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

Question

fcntl(fd, F_GETFD) is the canonical cheapest way to check that fd is a valid open file descriptor. If you need to batch-check a lot, using poll with a zero timeout and the events member set to 0 and checking for POLLNVAL in revents after it returns is more efficient.

With that said, the operation "check if a given resource handle is still valid" is almost always fundamentally incorrect. After a resource handle is freed (e.g. a fd is closed), its value may be reassigned to the next such resource you allocate. If there are any remaining references that might be used, they will wrongly operate on the new resource rather than the old one. Thus, the real answer is probably: If you don't already know by the logic of your program, you have major fundamental logic errors that need to be fixed.

You can use the fcntl() function:

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

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.

I don't think there is any function that can tell you if the descriptor is still valid. The descriptor is typically just a small integer like 6 and your libc can choose to reuse that number if you close the file and open a new one later.

Instead, you should consider using dup() to copy the file descriptor. By duplicating the file descriptor instead of using the same descriptor in multiple places, it might become easier for you to know whether the file descriptor is still valid. You just have to remember to close both the original descriptor and the duplicated one when you are done.