How to append to a file with fstream fstream::app flag seems not to work

i simply want to write (append) to a logfile. I looked it up here:

http://www.cplusplus.com/reference/iostream/fstream/open/

so this is what i did

#include <fstream>

fstream outfile;

//outfile.open("/tmp/debug开发者_C百科.txt" );  // works, simply for writing
outfile.open("/tmp/debug.txt", fstream::app );  // does nothing

outfile << "START" << endl;

outfile.close();

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fstream::app|fstream::out instead of fstream::app. app doesn't make sense without specifying out (one could think it should have implied out, but it doesn't).