sendfd - send a file descriptor to a local socket peer
## Synopsis
```**c++
#include <sys/socket.h>
int sendfd(int sockfd, int fd);
```
## Description
Send an open file descriptor to a local socket peer connected via `sockfd`. This is a non-blocking call that will fail if there are too many file descriptors already waiting to be received by the peer.
File descriptors are sent out-of-band and do not affect the regular data streams.
## Return value
If a file descriptor is successfully received, it is returned as a positive integer. Otherwise, -1 is returned and `errno` is set to indicate the error.
## Errors
*`EBADF`: `sockfd` or `fd` is not an open file descriptor.
*`ENOTSOCK`: `sockfd` does not refer to a socket.
*`EAFNOSUPPORT`: `sockfd` does not refer to a local domain socket.
*`ENOTCONN`: `sockfd` refers to a socket which is not connected.
*`EINVAL`: `sockfd` does not refer to a connected or accepted socket.
*`EBUSY`: There are too many file descriptors already waiting to be received by the peer.
