串口程序设计——struct termios结构体
tcflag_t c_iflag; /* 输入模式 */
tcflag_t c_oflag; /* 输出模式 */
tcflag_t c_cflag; /* 控制模式 */
tcflag_t c_lflag; /* 本地模式 */
cc_t c_cc[NCCS]; /* 控制字符 */
这个变量被用来提供一个健全的线路设置集合, 如果这个端口在被用户初始化前使用. 驱动初始化这个变量使用一个标准的数值集, 它拷贝自 tty_std_termios 变量. tty_std_termos 在 tty 核心被定义为:
struct termios tty_std_termios = { .c_iflag = ICRNL | IXON, .c_oflag = OPOST | ONLCR, .c_cflag = B38400 | CS8 | CREAD | HUPCL, .c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK | ECHOCTL | ECHOKE | IEXTEN, .c_cc = INIT_C_CC };
这个 struct termios 结构用来持有所有的当前线路设置, 给这个 tty 设备的一个特定端口. 这些线路设置控制当前波特率, 数据大小, 数据流控设置, 以及许多其他值.
三、成员的值
（一）c_iflag 标志常量：Input mode ( 输入模式)
input mode可以在输入值传给程序之前控制其处理的方式。其中输入值可能是由序列埠或键盘的终端驱动程序所接收到的字元。我们可以利用termios结构的c_iflag的标志来加以控制，其定义的方式皆以OR来加以组合。
- IGNBRK ：忽略输入中的 BREAK 状态。 （忽略命令行中的中断）
- BRKINT ：（命令行出现中断时，可产生一插断）如果设置了 IGNBRK，将忽略 BREAK。如果没有设置，但是设置了 BRKINT，那么 BREAK 将使得输入和输出队列被刷新，如果终端是一个前台进程组的控制终端，这个进程组中所有进程将收到 SIGINT 信号。如果既未设置 IGNBRK 也未设置 BRKINT，BREAK 将视为与 NUL 字符同义，除非设置了 PARMRK，这种情况下它被视为序列 377 � �。
- IGNPAR ：忽略桢错误和奇偶校验错。
- PARMRK ：如果没有设置 IGNPAR，在有奇偶校验错或桢错误的字符前插入 377 �。如果既没有设置 IGNPAR 也没有设置 PARMRK，将有奇偶校验错或桢错误的字符视为 �。
- INPCK ：启用输入奇偶检测。
- ISTRIP ：去掉第八位。
- INLCR :将输入中的 NL 翻译为 CR。（将收到的换行符号转换为Return）
- IGNCR :忽略输入中的回车。
- ICRNL :将输入中的回车翻译为新行 (除非设置了 IGNCR)(否则当输入信号有 CR 时不会终止输入)。
- IUCLC :(不属于 POSIX) 将输入中的大写字母映射为小写字母。
- IXON :启用输出的 XON/XOFF 流控制。
- IXANY :(不属于 POSIX.1；XSI) 允许任何字符来重新开始输出。(?)
- IXOFF :启用输入的 XON/XOFF 流控制。
- IMAXBEL:(不属于 POSIX) 当输入队列满时响零。Linux 没有实现这一位，总是将它视为已设置。
- OPOST ：启用具体实现自行定义的输出处理。
- OLCUC ：(不属于 POSIX) 将输出中的小写字母映射为大写字母。
- ONLCR ：(XSI) 将输出中的新行符映射为回车-换行。
- OCRNL ：将输出中的回车映射为新行符
- ONOCR ：不在第 0 列输出回车。
- ONLRET ：不输出回车。
- OFILL ：发送填充字符作为延时，而不是使用定时来延时。
- OFDEL ：(不属于 POSIX) 填充字符是 ASCII DEL (0177)。如果不设置，填充字符则是 ASCII NUL。
- NLDLY ：新行延时掩码。取值为 NL0 和 NL1。
- CRDLY ：回车延时掩码。取值为 CR0, CR1, CR2, 或 CR3。
- TABDLY ：水平跳格延时掩码。取值为 TAB0, TAB1, TAB2, TAB3 (或 XTABS)。取值为 TAB3，即 XTABS，将扩展跳格为空格 (每个跳格符填充 8 个空格)。(?)
