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7 .TH PTS 7D "Feb 29, 2020"
8 .SH NAME
9 pts \- STREAMS pseudo-tty slave driver
10 .SH DESCRIPTION
11 The pseudo-tty subsystem simulates a terminal connection, where the master side
12 represents the terminal and the slave represents the user process's special
13 device end point. In order to use the pseudo-tty subsystem, a node for the
14 master side driver \fB/dev/ptmx\fR and N nodes for the slave driver (N is
15 determined at installation time) must be installed. The names of the slave
16 devices are \fB/dev/pts/M\fR where \fBM\fR has the values 0 through N-1. When
17 the master device is opened, the corresponding slave device is automatically
18 locked out. No user may open that slave device until its permissions are
19 adjusted and the device unlocked by calling functions \fBgrantpt\fR(3C) and
20 \fBunlockpt\fR(3C). The user can then invoke the open system call with the name
21 that is returned by the \fBptsname\fR(3C) function. See the example below.
22 .sp
23 .LP
24 Only one open is allowed on a master device. Multiple opens are allowed on the
25 slave device. After both the master and slave have been opened, the user has
26 two file descriptors which are end points of a full duplex connection composed
27 of two streams automatically connected at the master and slave drivers. The
28 user may then push modules onto either side of the stream pair. Unless compiled
29 in XPG4v2 mode (see below), the consumer needs to push the \fBptem\fR(7M) and
30 \fBldterm\fR(7M) modules onto the slave side of the pseudo-terminal subsystem
31 to get terminal semantics.
32 .sp
33 .LP
34 The master and slave drivers pass all messages to their adjacent queues. Only
35 the \fBM_FLUSH\fR needs some processing. Because the read queue of one side is
36 connected to the write queue of the other, the \fBFLUSHR\fR flag is changed to
37 the \fBFLUSHW\fR flag and vice versa. When the master device is closed an
38 \fBM_HANGUP\fR message is sent to the slave device which will render the device
39 unusable. The process on the slave side gets the errno \fBEIO\fR when
40 attempting to write on that stream but it will be able to read any data
41 remaining on the stream head read queue. When all the data has been read, read
42 returns 0 indicating that the stream can no longer be used. On the last close
43 of the slave device, a 0-length message is sent to the master device. When the
44 application on the master side issues a \fBread()\fR or \fBgetmsg()\fR and 0 is
45 returned, the user of the master device decides whether to issue a
46 \fBclose()\fR that dismantles the pseudo-terminal subsystem. If the master
47 device is not closed, the pseudo-tty subsystem will be available to another
48 user to open the slave device. Since 0-length messages are used to indicate
49 that the process on the slave side has closed and should be interpreted that
50 way by the process on the master side, applications on the slave side should
51 not write 0-length messages. Unless the application is compiled in XPG4v2 mode
52 (see below) then any 0-length messages written on the slave side will be
53 discarded by the \fBptem\fR module.
54 .sp
55 .LP
56 The standard STREAMS system calls can access the pseudo-tty devices. The slave
57 devices support the \fBO_NDELAY\fR and \fBO_NONBLOCK\fR flags.
58 .SH XPG4v2 MODE
59 XPG4v2 requires that open of a slave pseudo terminal device provides the
60 process with an interface that is identical to the terminal interface (without
61 having to explicitly push any modules to achieve this). It also requires that
62 0-length messages written on the slave side will be propagated to the master.
63 .sp
64 Experience has shown, however, that most software does not expect slave pty
65 devices to operate in this manner and therefore this XPG4v2-compliant
66 behaviour is only enabled in XPG4v2/SUS (see \fBstandards\fR(5)) mode.
67 .SH EXAMPLES
68 .in +2
69 .nf
70 int fdm fds;
71 char *slavename;
72 extern char *ptsname();
73
74 fdm = open("/dev/ptmx", O_RDWR); /* open master */
75 grantpt(fdm); /* change permission of slave */
76 unlockpt(fdm); /* unlock slave */
77 slavename = ptsname(fdm); /* get name of slave */
78 fds = open(slavename, O_RDWR); /* open slave */
79 ioctl(fds, I_PUSH, "ptem"); /* push ptem */
80 ioctl(fds, I_PUSH, "ldterm"); /* push ldterm*/
81 .fi
82 .in -2
83
84 .SH FILES
85 .ne 2
86 .na
87 \fB\fB/dev/ptmx\fR\fR
88 .ad
89 .RS 14n
90 master clone device
91 .RE
92
93 .sp
94 .ne 2
95 .na
96 \fB\fB/dev/pts/M\fR\fR
97 .ad
98 .RS 14n
99 slave devices (M = 0 -> N-1)
100 .RE
101
102 .SH SEE ALSO
103 \fBgrantpt\fR(3C), \fBptsname\fR(3C), \fBunlockpt\fR(3C), \fBldterm\fR(7M),
104 \fBptm\fR(7D), \fBptem\fR(7M), \fBstandards\fR(5)
105 .sp
106 .LP
107 \fISTREAMS Programming Guide\fR