How is the file offset advanced by fstream::read?

Question

#include <fstream>
#include <iostream>


int main(int argc, char * argv [])
{
    std::ifstream f{ "test.syp" };
    std::cout << f.tellg() << '\n';
    char buffer[4];
    f.read(buffer, 4);
    std::cout << f.gcount() << '\n';
    std::cout << f.tellg() << '\n';
}

When I execute the above code, I get the following output:

0
4
20

If I change ifstream to fstream, I get the same thing except that the last number is 21.

I would expect the last number to be 4 in both instances. Why isn't it?

Edit: I get the expected result if I open the file with std::ios::binary; it must be a quirk of text-mode

Perhaps this is a bug in Visual Studio 2013. Let me try on OS X with clang... wut? I get -1 0 -1. Maybe I'm doing something wrong. — Mike Crawford, Dec 18 '14 at 12:24
Should the curly braces on line 7 - { "test.syp" } - be instead parentheses? — Mike Crawford, Dec 18 '14 at 12:27
The curly braces are the C++11 style initialization. It shouldn't make a difference. — Sydius, Dec 18 '14 at 16:56
@Mike: `-1 0 -1` hints that `f` wasn't successfully opened for read - perhaps you don't have a readable `test.syp` in your working directory? — Toby Speight, Nov 05 '19 at 15:56

score 0 · Answer 1 · answered Nov 05 '19 at 16:04

f.gcount returns the number of characters read by the last unformatted input operation (in this case, f.read()).

f.tellg() returns an input position indicator; that's an opaque value that has meaning to seekg(), but its numeric value isn't meaningful to user programs.

How is the file offset advanced by fstream::read?

1 Answers1