- BSDLY ：回退延时掩码。取值为 BS0 或 BS1。(从来没有被实现过)
- VTDLY ：竖直跳格延时掩码。取值为 VT0 或 VT1。
- FFDLY ：进表延时掩码。取值为 FF0 或 FF1。
- CBAUD ：(不属于 POSIX) 波特率掩码 (4+1 位)。
- CBAUDEX ：(不属于 POSIX) 扩展的波特率掩码 (1 位)，包含在 CBAUD 中。
- (POSIX 规定波特率存储在 termios 结构中，并未精确指定它的位置，而是提供了函数 cfgetispeed() 和 cfsetispeed() 来存取它。一些系统使用 c_cflag 中 CBAUD 选择的位，其他系统使用单独的变量，例如 sg_ispeed 和 sg_ospeed 。)
- CSIZE：字符长度掩码（传送或接收字元时用的位数）。取值为 CS5（传送或接收字元时用5bits）, CS6, CS7, 或 CS8。
- CSTOPB ：设置两个停止位，而不是一个。
- CREAD ：打开接受者。
- PARENB ：允许输出产生奇偶信息以及输入的奇偶校验（启用同位产生与侦测）。
- PARODD ：输入和输出是奇校验（使用奇同位而非偶同位）。
- HUPCL ：在最后一个进程关闭设备后，降低 modem 控制线 (挂断)。(?)
- CLOCAL ：忽略 modem 控制线。
- LOBLK :(不属于 POSIX) 从非当前 shell 层阻塞输出(用于 shl )。(?)
- CIBAUD :(不属于 POSIX) 输入速度的掩码。CIBAUD 各位的值与 CBAUD 各位相同，左移了 IBSHIFT 位。
- CRTSCTS :(不属于 POSIX) 启用 RTS/CTS (硬件) 流控制。
Local mode主要用来控制终端设备不同的特色。利用termios结构里的c_lflag的标志来设定局部模式。
在巨集中有两个比较重要的标志：1.ECHO：它可以让你阻止键入字元的回应。2.ICANON(正规模式)标志，它可以对所接收的字元在两种不同的终端设备模式之间来回切换。
（五）c_cc 数组： 特殊控制字元
- ISIG：当接受到字符 INTR, QUIT, SUSP, 或 DSUSP 时，产生相应的信号。
- ICANON：启用标准模式 (canonical mode)。允许使用特殊字符 EOF, EOL, EOL2, ERASE, KILL, LNEXT, REPRINT, STATUS, 和 WERASE，以及按行的缓冲。
- XCASE:(不属于 POSIX; Linux 下不被支持) 如果同时设置了 ICANON，终端只有大写。输入被转换为小写，除了有前缀的字符。输出时，大写字符被前缀（某些系统指定的特定字符） ，小写字符被转换成大写。
- ECHO ：回显输入字符。
- ECHOE ：如果同时设置了 ICANON，字符 ERASE 擦除前一个输入字符，WERASE 擦除前一个词。
- ECHOK ：如果同时设置了 ICANON，字符 KILL 删除当前行。
- ECHONL ：如果同时设置了 ICANON，回显字符 NL，即使没有设置 ECHO。
- ECHOCTL ：(不属于 POSIX) 如果同时设置了 ECHO，除了 TAB, NL, START, 和 STOP 之外的 ASCII 控制信号被回显为 ^X, 这里 X 是比控制信号大 0x40 的 ASCII 码。例如，字符 0x08 (BS) 被回显为 ^H。
- ECHOPRT ：(不属于 POSIX) 如果同时设置了 ICANON 和 IECHO，字符在删除的同时被打印。
- ECHOKE ：(不属于 POSIX) 如果同时设置了 ICANON，回显 KILL 时将删除一行中的每个字符，如同指定了 ECHOE 和 ECHOPRT 一样。
- DEFECHO :(不属于 POSIX) 只在一个进程读的时候回显。
- FLUSHO :(不属于 POSIX; Linux 下不被支持) 输出被刷新。这个标志可以通过键入字符 DISCARD 来开关。
- NOFLSH :禁止在产生 SIGINT, SIGQUIT 和 SIGSUSP 信号时刷新输入和输出队列，即关闭queue中的flush。
- TOSTOP :向试图写控制终端的后台进程组发送 SIGTTOU 信号（传送欲写入的信息到后台处理）。
- PENDIN :(不属于 POSIX; Linux 下不被支持) 在读入下一个字符时，输入队列中所有字符被重新输出。(bash 用它来处理 typeahead)
- IEXTEN :启用实现自定义的输入处理。这个标志必须与 ICANON 同时使用，才能解释特殊字符 EOL2，LNEXT，REPRINT 和 WERASE，IUCLC 标志才有效。
可提供使用者设定一些特殊的功能，如Ctrl+C的字元组合。特殊控制字元主要是利用termios结构里c_cc的阵列成员来做设定。
c_cc阵列主要用于正规与非正规两种环境，但要注意的是正规与非正规不可混为一谈。
MIN与TIME组合有以下四种：
- VINTR：(003, ETX, Ctrl-C, or also 0177, DEL, rubout) 中断字符。发出 SIGINT 信号。当设置 ISIG 时可被识别，不再作为输入传递。
- VQUIT ：(034, FS, Ctrl-) 退出字符。发出 SIGQUIT 信号。当设置 ISIG 时可被识别，不再作为输入传递。
- VERASE ：(0177, DEL, rubout, or 010, BS, Ctrl-H, or also #) 删除字符。删除上一个还没有删掉的字符，但不删除上一个 EOF 或行首。当设置 ICANON 时可被识别，不再作为输入传递。
