How to check if a file has been opened by another application in C++?

Question

I know, that there\'s the is_open() function in C++, but I want one program to check if a file hasn\'t been opened by another application. Is there any way to d

Answer 1

In Windows this little and dirty trick will work (if the file exists and you have the right permissions)

  if (  0 != rename("c:/foo.txt", "c:/foo.txt")  ) {
     printf("already opened\n");
  }

It's likely to work also in Linux.

Answer 2

Not only the standard library does not have this funcionality, it's not even possible in general. You could (on linux) check /proc/*/fd — but it is possible that your program does not have permission to do it on processes from other users (this is the default in Ubuntu, for instance).

Answer 3

No, the standard library has no such functionality.

Answer 4

The following code may work.

int main(int argc, char ** argv)
{
    int fd = open(argv[1], O_RDONLY);
    if (fd < 0) {
        perror("open");
        return 1;
    }

    if (fcntl(fd, F_SETLEASE, F_WRLCK) && EAGAIN == errno) {
        puts("file has been opened");
    }
    else {
        fcntl(fd, F_SETLEASE, F_UNLCK);
        puts("file has not been opened");
    }

    close(fd);
    return 0;
}

Answer 5

Perhaps you could just try and get a full write lock? It'll fail if anyone else has it open for reading or writing.

fopen("myfile.txt", "r+")

If it's not cross platform and is Win32, then you can request even more fine-grained set of locks.

See here and look at dwShareMode, value of 0, as well as the other parameters.

Answer 6

Nope. Unless other application uses advisory locks.

See http://docs.sun.com/app/docs/doc/816-0213/6m6ne37v5?a=view