Question

In researching forcing files to be written as quickly as possible, I have seen the following chunk of code in a few places:

_commit(outputFile.rdbuf()->fd());

where outputFile is an std::ofstream object. I am basically doing the following:

std::ofstream outputFile;
outputFile.write((char*)blocks, sizeof(blocks));
outputFile.flush();
_commit(outputFile.fd());
outputFile.close();

blocks is a simply an array of char arrays. What I am doing is writing to an array of char arrays to represent the contents of a binary file, then pushing that array out to the file itself. I need the files to get written as quickly as possible as the medium is somewhat less reliable flash memory and the chance of power outages is unusually high.

When I attempt to compile, I receive the following error on the _commit line of code:

error C2039: 'fd' : is not a member of 'basic_ofstream<char,struct std::char_traits<char> >'

How should I go about getting the file descriptor of outputFile and passing it to _commit?

EDIT: I have changed the above code to reflect using ofstream.fd(), which is supposed to be a public member of ofstream.

Answer 1

Someone should have written this as an answer. They have not so I will steal their glory. 🙂

The file descriptor is implementation dependent. Some systems might not have one. So the C++ Standard does not define any method to get it from a fstream or filebuf.

The _commit function in your example is definitely implementation dependent. One big hint is that the name starts with an underscore. A bit of Google shows that _commit only works on Windows.

To get what you want, you could write your own class derived from streambuf. This customized class would implement IO in the low-level terms that you want to use (Windows, POSIX) and can provide extra functions to return file descriptors or flush data to disk.

To use the custom class as a stream you would create a ostream and pass it a pointer to your streambuf class.