- VKILL ：(025, NAK, Ctrl-U, or Ctrl-X, or also @) 终止字符。删除自上一个 EOF 或行首以来的输入。当设置 ICANON 时可被识别，不再作为输入传递。
- VEOF ：(004, EOT, Ctrl-D) 文件尾字符。更精确地说，这个字符使得 tty 缓冲中的内容被送到等待输入的用户程序中，而不必等到 EOL。如果它是一行的第一个字符，那么用户程序的 read() 将返回 0，指示读到了 EOF。当设置 ICANON 时可被识别，不再作为输入传递。
- VMIN :非 canonical 模式读的最小字符数（MIN主要是表示能满足read的最小字元数）。
- VEOL :(0, NUL) 附加的行尾字符。当设置 ICANON 时可被识别。
- VTIME ：非 canonical 模式读时的延时，以十分之一秒为单位。
- VEOL2 ：(not in POSIX; 0, NUL) 另一个行尾字符。当设置 ICANON 时可被识别。
- VSWTCH ：(not in POSIX; not supported under Linux; 0, NUL) 开关字符。(只为 shl 所用。)
- VSTART ：(021, DC1, Ctrl-Q) 开始字符。重新开始被 Stop 字符中止的输出。当设置 IXON 时可被识别，不再作为输入传递。
- VSTOP ：(023, DC3, Ctrl-S) 停止字符。停止输出，直到键入 Start 字符。当设置 IXON 时可被识别，不再作为输入传递。
- VSUSP ：(032, SUB, Ctrl-Z) 挂起字符。发送 SIGTSTP 信号。当设置 ISIG 时可被识别，不再作为输入传递。
- VDSUSP ：(not in POSIX; not supported under Linux; 031, EM, Ctrl-Y) 延时挂起信号。当用户程序读到这个字符时，发送 SIGTSTP 信号。当设置 IEXTEN 和 ISIG，并且系统支持作业管理时可被识别，不再作为输入传递。
- VLNEXT ：(not in POSIX; 026, SYN, Ctrl-V) 字面上的下一个。引用下一个输入字符，取消它的任何特殊含义。当设置 IEXTEN 时可被识别，不再作为输入传递。
- VWERASE ：(not in POSIX; 027, ETB, Ctrl-W) 删除词。当设置 ICANON 和 IEXTEN 时可被识别，不再作为输入传递。
- VREPRINT ：(not in POSIX; 022, DC2, Ctrl-R) 重新输出未读的字符。当设置 ICANON 和 IEXTEN 时可被识别，不再作为输入传递。
- VDISCARD ：(not in POSIX; not supported under Linux; 017, SI, Ctrl-O) 开关：开始/结束丢弃未完成的输出。当设置 IEXTEN 时可被识别，不再作为输入传递。
- VSTATUS ：(not in POSIX; not supported under Linux; status request: 024, DC4, Ctrl-T).
- 这些符号下标值是互不相同的，除了 VTIME，VMIN 的值可能分别与 VEOL，VEOF 相同。 (在 non-canonical 模式下，特殊字符的含义更改为延时含义。MIN 表示应当被读入的最小字符数。TIME 是以十分之一秒为单位的计时器。如果同时设置了它们，read 将等待直到至少读入一个字符，一旦读入 MIN 个字符或者从上次读入字符开始经过了 TIME 时间就立即返回。如果只设置了 MIN，read 在读入 MIN 个字符之前不会返回。如果只设置了 TIME，read 将在至少读入一个字符，或者计时器超时的时候立即返回。如果都没有设置，read 将立即返回，只给出当前准备好的字符。)
1.原型int tcgetattr(int fd,struct termois & termios_p);2.功能取得终端介质（fd）初始值，并把其值 赋给temios_p; 函数可以从后台进程中调用；但是，终端属性可能被后来的前台进程所改变。
1.原型int tcsetattr(int fd,int actions,const struct termios *termios_p);2.功能
设置与终端相关的参数 (除非需要底层支持却无法满足)，使用 termios_p 引用的 termios 结构。optional_actions （tcsetattr函数的第二个参数）指定了什么时候改变会起作用：
- TCSANOW：改变立即发生
- TCSADRAIN：改变在所有写入 fd 的输出都被传输后生效。这个函数应当用于修改影响输出的参数时使用。(当前输出完成时将值改变)
- TCSAFLUSH ：改变在所有写入 fd 引用的对象的输出都被传输后生效，所有已接受但未读入的输入都在改变发生前丢弃(同TCSADRAIN，但会舍弃当前所有值)。
如果终端并非使用异步串行数据传输，tcsendbreak() 什么都不做。
（四）tcdrain()
（五）tcflush()
- TCIFLUSH ：刷新收到的数据但是不读
- TCOFLUSH ：刷新写入的数据但是不传送
- TCIOFLUSH ：同时刷新收到的数据但是不读，并且刷新写入的数据但是不传送
(七)波特率函数打开一个终端设备时的默认设置是输入和输出都没有挂起。
- TCOOFF ：挂起输出
- TCOON ：重新开始被挂起的输出
- TCIOFF ：发送一个 STOP 字符，停止终端设备向系统传送数据
- TCION ：发送一个 START 字符，使终端设备向系统传输数据
