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clarify 'Receiving sockets' section

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Christian Ulrich 2020-10-21 00:50:34 +02:00
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README.md
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@ -116,20 +116,23 @@ AA <- PA: ok|645
```
## Receiving sockets using recvmsg
## Receiving sockets using sendmsg/recvmsg
When ``punchd`` successfully has punched a hole, i.e. has a socket that is
connected to the other peer, it will report back status "ok" to the application.
The file descriptor representing that socket will be included in the status
message in a control message, a.k.a ancillary message.
But how can the application now use the socket? The answer is: ``punchd`` has to
pass the corresponding file descriptor to the application's process. On Unix
systems that is possible using the system calls ``sendmsg`` (on ``punchd``'s
side) and ``recvmsg`` (on the application's side). This allows ``punchd``
passing a control message, a.k.a. ancillary message, of type ``SCM_RIGHTS``
along with the actual contents of the status message. This control message will
contain the file descriptor.
The application has to use the ``recvmsg`` system call to receive that control
message (it has the type ``SCM_RIGHTS``) and access the file descriptor. This
mechanism is described in the man pages
[unix(7)](https://linux.die.net/man/7/unix),
The mechanism of passing file descriptors to other processes is described in the
man pages [unix(7)](https://linux.die.net/man/7/unix),
[cmsg(3)](https://linux.die.net/man/3/cmsg) and
[recv(2)](https://linux.die.net/man/2/recv). The example applications can be used
as a reference too. After receiving the file descriptor it can immediately be
used to send data to the other peer.
as a reference too. After receiving the file descriptor the application can
immediately use it to send data to the other peer.
# FAQ