how to tell if a stream is closed in C before calling fclose()

Question

I have a failing C program, and i've narrowed it down to a fork()ed child trying to close stdout and stderr, which were closed by its parent process before calling fork() - i assume those streams were passed on to the child process.

how can i tell if a stream is closed in C before attempting to close it using something like fclose(stdout)

Answer 1

If you're working at that level, you probably shouldn't be using the C standard library's buffered FILE pointers (stdin & co), but rather with the underlying file descriptor integers themselves.

You should be able to do some harmless operation, such as maybe a lseek(fd, 0, SEEK_SET) on the file to detect if the underlying descriptor is valid.

Answer 2

You can use ftell() to check that. If it returns -1, your stream is mostly closed.

Answer 3

After the call to fclose(), any use of stream results in undefined behavior.

So if it's the FILE* stdout, you can't use stdout anymore at all, not even to check if it's valid/open.

You could use the file descriptor for stdout directly, it's fd 1 .

struct stat stbuf;
if(fstat(1,&stbuf) == -1) {
  if(errno == EBADF) {
    stdout isn't open/valid
  }
}

Answer 4

C programs on UNIX expect to have a file descriptors 0, 1 and 2 open when they are started. If you do not want them to go anywhere, open /dev/null and dup it to those file descriptors.