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Pseudo Terminals

Pseudo Terminals

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Pseudo Terminals

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  1. Pseudo Terminals • Concept • Application • APIs

  2. Overview • 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 • 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

  3. Typical Pseudo Terminal Arrangement

  4. Typical Pseudo Terminal Arrangement • Steps • 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

  5. Applications • Network login (telnetd, sshd, etc) • The script utility • X Windows • etc

  6. Applications – Network Login

  7. Applications – Network Login • 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)

  8. Applications – script utility

  9. Applications – script utility • 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

  10. Pseudo Terminal Interfaces • Historically there were two main pseudo terminal APIs • BSD • System V (aka UNIX98) • LINUX supports both, but since kernel 2.4.4 BSD APIs are deprecated. Use UNIX98 interface instead

  11. UNIX98 Pseudo Terminals int posix_openpt(int flags); • Opens an unused UNIX98 PTY master • 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

  12. UNIX98 Pseudo Terminals • Limits on number of PTY devices • Default limit is 256 • Can be changed by re-compiling the kernel up to 4096 • Two files under /proc/sys/kernel/pty • max – the max pairs of PTYs • nr – number of PTYs currently in use

  13. BSD Pseudo Terminals • BSD style PTYs come as pre-created pairs • /dev/ptyXY (master) • /dev/ttyXY (slave) • 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

  14. GLIBC PTY Library Calls • glibc provides openpty and forkpty int openpty(int *amaster, int *aslave, char *name, struct termios *termp, struct winsize * winp); • 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); • Combines openpty and fork • Returns pid of 0 in child and actual pid in parent like regular fork function