Pseudo Terminals
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Concept
Application
APIs
Overview
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A pseudo terminal (PTY) is a user level
program that appears to be a terminal device
to another program
A pseudo terminal has two bidirectionally
connected ends, the master and the slave
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slave is connected to the program that thinks its
connected to a terminal device
master is connected to the program that is
providing this pseudo terminal service
Typical Pseudo Terminal
Arrangement
Typical Pseudo Terminal
Arrangement
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Steps
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Process opens pseudo terminal master and calls
fork
Child creates new session, opens corresponding
pseudo terminal slave, duplicates fd of terminal
slave to stdin, stdout, stderr and then calls exec
Pseudo terminal slave becomes the controlling
terminal for the child
It appears to the user process above the slave
that it is connected to a terminal device
Output from the master appears as input to the
slave and vice versa
Applications
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Network login (telnetd, sshd, etc)
The script utility
X Windows
etc
Applications – Network Login
Applications – Network Login
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Network login requests come in through the network
interface driver inetd. rlogind is started per request
 rlogind opens a pseudo terminal master and then
forks
 Child duplicates file descriptors 0, 1 and 2 to the
pseudo terminal then execs login
 Parent and child communicate through the pseudo
terminal
Note that the process connected to the master is also
reading/writing another I/O stream at the same time.
This implies multiplexed I/O (select or poll)
Applications – script utility
Applications – script utility
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The script utility places itself between
the terminal and a new copy of the
login shell
While running, all the output from the
terminal line descriptor above the PTY
slave is copied into a script file
Input that is echoed is also stored
Pseudo Terminal Interfaces
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Historically there were two main pseudo
terminal APIs
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BSD
System V (aka UNIX98)
LINUX supports both, but since kernel
2.4.4 BSD APIs are deprecated. Use
UNIX98 interface instead
UNIX98 Pseudo Terminals
int posix_openpt(int flags);
 Opens an unused UNIX98 PTY master
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flags can the OR zero or more of O_RDWR and O_NOCTTY
int grantpt(int fd);
 Sets permissions of the slave PTY
int unlockpt(int fd);
 Unlocks the slave PTY corresponding to the master
PTY designated by fd
char *ptsname(int fd);
 Returns the pathname of the PTY device based on
the FD of the master
UNIX98 Pseudo Terminals
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Limits on number of PTY devices
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Default limit is 256
Can be changed by re-compiling the kernel
up to 4096
Two files under /proc/sys/kernel/pty
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max – the max pairs of PTYs
nr – number of PTYs currently in use
BSD Pseudo Terminals
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BSD style PTYs come as pre-created pairs
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/dev/ptyXY (master)
/dev/ttyXY (slave)
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X is one letter from [p-z,a-e]
Y is one letter from [0-9,a-f]
Ex: /dev/ptyp1 and /dev/ttyp1 are a PTY pair
Process opens a PTY by trying master devices
sequentially until it can open one then
opening the corresponding slave
GLIBC PTY Library Calls
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glibc provides openpty and forkpty
int openpty(int *amaster, int *aslave, char *name, struct
termios *termp, struct winsize * winp);
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Opens a master slave pair and optionally uses
termios and winsize to set up the slave tty.
pid_t forkpty(int *amaster, char *name, struct termios *termp,
struct winsize *winp);
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Combines openpty and fork
Returns pid of 0 in child and actual pid in parent
like regular fork function