Check if a file that has been opened with fopen has been closed

Question

``` gcc (GCC) 4.7.2 c89 ``` Is it possible for check if a file has already closed? I have opened a file using `fopen()` and closed using `fclose(fd)`. This file gets opened and closed during the running of the program. However, the user can terminate the program by doing a `ctrl-c`. When I go to my cleanup routine, I don't know if the file is in a open or closed state. So if I try and do a `fclose(fd)` twice, it will stack dump. Some ideas I have been throwing around: - Add a state to the file to be either opened or closed, and then check that state, means more work for such a simple job. - Don't do anything as the OS will cleanup automatially when the program ends. - Is there is the access function, but that will just checks the mode. Many thanks in advance,

Accepted Answer

AFAIK, glibc doesn't provide a method to validate a `FILE*` variable.

Keeping some kind of state or just setting `fd` to `NULL` works, but you have to be careful not to get into your cleanup routine just after closing the file, but before resetting `fd`. As Ctrl+C sends a signal (SIGINT) to a program, you can just block the signal while the file is being closed and fd is being reset. So, your `fclose` in the main program should look like:

```
// 1. Block SIGINT
sigset_t old_mask, to_block;
sigemptyset(&to_block);
sigaddset(&to_block, SIGINT);
sigprocmask(SIG_BLOCK, &to_block, &old_mask);

// 2. Close the file and reset fd
fclose(fd);
fd = NULL;

// 3. Restore signal handling
sigprocmask(SIG_SETMASK, &old_mask, NULL);
```

And in your clean-up routine you should just check `fd`:

```
if (fd != NULL) fclose(fd);
```

If your program is multithreaded, you should use `pthread_sigmask` instead.

In your case, simple call of `fcloseall()` in the cleanup routine would be much easier.

As for the second option meant in your question, about doing nothing - well, the OS will clean-up everything for your program. Then only drawback is that if there were opened write streams, some data may be not written to disk. But maybe in case of Ctrl+C it's just ok.

Check if a file that has been opened with fopen has been closed

Solution

Problem