground to avoid rot, and the strength will come from the wall.
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