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POHow to get the double value using libpq?
 

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  1. POHow to get the double value using libpq?
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    <p>The <a href="http://www.postgresql.org/docs/9.3/interactive/libpq-example.html" rel="nofollow">examples</a> in the libpq documentation show how to get the the integer value by converting it to the host-endian representation.</p> <p>I am curious what must be done to get the double precision value using libpq (without libpqtyppes)? I have tried <code>reinterpret_cast</code> with no luck.</p> <p>Also why text and byte data doesn't need endian conversions?</p> <p>The DB runs locally on Windows 7, I am using Visual C++ 2013.</p> <p><code>pptr</code> is a double vaule I am trying to retrieve.</p> <pre><code>#include &lt;iostream&gt; #include &lt;memory&gt; #include &lt;vector&gt; #include &lt;libpq-fe.h&gt; #include &lt;Winsock2.h&gt; static void show_binary_results(PGresult *res) { int i, j; int i_fnum, n_fnum, p_fnum; /* Use PQfnumber to avoid assumptions about field order in result */ i_fnum = PQfnumber(res, "id"); n_fnum = PQfnumber(res, "name"); p_fnum = PQfnumber(res, "price"); for (i = 0; i &lt; PQntuples(res); i++) { char* iptr; char* nptr; char* pptr; int blen; int ival; /* Get the field values (we ignore possibility they are null!) */ iptr = PQgetvalue(res, i, i_fnum); nptr = PQgetvalue(res, i, n_fnum); pptr = PQgetvalue(res, i, p_fnum); /*THIS IS A VALUE I AM TRYING TO GET*/ /* * The binary representation of INT4 is in network byte order, which * we'd better coerce to the local byte order. */ ival = ntohl(*((uint32_t *) iptr)); /* * The binary representation of TEXT is, well, text, and since libpq * was nice enough to append a zero byte to it, it'll work just fine * as a C string. * * The binary representation of BYTEA is a bunch of bytes, which could * include embedded nulls so we have to pay attention to field length. */ //blen = PQgetlength(res, i, b_fnum); printf("tuple %d: got\n", i); printf(" i = (%d bytes) %d\n", PQgetlength(res, i, i_fnum), ival); printf(" t = (%d bytes) '%s'\n", PQgetlength(res, i, n_fnum), nptr); printf(" p = (%d bytes) %f\n", PQgetlength(res, i, p_fnum), *reinterpret_cast&lt;double*&gt;(pptr)); printf("\n\n"); } } int main(int argc, char* argv []) { auto conn_string = "postgresql://postgres:pwd@localhost/db"; auto conn_deleter = [](PGconn* c) { PQfinish(c); }; auto res_deleter = [](PGresult* r) { PQclear(r); std::cout &lt;&lt; "deleted" &lt;&lt; std::endl; }; std::unique_ptr&lt;PGconn, decltype(conn_deleter)&gt; conn(PQconnectdb(conn_string), conn_deleter); if (PQstatus(conn.get()) != ConnStatusType::CONNECTION_OK) std::cerr &lt;&lt; "Problem" &lt;&lt; std::endl; std::vector&lt;const char *&gt; params{ "1" }; std::unique_ptr &lt; PGresult, decltype(res_deleter)&gt; res(PQexecParams(conn.get(), "SELECT * FROM table_with_double WHERE id = $1", params.size(), /* one param */ NULL, /* let the backend deduce param type */ (const char * const *)&amp;params.front(), NULL, /* don't need param lengths since text */ NULL, /* default to all text params */ 1), /* ask for binary results */ res_deleter); if (PQresultStatus(res.get()) != ExecStatusType::PGRES_TUPLES_OK) std::cout &lt;&lt; "SELECT failed: " &lt;&lt; PQerrorMessage(conn.get()) &lt;&lt; std::endl; show_binary_results(res.get()); } </code></pre>
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    1. USStephane Rolland
    1. USKimi
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    1. COHow do you call PQgetvalue ?
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      1. POHow to get the double value using libpq?
      1. USStephane Rolland
    2. COAFAIK, `text` and `bytea` are basically glorified `char*` at this level so there is no endianness to worry about, similarly the bit layout of an IEEE double is machine-independent so there's no endianness there either.
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      1. USmu is too short
    3. CO@muistooshort, that is what I would expect, but the double value I'm getting is 0.
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      1. USKimi
 

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