构造iostream（c ++）是否将数据从硬盘驱动器读入内存？

Question

When I construct an iostream when say opening a file will this always read the entire file from the hard disk and then put it into memory, or is it streamed in and buffered by the OS on demand?

I ask because one way to check if a file exists is to see if opening it fails, but I fear if the files I am opening are very large then this take a long time if iostream must read the entire file in on open.

Answer 1

To check whether a file exists can be done like this if you want to use boost .

#include <boost/filesystem.hpp>

bool fileExists = boost::filesystem::exists("foo.txt");

Answer 2

No, it will not read the entire file into memory when you open it. It will read your file in chunks though, but I believe this process will not start until you read the first byte. Also these chunks are relatively small (on the order of 4-128 kibibytes in size), and the fact it does this will speed things up greatly if you are reading the file sequentially.

In a test on my Linux box (well, Linux VM) simply opening the file only results in the OS open system call, but no read system call. It doesn't start reading anything from the file until the first attempt to read from the stream. And then it reads 8191 (why 8191? that seems a very strange number) byte chunks as I read the file in.

Answer 3

Opening a file is a bad way of testing if the file exists - all it does is tell you if you can open it. Opening might fail for a number of reasons, typically because you don't have read permission, but the file will still exist. It is usually better to use an operating system specific function to test for existence. And no, opening an fstream will not cause the contents to be read.

Answer 4

What I think is, when you open a file, the corresponding data structures for the process opening the file are populated which include file pointer, file descriptor, v node etc.

Now one can read and write to a file using buffered streams (fwrite , fread) or using system calls (read and write).

When we use buffered streams, we buffer the data and then write or read it[This is done for efficiency puposes]. This statement itself means that the whole file is not read into memory but certain bytes are read into buffer and then made available.

In case of sys calls such as read and write , kernel level buffering is done (using fsync one can flush out kernel buffer too), but data is actually read and written to the device .file

checking existance of file

#include < sys/stat.h >

int main(){
struct stat file_i;
std::string f("myfile.txt");
if (stat(f.c_str(),&file_i) != 0){
   cout << "File not found" << endl;
}
return 0;
}

Hope this clarifies a bit.