CODEWITHSUNDEEP
c++windowsflushstd
vy32
vy32

Reputation: 29697

How do I flush a stdlib output file on win32?

I have a C++ program that is writing to a file on Windows 7. When I call f.flush() the NTFS file does not get bigger. Is there any way to force the file to get flushed?

Upvotes: 1

Views: 1757

Answers (2)

fghj
fghj

Reputation: 9404

You can look here:
How do I get the file HANDLE from the fopen FILE structure?
the code looks like this:

 FlushFileBuffers((HANDLE) _fileno(_file));

do not forget call fflush(file), before call FlusFileBuffers.

For std::fstream and gnu stl, I checked it on my linux box, have no windows at home, but should work with mingw, may be need some modifications:

#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <fstream>
#include <ext/stdio_filebuf.h>


int main(int argc, char *argv[])
{
    assert(argc == 2);
    std::ifstream f(argv[1]);
    assert(!!f);
/*
    //cin, cout etc
    __gnu_cxx::stdio_filebuf<char> *stdio_buf = dynamic_cast<__gnu_cxx::stdio_filebuf<char> *>(f.rdbuf());
    if (stdio_buf != 0) {
        printf("your fd is %d\n", stdio_buf->fd());
        return EXIT_SUCCESS;
    }
*/
    std::basic_filebuf<char> *file_buf = dynamic_cast<std::basic_filebuf<char> *>(f.rdbuf());
    if (file_buf != 0) {
        struct to_get_protected_member : public std::basic_filebuf<char> {
            int fd() { return _M_file.fd(); }
        };
        printf("your fd is %d\n", static_cast<to_get_protected_member *>(file_buf)->fd());
    }
    printf("what is going on?\n");
    return EXIT_FAILURE;
}

Upvotes: 2

Jerry Coffin
Jerry Coffin

Reputation: 490623

flush only flushes the internal buffers kept by the standard library code.

To flush the OS's buffers, you'd need/want to call FlushFileBuffers (after calling f.flush()).

Upvotes: 0

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