How to check if a given file descriptor stored in a variable is still valid?

Question

I have a file descriptor stored in a variable say var. How can I check whether that descriptor is valid at a later stage?

  fdvar1= open(.....);
  fdvar2 =          


        

Answer 1

From this forum article:

int is_valid_fd(int fd)
{
    return fcntl(fd, F_GETFL) != -1 || errno != EBADF;
}

fcntl(GETFL) is probably the cheapest and least likely to fail operation you can perform on a file descriptor. In particular, the specification suggests that it cannot be interrupted by signals, nor is it affected by any sort of lock held anywhere.