ifstream vs. fread for binary files

Question

Which is faster? ifstream or fread.
Which should I use to read binary files?

fread() puts the whole file into the memory.
So after fread, accessing the buffer it creates is fast.

Does ifstream::open() puts the whole file into the memory?
or does it access the hard disk every time we run ifstream::read()?

So... does ifstream::open() == fread()?
or (ifstream::open(); ifstream::read(file_length);) == fread()?

Or shall I use ifstream::rdbuf()->read()?

edit: My readFile() method now looks something like this:

void readFile()
{
    std::ifstream fin;
    fin.open("largefile.dat", ifstream::binary | ifstream::in);
    // in each of these small read methods, there are at least 1 fin.read()
    // call inside.
    readHeaderInfo(fin);
    readPreference(fin);
    readMainContent(fin);
    readVolumeData(fin);
    readTextureData(fin);
    fin.close();
}

Will the multiple fin.read() calls in the small methods slow down the program? Shall I only use 1 fin.read() in the main method and pass the buffer into the small methods? I guess I am going to write a small program to test.

Thanks!

Solution 1

Are you really sure about fread putting the whole file into memory? File access can be buffered, but I doubt that you really get the whole file put into memory. I think ifstream::read just uses fread under the hood in a more C++ conformant way (and is therefore the standard way of reading binary information from a file in C++). I doubt that there is a significant performance difference.

To use fread, the file has to be open. It doesn't take just a file and put it into memory at once. so ifstream::open == fopen and ifstream::read == fread.

Solution 2

As to which is faster, see my comment. For the rest:

• Neither of these methods automatically reads the whole file into memory. They both read as much as you specify.
• As least for ifstream I am sure that the IO is buffered, so there will not necessarily be a disk access for every read you make.
• See this question for the C++-way of reading binary files.

Solution 3

C++ stream api is usually a little bit slower then C file api if you use high level api, but it provides cleaner/safer api then C. If you want speed, consider using memory mapped files, though there is no portable way of doing this with standard library.

Comments

[Admin]

It will pull the whole file into memory if you tell it to, and if it can. And when it comes to I/O whether you doubt there is a performance difference or not is neither here nor there - the only thing to do is measure it.

[Admin]

Unfortunately, depending on "what it thinks is optimal" will not typically give you optimal performance.

[Christian Rau]

@Neil How do you tell it to, query the file's size and read a block of the whole size? You can do that with ifstream, too.

[Admin]

Yes you can do it with both? And so?

[Christian Rau]

I think, the OP thought it just takes a file and puts it into memory at a whole. But that's just wrong, fread and ifstream::read serve the same functionality and your comment didn't really help to clarify that to him.

[Admin]

How do you know "your comment didn't really help to clarify that to him"?

[Snowfish]

Thanks for the answer, I saw the question. The problem is the read() method was written by someone else. He ifstream::open() the file in the main read method, and pass the ifstream to small readContent() methods, which has fin.read() inside them. So I got confused if I should combine the fin.read() into 1 big fin.read() and pass the buffer to the small methods.

[Christian Rau]

@Neil I'll hopefully end this discussion now. I just meant don't state half-truths. Of course an fread can read the whole file, but an ifstream::read can just do exactly the same. And I know you always can and should measure things, that's why I said "doubt" and not "know".

[Snowfish]

Thanks! Now I understand that ifstream::read == fread. But then I figured out my true question from your answer, which is if I should ifstream::read once, and pass the buffer around, or ifstream::open, and pass the ifstream around to have multiple ifstream::read.

[Björn Pollex]

@Snowfish: I am not sure I understand. I think it would be best if you post this as a separate question and show the code you have.