= keyring:  getmsg, sendmsg, senderrmsg: send and receive messages on undelimited streams

    include "keyring.m";
    keyring := load Keyring Keyring->PATH;

    getmsg:  fn(fd: ref Sys->FD): array of byte;
    sendmsg: fn(fd: ref Sys->FD, buf: array of byte, n: int): int;
    senderrmsg: fn(fd: ref Sys->FD, s: string): int;

These functions allow arbitrary data, packed into  arrays  of
bytes,  to  be exchanged on network connections using connec‐
tion-oriented transport protocols that do not preserve record
boundaries  (eg,  TCP/IP  without  ssl(3)).  They are used to
implement   various   authentication   protocols,   including
auth(6), as implemented by keyring-auth(2).

Each data message is transmitted with a five-byte header con‐
taining a four-character zero-padded decimal count  n  termi‐
nated  by a newline, followed by n bytes of message data.  An
error message has a similar structure, except that the  first
character  of  the  count  is replaced by an exclamation mark
(!); the  message  data  following  contains  the  diagnostic
string in its UTF-8 encoding (see utf(6)).

Getmsg  reads  the  next message from fd and returns its data
content.

Sendmsg sends the first n bytes of buf as a  message  on  fd,
and returns n.

Senderrmsg sends the error message s.

== source
/libinterp/keyring.c

== diagnostics
Sendmsg  and senderrmsg return -1 if there was an error writ‐
ing to fd; they set the system error string.  Getmsg  returns
nil if there was an error reading from fd; it sets the system
error string to reflect the cause.  It also returns nil if an
error  message  was  received  instead of a data message; the
system error string will contain the error message's diagnos‐
tic.