设置速度为 B0 使得 modem "挂机"。与 B38400 相应的实际比特率可以用 setserial(8) 调整。
输入和输出波特率被保存于 termios 结构中。
cfmakeraw 设置终端属性如下：
termios_p->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL|IXON);
termios_p->c_oflag &= ~OPOST;
termios_p->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
termios_p->c_cflag &= ~(CSIZE|PARENB);
termios_p->c_cflag |= CS8;
2.cfsetospeed() 设置 termios_p 指向的 termios 结构中存储的输出波特率为 speed。取值必须是以下常量之一：
B0 B50 B75 B110 B134 B150 B200 B300 B600 B1200 B1800 B2400 B4800 B9600 B19200 B38400 B57600 B115200 B230400
其中：零值 B0 用来中断连接。如果指定了 B0，不应当再假定存在连接。通常，这样将断开连接。CBAUDEX 是一个掩码，指示高于 POSIX.1 定义的速度的那一些 (57600 及以上)。因此，B57600 & CBAUDEX 为非零。
3.cfgetispeed() 返回 termios 结构中存储的输入波特率。
4.cfsetispeed() 设置 termios 结构中存储的输入波特率为 speed。如果输入波特率被设为0，实际输入波特率将等于输出波特率。
五、RETURN VALUE 返回值
1.cfgetispeed() 返回 termios 结构中存储的输入波特率。
2.cfgetospeed() 返回 termios 结构中存储的输出波特率。
3.其他函数返回：
（1）0： 成功
(2)-1: 失败，
注意 tcsetattr() 返回成功，如果任何所要求的修改可以实现的话。因此，当进行多重修改时，应当在这个函数之后再次调用 tcgetattr() 来检测是否所有修改都成功实现。
六、NOTES 注意
Unix V7 以及很多后来的系统有一个波特率的列表，在十四个值 B0, ..., B9600 之后可以看到两个常数 EXTA, EXTB ("External A" and "External B")。很多系统将这个列表扩展为更高的波特率。
tcsendbreak 中非零的 duration 有不同的效果。SunOS 指定中断 duration*N 秒，其中 N 至少为 0.25，不高于 0.5 。Linux, AIX, DU, Tru64 发送 duration 微秒的 break 。FreeBSD, NetBSD, HP-UX 以及 MacOS 忽略 duration 的值。在 Solaris 和 Unixware 中， tcsendbreak 搭配非零的 duration 效果类似于 tcdrain。
SEE ALSO 参见
Name
termios, tcgetattr, tcsetattr, tcsendbreak, tcdrain, tcflush, tcflow, cfmakeraw, cfgetospeed, cfgetispeed, cfsetispeed, cfsetospeed, cfsetspeed - get and set terminal attributes, line control, get and set baud rate
Synopsis
#include <termios.h>
#include <unistd.h>
int tcgetattr(int fd, struct termios *termios_p);
int tcsetattr(int fd, int optional_actions,
const struct termios *termios_p);
int tcsendbreak(int fd, int duration);
int tcdrain(int fd);
int tcflush(int fd, int queue_selector);
int tcflow(int fd, int action);
void cfmakeraw(struct termios *termios_p);
speed_t cfgetispeed(const struct termios *termios_p);
speed_t cfgetospeed(const struct termios *termios_p);
int cfsetispeed(struct termios *termios_p, speed_t speed);
int cfsetospeed(struct termios *termios_p, speed_t speed);
int cfsetspeed(struct termios *termios_p, speed_t speed);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
- cfsetspeed(), cfmakeraw(): _BSD_SOURCE
Description
The termios functions describe a general terminal interface that is provided to control asynchronous communications ports.
