A problem about file reading

TPtrC strValue;
HBufC* str = HBufC::NewLC(50);
CleanupStack::PushL(str);
RFs fs;
User::LeaveIfError(fs.Connect());
CleanupClosePushL(fs);
RFile file;
User::LeaveIfError(file.Open(fs, filePath, EFileRead));
TFileText tfile;
tfile.Set(file);
TBuf<1024> buf;
tfile.Seek(ESeekStart);

while (tfile.Read(buf) == KErrNone)
{
　if (buf.Find(MainKey) >= 0)
　{
　　strValue.Set(buf);
　　str = strValue.AllocL();
　}
}
file.Close();
CleanupStack::PopAndDestroy();
CleanupStack::PopAndDestroy(str);