Adding lines to an open file?

Question

I have an std::ifstream object:

std::ifstream file(file_path);

I read lines from it:

std::getline(file, line);

Is it legal to add lines to this file from another place while the stream is open? E.g. even after I reach EOF in the C++ program, can lines be added through a text editor, and getline be called again to get the newly added lines? What does the standard say?

This post seems relevant? http://stackoverflow.com/questions/15385302/reading-a-file-while-it-is-being-written-by-another-process — taocp, Jun 14 '13 at 15:52
open system call may not succeed. what are you trying to do, if you want an app to write into a file, then another to read at the same time, try to used a pipe, search for "pipe c++" — aah134, Jun 14 '13 at 15:53
It is legal, but it's not likely to do what you want it to. You may get lucky, though... — twalberg, Jun 14 '13 at 16:11

score 0 · Answer 1 · answered Jul 19 '13 at 13:44

It is legal and it should do what you expect on most reasonable operating systems. Note that the input stream needs to be cleared in case you reach EOF. The output stream needs to be flushed one way or another. The code below illustrates that.

#include <iostream>
#include <fstream>
#include <string>

int main()
{
    const char *f = "f.txt";
    // create an empty file
    std::ofstream os(f);
    os.close();
    std::ifstream is(f);
    for (unsigned i = 1; 3 > i; ++i)
    {
        os.open(f, std::ios_base::out | std::ios_base::app);
        os << "line  " << i << std::endl;
        os.close();
        std::string s;
        is.clear();
        std::getline(is, s);
        std::cerr << s << std::endl;
    }
    os.open(f, std::ios_base::out | std::ios_base::app);
    for (unsigned i = 3; 6 > i; ++i)
    {
        os << "line  " << i << std::endl;
        os.flush();
        std::string s;
        is.clear();
        std::getline(is, s);
        std::cerr << s << std::endl;
    }
}

Adding lines to an open file?

1 Answers1