The termios structure
- Many of the functions described here have a termios_p argument that is a pointer to a termios structure. This structure contains at least the following members:
-
tcflag_t c_iflag; /* input modes */ tcflag_t c_oflag; /* output modes */ tcflag_t c_cflag; /* control modes */ tcflag_t c_lflag; /* local modes */ cc_t c_cc[NCCS]; /* special characters */
-
The values that may be assigned to these fields are described below. In the case of the first four bit-mask fields, the definitions of some of the associated flags that may be set are only exposed if a specific feature test macro (see
feature_test_macros(7)) is defined, as noted in brackets ("[]").
In the descriptions below, "not in POSIX" means that the value is not specified in POSIX.1-2001, and "XSI" means that the value is specified in POSIX.1-2001 as part of the XSI extension.
c_iflag flag constants:
-
IGNBRK
Ignore BREAK condition on input.
BRKINT
If IGNBRK is set, a BREAK is ignored. If it is not set but BRKINT is set, then a BREAK causes the input and output queues to be flushed, and if the terminal is the controlling terminal of a foreground process group, it will cause a SIGINT to be sent to this foreground process group. When neither IGNBRK nor BRKINT are set, a BREAK reads as a null byte ('\0'), except when PARMRK is set, in which case it reads as the sequence \377 \0 \0.
IGNPAR
Ignore framing errors and parity errors.
PARMRK
If IGNPAR is not set, prefix a character with a parity error or framing error with \377 \0. If neither IGNPAR nor PARMRK is set, read a character with a parity error or framing error as \0.
INPCK
Enable input parity checking.
ISTRIP
Strip off eighth bit.
INLCR
Translate NL to CR on input.
IGNCR
Ignore carriage return on input.
ICRNL
Translate carriage return to newline on input (unless IGNCR is set).
IUCLC
(not in POSIX) Map uppercase characters to lowercase on input.
IXON
Enable XON/XOFF flow control on output.
IXANY
(XSI) Typing any character will restart stopped output. (The default is to allow just the START character to restart output.)
IXOFF
Enable XON/XOFF flow control on input.
- IMAXBEL
- (not in POSIX) Ring bell when input queue is full. Linux does not implement this bit, and acts as if it is always set.
- IUTF8 (since Linux 2.6.4)
- (not in POSIX) Input is UTF8; this allows character-erase to be correctly performed in cooked mode.
- c_oflag flag constants defined in POSIX.1:
-
OPOST
Enable implementation-defined output processing.
- The remaining c_oflag flag constants are defined in POSIX.1-2001, unless marked otherwise.
-
OLCUC
(not in POSIX) Map lowercase characters to uppercase on output.
ONLCR
(XSI) Map NL to CR-NL on output.
OCRNL
Map CR to NL on output.
ONOCR
Don't output CR at column 0.
ONLRET
Don't output CR.
OFILL
Send fill characters for a delay, rather than using a timed delay.
OFDEL
(not in POSIX) Fill character is ASCII DEL (0177). If unset, fill character is ASCII NUL ('\0'). (Not implemented on Linux.)
NLDLY
Newline delay mask. Values are NL0 and NL1. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]
CRDLY
Carriage return delay mask. Values are CR0, CR1, CR2, or CR3. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]
TABDLY
Horizontal tab delay mask. Values are TAB0, TAB1, TAB2, TAB3 (or XTABS). A value of TAB3, that is, XTABS, expands tabs to spaces (with tab stops every eight columns). [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]
BSDLY
Backspace delay mask. Values are BS0 or BS1. (Has never been implemented.) [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]
VTDLY
Vertical tab delay mask. Values are VT0 or VT1.
FFDLY
Form feed delay mask. Values are FF0 or FF1. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]
- c_cflag flag constants:
-
CBAUD
(not in POSIX) Baud speed mask (4+1 bits). [requires _BSD_SOURCE or _SVID_SOURCE]
- CBAUDEX
-
(not in POSIX) Extra baud speed mask (1 bit), included in
CBAUD. [requires
_BSD_SOURCE or
_SVID_SOURCE]
(POSIX says that the baud speed is stored in the termios structure without specifying where precisely, and provides cfgetispeed() and cfsetispeed() for getting at it. Some systems use bits selected by CBAUD in c_cflag, other systems use separate fields, for example, sg_ispeed and sg_ospeed.)
-
CSIZE
Character size mask. Values are CS5, CS6, CS7, or CS8.
CSTOPB
Set two stop bits, rather than one.
CREAD
Enable receiver.
PARENB
Enable parity generation on output and parity checking for input.
PARODD
If set, then parity for input and output is odd; otherwise even parity is used.
HUPCL
Lower modem control lines after last process closes the device (hang up).
CLOCAL
Ignore modem control lines.
LOBLK
(not in POSIX) Block output from a noncurrent shell layer. For use by shl (shell layers). (Not implemented on Linux.)
