Can't write to file in a loop containing sleep()

The essence of my problem is that I can't write to a file in a loop with sleep(). If I have the following code:

ofstream file
file.open("file.name");

for(;;) {
    file << "HELLO\n";
}

This code works perfectl开发者_JAVA百科y and prints HELLO repeatedly into "file.name". However, I want to do something like this (I'm recording data from a real-time application):

for(;;) {
    file << "HELLO\n";
    sleep(1);
}

This doesn't seem to print anything into my file. Any ideas?

You need to flush the output. The output stream is buffering your data into memory but not writing it out to disk. You should either use std::endl (which prints a newline and flushes) instead of the string literal '\n', or explicitly flush the stream with std::flush:

for(;;) {
    file << "HELLO" << endl;
}
// or
for(;;) {
    file << "HELLO\n" << flush;
}

The magic word you are looking for is "flush".

c++ std::ofstream flush() but not close()

before the sleep, flush the file so that it isn't pending in a buffer waiting for there to be enough of a change to bother writing out.

It's probably just a buffering issue. Because you are now writing much slower, the output buffer wont fill up so fast so you may not 'see' the written data. Try adding a flush() before the sleep.

file.flush()