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When you have opened a resource fork, you can write resources to it (assuming it is the current resource file).
A call to AddResource creates a new entry for a resource in the resource map in memory (but not on the disk) and sets that entry's location to refer to that resource's data. A call to either UpdateResFile or WriteResFile will then write the resource to disk. Note that you usually need to set the current resource file to the desired file before calling AddResource because AddResource always adds the resource to the resource map in memory, which corresponds to the current resource file.