Hi Saman,
thank you for your answer, i've not been at work so i didn't read your reply.
Compiling with a 32 bit compiler is what i've done for first, i'm using Qt 5.0.2 MinGW 32bit
I don't understand from your reply if i have to put in my application folder both the 32 and 64 bit compiled dll.
Do i need two versions of the same library?
I thought about some type conversions of the parameter i'm passing to the library but i'm using only pointers, int and chars. I don't know where it could be fail.
Follow parts of my code
QTR.pro
Qt Code: 
  1. #-------------------------------------------------
  2. #
  3. # Project created by QtCreator 2012-11-13T11:23:02
  4. #
  5. #-------------------------------------------------
  6.  
  7. TARGET = QTR
  8. TEMPLATE = lib
  9. QMAKE_CFLAGS += /Gz QMAKE_CXXFLAGS += /Gz
  10. DEFINES += QTR_LIBRARY
  11.  
  12. SOURCES += qtr.cpp
  13.  
  14. HEADERS += qtr.h\
  15.         QTR_global.h
  16.  
  17. symbian {
  18.     MMP_RULES += EXPORTUNFROZEN
  19.     TARGET.UID3 = 0xE3A9118A
  20.     TARGET.CAPABILITY = 
  21.     TARGET.EPOCALLOWDLLDATA = 1
  22.     addFiles.sources = QTR.dll
  23.     addFiles.path = !:/sys/bin
  24.     DEPLOYMENT += addFiles
  25. }
  26.  
  27. unix:!symbian {
  28.     maemo5 {
  29.         target.path = /opt/usr/lib
  30.     } else {
  31.         target.path = /usr/lib
  32.     }
  33.     INSTALLS += target
  34. }
  35.  
  36. CONFIG += QwtDll
  37.  
  38.  
  39. win32 {
  40. QMAKE_LFLAGS += -static-libgcc
  41. }
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header file

Qt Code: 
  1. #-------------------------------------------------
  2. #
  3. # Project created by QtCreator 2012-11-13T11:23:02
  4. #
  5. #-------------------------------------------------
  6.  
  7. TARGET = QTR
  8. TEMPLATE = lib
  9. QMAKE_CFLAGS += /Gz QMAKE_CXXFLAGS += /Gz
  10. DEFINES += QTR_LIBRARY
  11.  
  12. SOURCES += qtr.cpp
  13.  
  14. HEADERS += qtr.h\
  15.         QTR_global.h
  16.  
  17. symbian {
  18.     MMP_RULES += EXPORTUNFROZEN
  19.     TARGET.UID3 = 0xE3A9118A
  20.     TARGET.CAPABILITY = 
  21.     TARGET.EPOCALLOWDLLDATA = 1
  22.     addFiles.sources = QTR.dll
  23.     addFiles.path = !:/sys/bin
  24.     DEPLOYMENT += addFiles
  25. }
  26.  
  27. unix:!symbian {
  28.     maemo5 {
  29.         target.path = /opt/usr/lib
  30.     } else {
  31.         target.path = /usr/lib
  32.     }
  33.     INSTALLS += target
  34. }
  35.  
  36. CONFIG += QwtDll
  37.  
  38.  
  39. win32 {
  40. QMAKE_LFLAGS += -static-libgcc
  41. }
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can anybody help?
thank you in advance
daniel