libpq文档中的examples显示如何通过将整数值转换为host-endian表示来获取整数值。
我很想知道使用libpq(没有libpqtyppes)获取双精度值必须做些什么?我试过reinterpret_cast
没有运气。
为什么文本和字节数据不需要字节序转换呢?
数据库在Windows 7上本地运行,我使用的是Visual C ++ 2013。
pptr
是我想要检索的双重vaule。
#include <iostream>
#include <memory>
#include <vector>
#include <libpq-fe.h>
#include <Winsock2.h>
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 < 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<double*>(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 << "deleted" << std::endl; };
std::unique_ptr<PGconn, decltype(conn_deleter)> conn(PQconnectdb(conn_string), conn_deleter);
if (PQstatus(conn.get()) != ConnStatusType::CONNECTION_OK)
std::cerr << "Problem" << std::endl;
std::vector<const char *> params{ "1" };
std::unique_ptr < PGresult, decltype(res_deleter)> 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 *)¶ms.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 << "SELECT failed: " << PQerrorMessage(conn.get()) << std::endl;
show_binary_results(res.get());
}
答案 0 :(得分:4)
显然双列的数据是大端的,必须转换为小端。与整数相同。基于此article的优秀answer,我使用了一个简单的函数来交换double值。
double double_swap(double d)
{
union
{
double d;
unsigned char bytes[8];
} src, dest;
src.d = d;
dest.bytes[0] = src.bytes[7];
dest.bytes[1] = src.bytes[6];
dest.bytes[2] = src.bytes[5];
dest.bytes[3] = src.bytes[4];
dest.bytes[4] = src.bytes[3];
dest.bytes[5] = src.bytes[2];
dest.bytes[6] = src.bytes[1];
dest.bytes[7] = src.bytes[0];
return dest.d;
}
应用此功能可从数据库中检索正确的值。
printf(" p = (%d bytes) %lf\n",
PQgetlength(res, i, p_fnum), double_swap(*((double*) pptr)));