PTY(4) PTY(4) NAME pty - pseudo terminal driver SYNOPSIS pseudo-device pty [ count ] DESCRIPTION The _p_t_y driver provides support for a device-pair termed a _p_s_e_u_d_o _t_e_r_‐ _m_i_n_a_l. A pseudo terminal is a pair of character devices, a _m_a_s_t_e_r device and a _s_l_a_v_e device. The slave device provides processes an interface identical to that described in _t_t_y(4). However, whereas all other devices which provide the interface described in _t_t_y(4) have a hardware device of some sort behind them, the slave device has, instead, another process manipulating it through the master half of the pseudo terminal. That is, anything written on the master device is given to the slave device as input and anything written on the slave device is presented as input on the master device. In configuring, if an optional ‘‘count’’ is given in the specification, that number of pseudo terminal pairs are configured; the default count is 32. The following _i_o_c_t_l calls apply only to pseudo terminals: TIOCSTOP Stops output to a terminal (e.g. like typing ^S). Takes no parameter. TIOCSTART Restarts output (stopped by TIOCSTOP or by typing ^S). Takes no parameter. TIOCPKT Enable/disable _p_a_c_k_e_t mode. Packet mode is enabled by specify‐ ing (by reference) a nonzero parameter and disabled by specify‐ ing (by reference) a zero parameter. When applied to the master side of a pseudo terminal, each subsequent _r_e_a_d from the termi‐ nal will return data written on the slave part of the pseudo terminal preceded by a zero byte (symbolically defined as TIOCPKT_DATA), or a single byte reflecting control status infor‐ mation. In the latter case, the byte is an inclusive-or of zero or more of the bits: TIOCPKT_FLUSHREAD whenever the read queue for the terminal is flushed. TIOCPKT_FLUSHWRITE whenever the write queue for the terminal is flushed. TIOCPKT_STOP whenever output to the terminal is stopped a la ^S. TIOCPKT_START whenever output to the terminal is restarted. TIOCPKT_DOSTOP whenever _t__s_t_o_p_c is ^S and _t__s_t_a_r_t_c is ^Q. TIOCPKT_NOSTOP whenever the start and stop characters are not ^S/^Q. While this mode is in use, the presence of control status infor‐ mation to be read from the master side may be detected by a _s_e_l_e_c_t for exceptional conditions. This mode is used by _r_l_o_g_i_n(1C) and _r_l_o_g_i_n_d(8C) to implement a remote-echoed, locally ^S/^Q flow-controlled remote login with proper back-flushing of output; it can be used by other similar programs. TIOCUCNTL Enable/disable a mode that allows a small number of simple user _i_o_c_t_l commands to be passed through the pseudo-terminal, using a protocol similar to that of TIOCPKT. The TIOCUCNTL and TIOCPKT modes are mutually exclusive. This mode is enabled from the master side of a pseudo terminal by specifying (by reference) a nonzero parameter and disabled by specifying (by reference) a zero parameter. Each subsequent _r_e_a_d from the master side will return data written on the slave part of the pseudo terminal preceded by a zero byte, or a single byte reflecting a user con‐ trol operation on the slave side. A user control command con‐ sists of a special _i_o_c_t_l operation with no data; the command is given as UIOCCMD(_n), where _n is a number in the range 1-255. The operation value _n will be received as a single byte on the next _r_e_a_d from the master side. The _i_o_c_t_l UIOCCMD(0) is a no-op that may be used to probe for the existence of this facility. As with TIOCPKT mode, command operations may be detected with a _s_e_l_e_c_t for exceptional conditions. TIOCREMOTE A mode for the master half of a pseudo terminal, independent of TIOCPKT. This mode causes input to the pseudo terminal to be flow controlled and not input edited (regardless of the terminal mode). Each write to the control terminal produces a record boundary for the process reading the terminal. In normal usage, a write of data is like the data typed as a line on the termi‐ nal; a write of 0 bytes is like typing an end-of-file character. TIOCREMOTE can be used when doing remote line editing in a win‐ dow manager, or whenever flow controlled input is required. FILES /dev/pty[p-r][0-9a-f] master pseudo terminals /dev/tty[p-r][0-9a-f] slave pseudo terminals DIAGNOSTICS None. 4.2 Berkeley Distribution May 19, 1986 PTY(4)