pty.7 (2010 09)

p
pty(7) pty(7)
the slave side. In normal usage, writing data is like typing the data as a line on a termi-
nal; writing zero bytes is equivalent to typing an end-of-file character (that is, the EOF
character as defined in termio (7)). The data read by the slave side is identical to the data
written on the master side. Data written on the slave side and read on the master side
with TIOCREMOTE enabled is still subject to the normal termio (7) processing.
TIOCREMOTE can be used when doing remote line editing in a window manager, or
whenever flow-controlled input is required. Issuing a
TIOCMONITOR ioctl() request
flushes all data buffered in the pseudo terminal.
The following
ioctl() requests, defined in
<sys/ptyio.h>, apply only to the master side of
pty.In
particular, these
ioctl() requests are only recognized when
TIOCTTY is enabled. When TIOCTTY is
disabled, these
ioctl() requests set and return the external variable
errno to [EINVAL].
TIOCBREAK Cause a break operation to be done on the slave side of the
pty, as if a user had pressed
the break key on a real terminal. Takes no parameter.
TIOCSTOP Stop data flowing from the slave side of the
pty to the master side (equivalent to typing
^S). Takes no parameter.
TIOCSTART Restart output (stopped by TIOCSTOP or by typing
ˆS). Takes no parameter.
Flow-Control Input and Output Processing
The following terms are used to describe the flow of data through pseudo terminals. INPUT refers to
data flowing from the master side of a
pty to the slave side. OUTPUT refers to data flowing from the
slave side of a pty to the master side.
When packet mode (
TIOCPKT) is disabled and INPUT is stopped (see IXOFF, input modes, in termio (7)),
the next
read() from the master side of a pty returns a STOP character. When INPUT is restarted,
the next read() from the master side returns a START character. If packet mode (
TIOCPKT)is
enabled, the STOP or START character is preceded by a data packet indicator (
TIOCPKTDATA).
select() should be used by the master-side server before each write() request to properly handle
INPUT flow control (see select (2)).
When INPUT flow control is enabled,
write() and select() are handled as follows: Write-selects on
the master side of a pty return true only if INPUT has not been stopped. If INPUT becomes stopped
while data is being written into the master side of a pty, the write returns with the number of bytes writ-
ten before INPUT was stopped. Writes done after INPUT is stopped return immediately with zero bytes
written.
When packet mode (
TIOCPKT) is disabled and OUTPUT is stopped (see IXON, input modes in termio (7)),
each subsequent
read() from the master side of a pty returns with no data read. When OUTPUT is
restarted, each subsequent read() from the master side returns data written on the slave side. If
packet mode (TIOCPKT) is enabled, the first read() after OUTPUT has been stopped returns a
TIOCPKTSTOP packet. All subsequent reads from the master side while OUTPUT is stopped returns a
TIOCPKTDATA packet with no data. When OUTPUT is restarted, the next read() from the master side
returns a TIOCPKTSTART packet. All subsequent reads from the master side return data written on the
slave side preceded by a TIOCPKTDATA packet. select() should be used by the master-side server
before each read() to properly handle OUTPUT flow control. Otherwise, reads from the master side of
a pty will not be prevented when OUTPUT is stopped.
Trapping ioctl(), open(), close() Requests
When trapping is enabled, the master side is notified when the application on its slave side makes an
ioctl(), open(),orclose() request. For trapped ioctl() and open() requests, the slave side is
blocked (that is, the request does not complete) until the server on its master side acknowledges the
trapped request. For trapped close() requests, the slave slave does not block for an acknowledgement.
select() should be used by the master side server to receive notification of trapped ioctl(),
open(), and close() requests. When one of these requests is trapped, the select() returns with an
"exceptional condition" indicated for the slave side’s file descriptor. Other mechanisms for receiving
notification of trapped requests are defined below, but these mechanisms should be used only if
select() is not available.
When trapping is disabled (default condition), unrecognized slave
ioctl() requests return an error,
with the external variable errno set to [EINVAL]. The only ioctl() requests recognized by the slave
side are those defined by termio (7) and only when TIOCTTY is enabled. When TIOCTTY is disabled, no
ioctl() requests are recognized by the slave side. If trapping is enabled and the master side closes,
trapping is disabled. If the master closes during the middle of a handshake with the slave, the
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