CIBAUD
(not in POSIX) Mask for input speeds. The values for the CIBAUD bits are the same as the values for the CBAUD bits, shifted left IBSHIFT bits. [requires _BSD_SOURCE or_SVID_SOURCE] (Not implemented on Linux.)
CMSPAR
(not in POSIX) Use "stick" (mark/space) parity (supported on certain serial devices): if PARODD is set, the parity bit is always 1; if PARODD is not set, then the parity bit is always 0). [requires _BSD_SOURCE or _SVID_SOURCE]
- CRTSCTS
- (not in POSIX) Enable RTS/CTS (hardware) flow control. [requires _BSD_SOURCE or _SVID_SOURCE]
- c_lflag flag constants:
-
ISIG
When any of the characters INTR, QUIT, SUSP, or DSUSP are received, generate the corresponding signal.
ICANON
Enable canonical mode (described below).
XCASE
(not in POSIX; not supported under Linux) If ICANON is also set, terminal is uppercase only. Input is converted to lowercase, except for characters preceded by \. On output, uppercase characters are preceded by \ and lowercase characters are converted to uppercase. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]
ECHO
Echo input characters.
ECHOE
If ICANON is also set, the ERASE character erases the preceding input character, and WERASE erases the preceding word.
ECHOK
If ICANON is also set, the KILL character erases the current line.
ECHONL
If ICANON is also set, echo the NL character even if ECHO is not set.
- ECHOCTL
- (not in POSIX) If ECHO is also set, terminal special characters other than TAB, NL, START, and STOP are echoed as ^X, where X is the character with ASCII code 0x40 greater than the special character. For example, character 0x08 (BS) is echoed as ^H. [requires _BSD_SOURCE or _SVID_SOURCE]
- ECHOPRT
- (not in POSIX) If ICANON and IECHO are also set, characters are printed as they are being erased. [requires _BSD_SOURCE or _SVID_SOURCE]
-
ECHOKE
(not in POSIX) If ICANON is also set, KILL is echoed by erasing each character on the line, as specified by ECHOE and ECHOPRT. [requires _BSD_SOURCE or_SVID_SOURCE]
- DEFECHO
- (not in POSIX) Echo only when a process is reading. (Not implemented on Linux.)
-
FLUSHO
(not in POSIX; not supported under Linux) Output is being flushed. This flag is toggled by typing the DISCARD character. [requires _BSD_SOURCE or _SVID_SOURCE]
NOFLSH
Disable flushing the input and output queues when generating signals for the INT, QUIT, and SUSP characters.
TOSTOP
Send the SIGTTOU signal to the process group of a background process which tries to write to its controlling terminal.
PENDIN
(not in POSIX; not supported under Linux) All characters in the input queue are reprinted when the next character is read. (bash(1) handles typeahead this way.) [requires_BSD_SOURCE or _SVID_SOURCE]
IEXTEN
Enable implementation-defined input processing. This flag, as well as ICANON must be enabled for the special characters EOL2, LNEXT, REPRINT, WERASE to be interpreted, and for the IUCLC flag to be effective.
- The c_cc array defines the terminal special characters. The symbolic indices (initial values) and meaning are:
- VDISCARD
- (not in POSIX; not supported under Linux; 017, SI, Ctrl-O) Toggle: start/stop discarding pending output. Recognized when IEXTEN is set, and then not passed as input.
-
VDSUSP
(not in POSIX; not supported under Linux; 031, EM, Ctrl-Y) Delayed suspend character (DSUSP): send SIGTSTP signal when the character is read by the user program. Recognized when IEXTEN and ISIG are set, and the system supports job control, and then not passed as input.
VEOF
(004, EOT, Ctrl-D) End-of-file character (EOF). More precisely: this character causes the pending tty buffer to be sent to the waiting user program without waiting for end-of-line. If it is the first character of the line, the read(2) in the user program returns 0, which signifies end-of-file. Recognized when ICANON is set, and then not passed as input.
VEOL
(0, NUL) Additional end-of-line character (EOL). Recognized when ICANON is set.
VEOL2
(not in POSIX; 0, NUL) Yet another end-of-line character (EOL2). Recognized when ICANON is set.
VERASE
(0177, DEL, rubout, or 010, BS, Ctrl-H, or also #) Erase character (ERASE). This erases the previous not-yet-erased character, but does not erase past EOF or beginning-of-line. Recognized when ICANON is set, and then not passed as input.
VINTR
(003, ETX, Ctrl-C, or also 0177, DEL, rubout) Interrupt character (INTR). Send a SIGINT signal. Recognized when ISIG is set, and then not passed as input.
VKILL
(025, NAK, Ctrl-U, or Ctrl-X, or also @) Kill character (KILL). This erases the input since the last EOF or beginning-of-line. Recognized when ICANON is set, and then not passed as input.
