tcgetattr function and tcsetattr function control terminal

6.4.4 using the Tcgetattr function and the TCSETATTR function to control the terminal

To facilitate the process of obtaining and modifying terminal parameters, Linux also provides tcgetattr functions and tcsetattr functions. The tcgetattr is used to obtain the relevant parameters of the terminal, while the TCSETATTR function sets the terminal parameters. The specific information for these two functions is shown in Table 6.2.

Table 6.2 tcgetattr functions and TCSETATTR functions

Header file	<termios.h> <unistd.h>
function form	int tcgetattr (int fd, struct termios *termios_p); int tcsetattr (int fd, int optional_actions, const struct Termios *termios_p);
return value	Success	Failed	Whether to set errno
	0	−1	Is

Description: The TCGETATTR function is used to obtain terminal-related parameters. The parameter fd is the file descriptor of the terminal, and the returned result is stored in the Termios structure, which generally includes the following members:

tcflag_t C_iflag; tcflag_t C_oflag; tcflag_t C_cflag; tcflag_t C_lflag; cc_t C_cc[nccs];

Its specific meaning is as follows.
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C_iflag: Input mode flag, control the terminal input method, the specific parameters as shown in table 6.3.

Table 6.3 C_iflag Parameter table

Key Value	Speak clearly
Ignbrk	Ignore break key input
Brkint	If the input setting of the Ignbrk,break key is ignored, if Brkint is set, a SIGINT interrupt is generated
Ignpar	Ignore parity errors
Parmrk	Identify parity errors
Inpck	Allow input parity
Istrip	Remove the 8th bit of a character
Inlcr	Converts the input NL (line break) to CR (carriage return)
Igncr	Ignore input carriage return
Icrnl	Converts the entered carriage return to a newline (if IGNCR is not set)
Iuclc	Convert uppercase characters entered into lowercase characters (non-POSIX)
Ixon	Allow control of xon/xoff flow at input
Ixany	Enter any characters that will restart the stopped output
Ixoff	Allow control of xon/xoff flow at input
Imaxbel	When the input queue starts ringing when it is full, Linux uses this parameter instead to assume that the parameter is always set

C_oflag: Output mode flag, control the terminal output mode, the specific parameters as shown in Table 6.4.

Table 6.4 C_oflag Parameters

Key Value	Speak clearly
Opost	Post-processing output
Olcuc	Converts the input lowercase characters to uppercase characters (non-POSIX)
Onlcr	Converts the input NL (line break) to CR (carriage return) and NL (line break)
Ocrnl	Converts the input CR (carriage return) to NL (line break)
Onocr	The first line does not output a carriage return character
Onlret	Do not output carriage return characters
Ofill	Send padding characters to delay terminal output
Ofdel	With the ASCII code del as the fill character, if the parameter is not set, the padding character will be nul (' + ') (non-POSIX)
nldly	NewLine output delay, NL0 (no delay) or NL1 (delay of 0.1s) can be taken
crdly	Carriage return delay, value range: CR0, CR1, CR2, and CR3
tabdly	Horizontal tab output delay, value range: TAB0, TAB1, TAB2, and TAB3
bsdly	Space output delay, can take BS0 or BS1
vtdly	Vertical tab Output delay, can take VT0 or VT1
ffdly	Page change delay, can take FF0 or FF1

C_cflag: Control mode flag, specify the terminal hardware control information, the specific parameters as shown in table 6.5.

Table 6.5 C_oflag Parameters

Key Value	Speak clearly
Cbaud	Baud rate (4+1 bit) (non-POSIX)
Cbaudex	Additional baud rate (1-bit) (non-POSIX)
CSIZE	Character length, with values ranging from CS5, CS6, CS7, or CS8
Cstopb	Set two stop bits
Cread	Using receivers
Parenb	Using the Parity check
Parodd	Using parity for input, using even parity for output
Hupcl	Hang when device is turned off
Clocal	Ignore Modem line Status
Crtscts	Using Rts/cts flow control

C_lflag: Local mode flag, control the terminal editing function, the specific parameters as shown in Table 6.6.

