Qt: cannot open file for writing -

i try access simple text file qt-widget application qfile class reading writing. reading file line line string works fine. opening ready write fails. following code checks if file exists , tries set proper permissions, in end file won't open. here failing piece of code:

#include "mainwindow.h" #include <qapplication> #include <qfile> #include <qdebug>  int main(int argc, char *argv[]){   qapplication app(argc, argv);   mainwindow w;   w.show();      qfile file(":/test.dat");     qdebug() << "exists?              " << file.exists();     qdebug() << "writable?            " << file.iswritable();     qdebug() << "permissions before?  " << file.permissions();     qdebug() << "permissions set?     " << file.setpermissions(qfiledevice::writeother | qfiledevice::readother);     qdebug() << "permissions after?   " << file.permissions();      qdebug() << "opened?              " << file.open(qiodevice::append);     qdebug() << "errors?              " << file.errorstring();     qdebug() << "errnum?              " << file.error();     qtextstream out(&file);     out << "something append";     file.close();    return app.exec(); } 

qt returns message:

exists?               true writable?             false permissions before?   qflags(0x4|0x40|0x400|0x4000) permissions set?      false permissions after?    qflags(0x4|0x40|0x400|0x4000) opened?               false errors?               "unknown error" errnum?               5 qiodevice::write (qfile, ":/test.dat"): device not open 

if change parameter in open-function qiodevice::readonly file readable without problems, failing qiodevice::writeonly. why doesn't same thing work writing well? permission? , why don't permissions change after called setpermissions? run qt root on ubuntu 14.04. , test.dat has full rights -rwxrwxrwx owned user. can help? thanks!

the author having linux-related problem writing file created console process elevated privileges. have reproduced problem , when attempted remove file with:

vi \home\myuser\documents\f.txt // create file console rm \home\myuser\documents\f.txt // try remove console 

i got "rm: remove write-protected regular file "\home\myuser\documents\f.txt" , responded "yes" , code above shows after creating new file in context of program's process:

opened?               true exists?               true writable?             true permissions before?   qflags(0x4|0x20|0x40|0x200|0x400|0x2000|0x4000) permissions set?      true permissions after?    qflags(0x4|0x20|0x40|0x200|0x400|0x2000|0x4000) errors?               "unknown error" errnum?               0 

i run qt creator root on ubuntu 14.04.

it not ensure privileges of program run it, guess. update: make sure run program appropriate permissions e.g. root in case.