1TCSETATTR(3P) POSIX Programmer's Manual TCSETATTR(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 tcsetattr — set the parameters associated with the terminal
13
15 #include <termios.h>
16 int tcsetattr(int fildes, int optional_actions,
18 const struct termios *termios_p);
19
21 The tcsetattr() function shall set the parameters associated with the
22 terminal referred to by the open file descriptor fildes (an open file
23 descriptor associated with a terminal) from the termios structure ref‐
24 erenced by termios_p as follows:
25 * If optional_actions is TCSANOW, the change shall occur immediately.
27 * If optional_actions is TCSADRAIN, the change shall occur after all
29 output written to fildes is transmitted. This function should be
30 used when changing parameters that affect output.
31 * If optional_actions is TCSAFLUSH, the change shall occur after all
33 output written to fildes is transmitted, and all input so far
34 received but not read shall be discarded before the change is made.
35 If the output baud rate stored in the termios structure pointed to by
37 termios_p is the zero baud rate, B0, the modem control lines shall no
38 longer be asserted. Normally, this shall disconnect the line.
39 If the input baud rate stored in the termios structure pointed to by
41 termios_p is 0, the input baud rate given to the hardware is the same
42 as the output baud rate stored in the termios structure.
43 The tcsetattr() function shall return successfully if it was able to
45 perform any of the requested actions, even if some of the requested
46 actions could not be performed. It shall set all the attributes that
47 the implementation supports as requested and leave all the attributes
48 not supported by the implementation unchanged. If no part of the
49 request can be honored, it shall return -1 and set errno to [EINVAL].
50 If the input and output baud rates differ and are a combination that is
51 not supported, neither baud rate shall be changed. A subsequent call to
52 tcgetattr() shall return the actual state of the terminal device
53 (reflecting both the changes made and not made in the previous tcse‐
54 tattr() call). The tcsetattr() function shall not change the values
55 found in the termios structure under any circumstances.
56 The effect of tcsetattr() is undefined if the value of the termios
58 structure pointed to by termios_p was not derived from the result of a
59 call to tcgetattr() on fildes; an application should modify only fields
60 and flags defined by this volume of POSIX.1‐2017 between the call to
61 tcgetattr() and tcsetattr(), leaving all other fields and flags unmodi‐
62 fied.
63 No actions defined by this volume of POSIX.1‐2017, other than a call to
65 tcsetattr(), a close of the last file descriptor in the system associ‐
66 ated with this terminal device, or an open of the first file descriptor
67 in the system associated with this terminal device (using the
68 O_TTY_INIT flag if it is non-zero and the device is not a pseudo-termi‐
69 nal), shall cause any of the terminal attributes defined by this volume
70 of POSIX.1‐2017 to change.
71 If tcsetattr() is called from a process which is a member of a back‐
73 ground process group on a fildes associated with its controlling termi‐
74 nal:
75 * If the calling thread is blocking SIGTTOU signals or the process is
77 ignoring SIGTTOU signals, the operation completes normally and no
78 signal is sent.
79 * Otherwise, a SIGTTOU signal shall be sent to the process group.
81
83 Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
84 returned and errno set to indicate the error.
85
87 The tcsetattr() function shall fail if:
88 EBADF The fildes argument is not a valid file descriptor.
90 EINTR A signal interrupted tcsetattr().
92 EINVAL The optional_actions argument is not a supported value, or an
94 attempt was made to change an attribute represented in the
95 termios structure to an unsupported value.
96 EIO The process group of the writing process is orphaned, the call‐
98 ing thread is not blocking SIGTTOU, and the process is not
99 ignoring SIGTTOU.
100 ENOTTY The file associated with fildes is not a terminal.
102 The following sections are informative.
104
106 None.
107
109 If trying to change baud rates, applications should call tcsetattr()
110 then call tcgetattr() in order to determine what baud rates were actu‐
111 ally selected.
112 In general, there are two reasons for an application to change the
114 parameters associated with a terminal device:
115 1. The device already has working parameter settings but the applica‐
117 tion needs a different behavior, such as non-canonical mode instead
118 of canonical mode. The application sets (or clears) only a few
119 flags or c_cc[] values. Typically, the terminal device in this case
120 is either the controlling terminal for the process or a pseudo-ter‐
121 minal.
122 2. The device is a modem or similar piece of equipment connected by a
124 serial line, and it was not open before the application opened it.
125 In this case, the application needs to initialize all of the param‐
126 eter settings ``from scratch''. However, since the termios struc‐
127 ture may include both standard and non-standard parameters, the
128 application cannot just initialize the whole structure in an arbi‐
129 trary way (e.g., using memset()) as this may cause some of the non-
130 standard parameters to be set incorrectly, resulting in non-con‐
131 forming behavior of the terminal device. Conversely, the applica‐
132 tion cannot just set the standard parameters, assuming that the
133 non-standard parameters will already have suitable values, as the
134 device might previously have been used with non-conforming parame‐
135 ter settings (and some implementations retain the settings from one
136 use to the next). The solution is to open the terminal device using
137 the O_TTY_INIT flag to initialize the terminal device to have con‐
138 forming parameter settings, obtain those settings using tcge‐
139 tattr(), and then set all of the standard parameters to the desired
140 settings.
141
143 The tcsetattr() function can be interrupted in the following situa‐
144 tions:
145 * It is interrupted while waiting for output to drain.
147 * It is called from a process in a background process group and SIGT‐
149 TOU is caught.
150 See also the RATIONALE section in tcgetattr().
152
154 Using an input baud rate of 0 to set the input rate equal to the output
155 rate may not necessarily be supported in a future version of this vol‐
156 ume of POSIX.1‐2017.
157
159 cfgetispeed(), tcgetattr()
160 The Base Definitions volume of POSIX.1‐2017, Chapter 11, General Termi‐
162 nal Interface, <termios.h>
163
165 Portions of this text are reprinted and reproduced in electronic form
166 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
167 table Operating System Interface (POSIX), The Open Group Base Specifi‐
168 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
169 Electrical and Electronics Engineers, Inc and The Open Group. In the
170 event of any discrepancy between this version and the original IEEE and
171 The Open Group Standard, the original IEEE and The Open Group Standard
172 is the referee document. The original Standard can be obtained online
173 at http://www.opengroup.org/unix/online.html .
174 Any typographical or formatting errors that appear in this page are
176 most likely to have been introduced during the conversion of the source
177 files to man page format. To report such errors, see https://www.ker‐
178 nel.org/doc/man-pages/reporting_bugs.html .
179IEEE/The Open Group 2017 TCSETATTR(3P)