Table 6.6 C_lflag Parameters

Key Value	Speak clearly
Isig	Generates a corresponding signal when entering Intr, QUIT, Susp, or Dsusp
Icanon	Use standard input mode
Xcase	In cases where Icanon and xcase are set simultaneously, the terminal only uses uppercase. If only xcase is set, the input character is converted to lowercase characters unless the character uses an escape character (not POSIX, and Linux does not support this parameter)
ECHO	Show input characters
Echoe	If Icanon is set at the same time, erase will delete the input characters, werase will delete the input words
Echok	If Icanon is set at the same time, kill deletes the forward
Echonl	If Icanon is set at the same time, even if Echo is not set, the line break is still displayed
Echoprt	If Echo and Icanon are set simultaneously, the printed characters (non-POSIX) will be deleted
Tostop	Send Sigttou signal to background output

C_CC[NCCS]: A control character used to hold special characters in the terminal driver, such as an input terminator. The control characters shown in table 6.7 are defined in C_CC.

Table 6.7 C_CC supported control characters

Macro	Speak clearly	Macro	Speak clearly
Vintr	Interrupt characters	Veol	Additional End-of-file characters
Vquit	Quit character	Vtime	Timeout for non-canonical mode reads
Verase	Erase characters	Vstop	Stop character
Vkill	Kill character	Vstart	Start character
Veof	End-of-file characters	Vsusp	Suspend characters
VMIN	Minimum number of characters when reading non-canonical mode

The TCSETATTR function is used to set the relevant parameters for the terminal. The parameter FD is an open terminal file descriptor, and the parameter optional_actions is used to control when the modification is in effect, while the struct termios_p holds the parameters to be modified.
Optional_actions can take the following values.
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Tcsanow: Changes the properties immediately after the data transfer is complete.
Tcsadrain: Wait for all data transfers to end before changing the properties.
Tcsaflush: Clears the input and output buffers before changing the properties.

Error message:
EBADF: Illegal file descriptor.
The EINTR:TCSETATTR function call was interrupted by the signal.
EINVAL: The parameter optional_actions used an illegal value, or an illegal value was used in the parameter Termios.
Enctty: A non-terminal file descriptor.

Example Walkthrough:
Program p6.2.c by modifying the terminal control character, the terminal input terminator from "Ctrl+d", modified to "Ctrl+g". First, the program calls the TCGETATTR function to obtain the Termios information of the standard input, modifying the c_cc[veof] control character in the Termios struct to 0x07 (that is, ctrl+g); Use the TCSETATTR function to set the modified Termios parameter to the terminal. The specific code looks like this:

P6.2.C Modify Terminal Control Word Descriptor example #include <stdio.h> #include <termios.h> #include <unistd.h> #include <errno.h> int main (void) { Term used to store the obtained terminal parameter information struct Termios term; int err; Get the terminal parameters of the standard input and save the information in the term variable if (tcgetattr (stdin_fileno,&term) ==-1) { Perror ("Cannot get standard input description"); return 1; } Modify the end control character of the obtained terminal information term.c_cc[veof]= (cc_t) 0x07; Use the TCSETATTR function to set the modified terminal parameter to standard input Err is used to save the result after a function call Err=tcsetattr (Stdin_fileno,tcsaflush,&term); If err is-1 or a EINTR error occurs (function execution is interrupted by a signal), Give a related error message if (err==-1 && err==eintr) { Perror ("Failed to change EOF character"); return 1; } return 0; }

Compile the P6.2.C program with GCC and get the executable program named p6.2. Press "Ctrl+d" to end the terminal before executing the p6.2 program. After the execution of the p6.2 program, press "Ctrl+d" lost its role, and input "ctrl+g" to achieve the original "ctrl+d" function.

tcgetattr function and tcsetattr function control terminal