VLNEXT
(not in POSIX; 026, SYN, Ctrl-V) Literal next (LNEXT). Quotes the next input character, depriving it of a possible special meaning. Recognized when IEXTEN is set, and then not passed as input.
VMIN
Minimum number of characters for noncanonical read (MIN).
VQUIT
(034, FS, Ctrl-\) Quit character (QUIT). Send SIGQUIT signal. Recognized when ISIG is set, and then not passed as input.
- VREPRINT
- (not in POSIX; 022, DC2, Ctrl-R) Reprint unread characters (REPRINT). Recognized when ICANON and IEXTEN are set, and then not passed as input.
-
VSTART
(021, DC1, Ctrl-Q) Start character (START). Restarts output stopped by the Stop character. Recognized when IXON is set, and then not passed as input.
- VSTATUS
- (not in POSIX; not supported under Linux; status request: 024, DC4, Ctrl-T). Status character (STATUS). Display status information at terminal, including state of foreground process and amount of CPU time it has consumed. Also sends a SIGINFO signal (not supported on Linux) to the foreground process group.
-
VSTOP
(023, DC3, Ctrl-S) Stop character (STOP). Stop output until Start character typed. Recognized when IXON is set, and then not passed as input.
VSUSP
(032, SUB, Ctrl-Z) Suspend character (SUSP). Send SIGTSTP signal. Recognized when ISIG is set, and then not passed as input.
VSWTCH
(not in POSIX; not supported under Linux; 0, NUL) Switch character (SWTCH). Used in System V to switch shells in shell layers, a predecessor to shell job control.
VTIME
Timeout in deciseconds for noncanonical read (TIME).
- VWERASE
- (not in POSIX; 027, ETB, Ctrl-W) Word erase (WERASE). Recognized when ICANON and IEXTEN are set, and then not passed as input.
-
An individual terminal special character can be disabled by setting the value of the corresponding
c_cc element to
_POSIX_VDISABLE.
The above symbolic subscript values are all different, except that VTIME, VMIN may have the same value as VEOL, VEOF, respectively. In noncanonical mode the special character meaning is replaced by the timeout meaning. For an explanation of VMIN and VTIME, see the description of noncanonical mode below.
Retrieving and changing terminal settings
-
tcgetattr() gets the parameters associated with the object referred by
fd and stores them in the
termios structure referenced by
termios_p. This function may be invoked from a background process; however, the terminal attributes may be subsequently changed by a foreground process.
tcsetattr() sets the parameters associated with the terminal (unless support is required from the underlying hardware that is not available) from the termios structure referred to bytermios_p. optional_actions specifies when the changes take effect:
- TCSANOW
- the change occurs immediately.
- TCSADRAIN
- the change occurs after all output written to fd has been transmitted. This function should be used when changing parameters that affect output.
- TCSAFLUSH
- the change occurs after all output written to the object referred by fd has been transmitted, and all input that has been received but not read will be discarded before the change is made.
Canonical and noncanonical mode
-
The setting of the
ICANON canon flag in
c_lflag determines whether the terminal is operating in canonical mode (
ICANON set) or noncanonical mode (
ICANON unset). By default,
ICANON set.
In canonical mode:
-
*
Input is made available line by line. An input line is available when one of the line delimiters is typed (NL, EOL, EOL2; or EOF at the start of line). Except in the case of EOF, the line delimiter is included in the buffer returned by read(2).
*
Line editing is enabled (ERASE, KILL; and if the IEXTEN flag is set: WERASE, REPRINT, LNEXT). A read(2) returns at most one line of input; if the read(2) requested fewer bytes than are available in the current line of input, then only as many bytes as requested are read, and the remaining characters will be available for a future read(2).
- In noncanonical mode input is available immediately (without the user having to type a line-delimiter character), and line editing is disabled. The settings of MIN ( c_cc[VMIN]) and TIME ( c_cc[VTIME]) determine the circumstances in which a read(2) completes; there are four distinct cases:
-
*
MIN == 0; TIME == 0: If data is available, read(2) returns immediately, with the lesser of the number of bytes available, or the number of bytes requested. If no data is available,read(2) returns 0.
*
MIN > 0; TIME == 0: read(2) blocks until the lesser of MIN bytes or the number of bytes requested are available, and returns the lesser of these two values.
*
MIN == 0; TIME > 0: TIME specifies the limit for a timer in tenths of a second. The timer is started when read(2) is called. read(2) returns either when at least one byte of data is available, or when the timer expires. If the timer expires without any input becoming available, read(2) returns 0.
