error "unknow error" occurs when using open function in QFile

i'am under Ubuntu 9.04 with QT4 and eclipse, here is my code to open a existent file.

Code:

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QFile file("SC");
if(!file.open(QIODevice::ReadOnly|QIODevice::Text))
       qDebug()<<"error"<<file.errorString();

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and when i run the code , output is error "Unknown error".
what's wrong with it?
Thanks