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extern "C"
{
    char* result(char* );
}

char* result(char* a)
{
    return a;
}

#include <iostrem>

#include <QtCore/QtGlobal>
#include <math.h>

#ifdef Q_OS_WIN 
#	if defined( QT_BUILD_MATH_LIB )
#		define Q_MATH_EXPORT __declspec(dllexport)
#	else 
#		define Q_MATH_EXPORT __declspec(dllimport)
#	endif
#else 
#		define Q_MATH_EXPORT
#endif  	


extern "C" Q_MATH_EXPORT double ib_udf_round_to(double *x, int *y)
{
 double p = pow(10.0, *y);
 return qRound64(*x * p) / p;
}

...
win32 {
 DEFINES +=  QT_BUILD_MATH_LIB
}
...

QString str;

#include <string>
std::string str;

QByteArray::toString()

extern "C"  __declspec(dllexport) char* ib_udf_createUUID()
{
  char *p; 
  
  p = QUuid::createUuid().toString().toLatin1().data();
  return p;
}

#include <QtCore/QtGlobal>
#include <QUuid>

#ifdef Q_OS_WIN
#	if defined( QT_BUILD_MATH_LIB )
#		define Q_MATH_EXPORT __declspec(dllexport)
#	else
#		define Q_MATH_EXPORT __declspec(dllimport)
#	endif
#else
#		define Q_MATH_EXPORT
#endif

extern "C"  __declspec(dllexport) char* ib_udf_createUUID()
{
  char *p;

  p = QUuid::createUuid().toString().toLatin1().data();
  return p;
}

DECLARE EXTERNAL FUNCTION UDF
RETURNS CSTRING(254) FREE_IT
ENTRY_POINT 'ib_udf_createUUID' MODULE_NAME 'crqphudf';

select UDF() from Object

#include <QtCore/QtGlobal>
#include <QtCore/QUuid>
#include <ib_util.h>

extern "C"  __declspec(dllexport) char* ib_udf_createUUID()
{
  char *p, *buf;

  p = QUuid::createUuid().toString().toLatin1().data();
  buf = (char*) ib_util_malloc(64);

  затем перегоняешь из p в buf

  return buf;
}

#include <QtCore/QtGlobal>
#include <QDebug>

extern "C" __declspec(dllexport) char* tostring(char* str)
{
  QByteArray data(str);
  data = data.toBase64();

  char *p = new char();

  p = data.data();

  qDebug() << p;

  return p;
}

QT -= gui
QT += core
TARGET = crqphudf
TEMPLATE = lib
DEFINES += CRQPHUDF_LIBRARY
SOURCES += crqphudf.cpp
HEADERS += 
CONFIG += release

    typedef char* (*toBase)(char *);
    toBase tobase = (toBase) QLibrary::resolve("crqphudf","tostring");

    char* base = tobase("qwerty");

    qDebug() << "key" << base;

qDebug() << p;

qDebug() << "key" << base;

  char *p = new char();
  p = str;
  return p;

kernel32.dll
mingw10.dll
msvctrl.dll
QtCore4.dll

extern "C" __declspec(dllexport) char* uid()
{
  char *p;

  p = QUuid::createUuid().toString().toAscii().data();
  return p;
}

DECLARE EXTERNAL FUNCTION UDF
  RETURNS CSTRING(254)
  ENTRY_POINT 'uid'
  MODULE_NAME 'crqphudf'

select UDF() from Object