Is there a way to write into a file while another process is reading?

Question

I have a writing process on C++ application, another C# application reads data continuously whenever the file has been changed.

On C++:

FILE *fp = fopen(result_file, "a");
if (fp) {
    // Write some thing
    fclose(fp);
}

On C#:

private void Init() {
    FileSystemWatcher watcher = new FileSystemWatcher(ResultFolder);
    watcher.Changed += new FileSystemEventHandler(OnResultChanged);
    watcher.EnableRaisingEvents = true;
}

private void OnResultChanged(object sender, FileSystemEventArgs e) {
    if (e.ChangeType == WatcherChangeTypes.Changed) {
        // Check file ready to read

        // Ready all lines
        string[] lines = File.ReadAllLines(e.FullPath);
        // Process lines
    }
}

But sometimes the code on C++ cannot open the file to read, how can I fix it?

P/S: I found that on C# we have a way to share file access such as below command

File.Open(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)

But cannot find similar way in C++.

score 1 · Accepted Answer · edited May 23 '17 at 10:28

You're on Windows, so the C++ way is

CreateFile(.... FILE_SHARE_READ | FILE_SHARE_WRITE, ....)

It returns a Win32 HANDLE, which is not the easiest thing to use in C++ (there are no convenience functions for formatted I/O). But you can turn it into a FILE* or fstream.

See this question:

Can I use CreateFile, but force the handle into a std::ofstream?

Or, you can use the shflag parameter of _fsopen() or the _Prot parameter of an fstream constructor:

The argument shflag is a constant expression consisting of one of the following manifest constants, defined in Share.h.

Term Definition

_SH_COMPAT Sets Compatibility mode for 16-bit applications.

_SH_DENYNO Permits read and write access.

_SH_DENYRD Denies read access to the file.

_SH_DENYRW Denies read and write access to the file.

_SH_DENYWR Denies write access to the file.

MSDN has the documentation and examples

Is there a way to write into a file while another process is reading?

1 Answers1