*
MIN > 0; TIME > 0: TIME specifies the limit for a timer in tenths of a second. Once an initial byte of input becomes available, the timer is restarted after each further byte is received. read(2) returns either when the lesser of the number of bytes requested or MIN byte have been read, or when the inter-byte timeout expires. Because the timer is only started after the initial byte becomes available, at least one byte will be read.
Raw mode
-
cfmakeraw() sets the terminal to something like the "raw" mode of the old Version 7 terminal driver: input is available character by character, echoing is disabled, and all special processing of terminal input and output characters is disabled. The terminal attributes are set as follows:
termios_p->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON); termios_p->c_oflag &= ~OPOST; termios_p->c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN); termios_p->c_cflag &= ~(CSIZE | PARENB); termios_p->c_cflag |= CS8;
Line control
-
tcsendbreak() transmits a continuous stream of zero-valued bits for a specific duration, if the terminal is using asynchronous serial data transmission. If
duration is zero, it transmits zero-valued bits for at least 0.25 seconds, and not more that 0.5 seconds. If
duration is not zero, it sends zero-valued bits for some implementation-defined length of time.
If the terminal is not using asynchronous serial data transmission, tcsendbreak() returns without taking any action.
tcdrain() waits until all output written to the object referred to by fd has been transmitted.
tcflush() discards data written to the object referred to by fd but not transmitted, or data received but not read, depending on the value of queue_selector:
- TCIFLUSH
- flushes data received but not read.
- TCOFLUSH
- flushes data written but not transmitted.
- TCIOFLUSH
- flushes both data received but not read, and data written but not transmitted.
- tcflow() suspends transmission or reception of data on the object referred to by fd, depending on the value of action:
-
TCOOFF
suspends output.
TCOON
restarts suspended output.
TCIOFF
transmits a STOP character, which stops the terminal device from transmitting data to the system.
TCION
transmits a START character, which starts the terminal device transmitting data to the system.
- The default on open of a terminal file is that neither its input nor its output is suspended.
Line speed
-
The baud rate functions are provided for getting and setting the values of the input and output baud rates in the
termios structure. The new values do not take effect until
tcsetattr() is successfully called.
Setting the speed to B0 instructs the modem to "hang up". The actual bit rate corresponding to B38400 may be altered with setserial(8).
The input and output baud rates are stored in the termios structure.
cfgetospeed() returns the output baud rate stored in the termios structure pointed to by termios_p.
cfsetospeed() sets the output baud rate stored in the termios structure pointed to by termios_p to speed, which must be one of these constants:
-
B0
B50
B75
B110
B134
B150
B200
B300
B600
B1200
B1800
B2400
B4800
B9600
B19200
B38400
B57600
B115200
B230400
-
The zero baud rate,
B0, is used to terminate the connection. If B0 is specified, the modem control lines shall no longer be asserted. Normally, this will disconnect the line.
CBAUDEX is a mask for the speeds beyond those defined in POSIX.1 (57600 and above). Thus,
B57600 &
CBAUDEX is nonzero.
cfgetispeed() returns the input baud rate stored in the termios structure.
cfsetispeed() sets the input baud rate stored in the termios structure to speed, which must be specified as one of the Bnnn constants listed above for cfsetospeed(). If the input baud rate is set to zero, the input baud rate will be equal to the output baud rate.
cfsetspeed() is a 4.4BSD extension. It takes the same arguments as cfsetispeed(), and sets both input and output speed.
Return Value
cfgetispeed() returns the input baud rate stored in the termios structure.
cfgetospeed() returns the output baud rate stored in the termios structure.
All other functions return:
-
0
on success.
-1
on failure and set errno to indicate the error.
- Note that tcsetattr() returns success if any of the requested changes could be successfully carried out. Therefore, when making multiple changes it may be necessary to follow this call with a further call to tcgetattr() to check that all changes have been performed successfully.
Conforming To
tcgetattr(), tcsetattr(), tcsendbreak(), tcdrain(), tcflush(), tcflow(), cfgetispeed(), cfgetospeed(), cfsetispeed(), and cfsetospeed() are specified in POSIX.1-2001.
cfmakeraw() and cfsetspeed() are nonstandard, but available on the BSDs.
Notes
UNIX V7 and several later systems have a list of baud rates where after the fourteen values B0, ..., B9600 one finds the two constants EXTA, EXTB ("External A" and "External B"). Many systems extend the list with much higher baud rates.
The effect of a nonzero duration with tcsendbreak() varies. SunOS specifies a break of duration * N seconds, where N is at least 0.25, and not more than 0.5. Linux, AIX, DU, Tru64 send a break of duration milliseconds. FreeBSD and NetBSD and HP-UX and MacOS ignore the value of duration. Under Solaris and UnixWare, tcsendbreak() with nonzero duration behaves like tcdrain().
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