Thanks for your help. I'd be happy to show you my code:

My *.h file

Qt Code: 
  1. class MyWidget : public QWidget
  2. {
  3. ..........
  4. public:
  5.   void function1();
  6.   void function2();
  7.  
  8. private:
  9.   QStringList list2;
  10. ..........
  11. }
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There's nothing in the constructor for the QStringList.
My main *.cpp file:

Qt Code: 
  1. void MyWidget::function1(){
  2.                     QStringList list; // declared and filled with data, everything OK here
  3.                     .....................
  4.                     i =4;
  5.                     QStringList list2(list);
  6.                     for (int j =0; j <list.size(); j++)
  7.                     {
  8.                      list2[j] = list[i];
  9.                      i +=1;
  10.                      if (i>=list.size()) i=0;
  11.                     }
  12.  
  13.                     function2();
  14.  }
  15.  
  16. void MyWidget::function2(){
  17.     // contains my C code to test if the right thing happened
  18.     //nothing is written into the output file
  19.     // but it works if I put this code inside function1()
  20.     //so I know there are no logic or syntax error
  21.           FILE *my;
  22.           if ((my=fopen("output.txt","w")) == 0) {exit(1);}	
  23.           char filename2[70];
  24.           for (int k =0; k <list2.size(); k++){ 
  25.              strncpy(filename2, list2[k].toLocal8Bit().constData(), 70);                                    
  26.              fprintf(my, "%s\n", filename2);  
  27.            }
  28.  
  29.                     fclose(my);
  30. }
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