tcgetattr function and tcsetattr function __ function

Linux also provides tcgetattr functions and TCSETATTR functions to facilitate the acquisition and modification of terminal parameters through programs. Tcgetattr is used to get the relevant parameters for the terminal, and the TCSETATTR function is used to set 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 a terminal file descriptor, and the returned result is stored in the Termios structure body, 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.

C_iflag: Input mode flag, control terminal input mode, specific parameters as shown in table 6.3.

Table 6.3 C_iflag Parameter table

Key value	Description
Ignbrk	Ignore break key input
Brkint	If the input with the Ignbrk,break key is set to be 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 the character
Inlcr	Converts the entered NL (newline) 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	Control of Xon/xoff flow when input is allowed
Ixany	Enter any character to reboot the stopped output
Ixoff	Control of Xon/xoff flow when input is allowed
Imaxbel	When the input queue starts ringing when it is full, Linux uses this parameter instead of the argument that it 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	Description
Opost	After processing output
Olcuc	Converts the entered lowercase characters to uppercase characters (non-POSIX)
Onlcr	Converts the input NL (newline) to CR (carriage return) and NL (linefeed).
Ocrnl	Converts the input CR (carriage return) to NL (line wrap)
Onocr	The first line does not output a return character
Onlret	Do not output carriage return characters
Ofill	Send fill characters to delay terminal output
Ofdel	Fills the character with the ASCII key, and if this argument is not set, the padding character is nul (' ") (non-POSIX)
nldly	NewLine output delay, can take NL0 (without delay) or NL1 (delay 0.1s)
crdly	Carriage return delay with a value range of: CR0, CR1, CR2 and CR3
tabdly	Horizontal tab output delay, values range: TAB0, TAB1, TAB2, and TAB3
bsdly	Space output delay, can take BS0 or BS1
vtdly	Vertical tab Output delay, you can take VT0 or VT1
ffdly	Change page delay, can take FF0 or FF1

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

Key value	Description
Cbaud	Baud rate (4+1 bit) (non-POSIX)
Cbaudex	Additional baud rate (1-bit) (non-POSIX)
CSize	Character length, which ranges from CS5, CS6, CS7, or CS8
Cstopb	Set two stop bits
Cread	Using the receiver
Parenb	Using parity
Parodd	Use parity for input, parity on output
Hupcl	Suspend 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	Description
Isig	When input intr, QUIT, Susp or DSUSP, the corresponding signal is generated
Icanon	Use standard input mode
Xcase	When Icanon and Xcase are set simultaneously, the terminal uses only 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	Display input characters
Echoe	If Icanon is set at the same time, erase will delete the entered characters and Werase will delete the entered words
Echok	If the Icanon is set at the same time, kill will delete 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 will be deleted (non-POSIX)
Tostop	Send Sigttou signal to background output

C_CC[NCCS]: A control character that holds special characters in a terminal driver, such as an input terminator. The control characters as shown in table 6.7 are defined in C_CC.

Table 6.7 C_CC supported control characters

Macro	Description	Macro	Description
Vintr	Interrupt characters	Veol	Additional End-of-file characters
Vquit	Quit characters	Vtime	Timeout when 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 non-canonical mode reads

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 takes effect, and the parameters to be modified are saved in the termios_p of the struct body.
Optional_actions can take the following values.

Tcsanow: Change the attribute immediately after the data transfer is complete.
Tcsadrain: Wait for all data transfer to end before changing properties.
Tcsaflush: Clears the input-output buffer before changing the property.

Error message:
EBADF: Illegal file descriptor.
The EINTR:TCSETATTR function call was interrupted by a signal.
Einval: Parameter optional_actions uses an illegal value, or an illegal value is used in Parameter Termios.
Enctty: A non-terminal file descriptor.

Instance Walkthrough:
By modifying the terminal control characters, the program p6.2.c the terminal input terminator from "Ctrl+d" to "ctrl+g". First, the program calls the TCGETATTR function to obtain the standard input Termios information, modifies the c_cc[veof in Termios structure to 0x07 (i.e. ctrl+g); then, use tcsetattr The function sets 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; Obtains the standard input terminal parameter, saves the obtained 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 results 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 the relevant 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 executing p6.2 program, press "Ctrl+d" to lose function, and input "ctrl+g" realize original "ctrl+d" function