Retrieve all the returned rows in one call

I am right now using the libpq interface and struggling with efficiently
retrieve the result from a query.

Let's say I run a select statement with some where clauses and then retrieve
the result into a PGresult. I want to know if there is an efficient way to
retrieve all the rows at once? I am going to return a char*** from my
function, i.e. a two-dimensional array of char pointers.
I have seen the COPY sql command, but up to v8.1 i cannot use a select
statement with it, the only way to customize COPY is to choose which columns
to return, but still, the only option is to return the whole table.
Furthermore, when using PQgetCopyData I get the whole row at once, but I
still have to loop through all rows.
I have also seen in the PGresult structure that it has a member called
tuples, which is a two-dimensional array of the structure PGresAttValue.
This is a way to recieve all the data at once, but it is in a format which
is specific for PostgreSQL and I am developing a database interface
including other databases two, so I want to use a general structure returned
from the function, i.e. a char***. This means that I would have to loop
through the tuples array to convert into the char array structure, and hence
lose the whole meaning with recieving all the rows at once.

So my question is if it is possible in any way to retrieve the result in the
form of char***?

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