cfgetispeed(3) -- Linux man page
NAMEtermios, tcgetattr, tcsetattr, tcsendbreak, tcdrain, tcflush, tcflow, cfmakeraw, cfgetospeed, cfgetispeed, cfsetispeed, cfsetospeed - get and set terminal attributes, line control, get and set baud rate
int tcgetattr(int fd, struct termios *termios_p);
int tcsetattr(int fd, int optional_actions, 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);
int cfmakeraw(struct termios *termios_p);
speed_t cfgetispeed(struct termios *termios_p);
speed_t cfgetospeed(struct termios *termios_p);
int cfsetispeed(struct termios *termios_p, speed_t speed);
DESCRIPTIONThe termios functions describe a general terminal interface that is provided to control asynchronous communications ports.
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]; /* control chars */
c_iflag flag constants:
- Ignore BREAK condition on input.
- 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 NUL character, except when PARMRK is set, in which case it reads as the sequence \377 \0 \0.
- Ignore framing errors and parity errors.
- 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.
- Enable input parity checking.
- Strip off eighth bit.
- Translate NL to CR on input.
- Ignore carriage return on input.
- Translate carriage return to newline on input (unless IGNCR is set).
- (not in POSIX) Map uppercase characters to lowercase on input.
- Enable XON/XOFF flow control on output.
- (not in POSIX.1; XSI) Enable any character to restart output.
- Enable XON/XOFF flow control on input.
- (not in POSIX) Ring bell when input queue is full. Linux does not implement this bit, and acts as if it is always set.
c_oflag flag constants defined in POSIX.1:
- Enable implementation-defined output processing.
The remaining c_oflag flag constants are defined in POSIX 1003.1-2001, unless marked otherwise.
- (not in POSIX) Map lowercase characters to uppercase on output.
- (XSI) Map NL to CR-NL on output.
- Map CR to NL on output.
- Don't output CR at column 0.
- Don't output CR.
- Send fill characters for a delay, rather than using a timed delay.
- (not in POSIX) Fill character is ASCII DEL (0177). If unset, fill character is ASCII NUL.
- Newline delay mask. Values are NL0 and NL1.
- Carriage return delay mask. Values are CR0, CR1, CR2, or CR3.
- 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).
- Backspace delay mask. Values are BS0 or BS1. (Has never been implemented.)
- Vertical tab delay mask. Values are VT0 or VT1.
- Form feed delay mask. Values are FF0 or FF1.
c_cflag flag constants:
- (not in POSIX) Baud speed mask (4+1 bits).
- (not in POSIX) Extra baud speed mask (1 bit), included in CBAUD.
(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, e.g. sg_ispeed and sg_ospeed.)
- Character size mask. Values are CS5, CS6, CS7, or CS8.
- Set two stop bits, rather than one.
- Enable receiver.
- Enable parity generation on output and parity checking for input.
- Parity for input and output is odd.
- Lower modem control lines after last process closes the device (hang up).
- Ignore modem control lines.
- (not in POSIX) Block output from a noncurrent shell layer. (For use by shl.)
- (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.
- (not in POSIX) Enable RTS/CTS (hardware) flow control.
c_lflag flag constants:
- When any of the characters INTR, QUIT, SUSP, or DSUSP are received, generate the corresponding signal.
- Enable canonical mode. This enables the special characters EOF, EOL, EOL2, ERASE, KILL, LNEXT, REPRINT, STATUS, and WERASE, and buffers by lines.
- (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.
- Echo input characters.
- If ICANON is also set, the ERASE character erases the preceding input character, and WERASE erases the preceding word.
- If ICANON is also set, the KILL character erases the current line.
- If ICANON is also set, echo the NL character even if ECHO is not set.
- (not in POSIX) If ECHO is also set, ASCII control signals other than TAB, NL, START, and STOP are echoed as ^X, where X is the character with ASCII code 0x40 greater than the control signal. For example, character 0x08 (BS) is echoed as ^H.
- (not in POSIX) If ICANON and IECHO are also set, characters are printed as they are being erased.
- (not in POSIX) If ICANON is also set, KILL is echoed by erasing each character on the line, as specified by ECHOE and ECHOPRT.
- (not in POSIX) Echo only when a process is reading.
- (not in POSIX; not supported under Linux) Output is being flushed. This flag is toggled by typing the DISCARD character.
- Disable flushing the input and output queues when generating the SIGINT, SIGQUIT and SIGSUSP signals.
- Send the SIGTTOU signal to the process group of a background process which tries to write to its controlling terminal.
- (not in POSIX; not supported under Linux) All characters in the input queue are reprinted when the next character is read. (bash handles typeahead this way.)
- 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 special control characters. The symbolic indices (initial values) and meaning are:
- (003, ETX, Ctrl-C, or also 0177, DEL, rubout) Interrupt character. Send a SIGINT signal. Recognized when ISIG is set, and then not passed as input.
- (034, FS, Ctrl-\) Quit character. Send SIGQUIT signal. Recognized when ISIG is set, and then not passed as input.
- (0177, DEL, rubout, or 010, BS, Ctrl-H, or also #) Erase character. 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.
- (025, NAK, Ctrl-U, or Ctrl-X, or also @) Kill character. This erases the input since the last EOF or beginning-of-line. Recognized when ICANON is set, and then not passed as input.
- (004, EOT, Ctrl-D) End-of-file character. 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() in the user program returns 0, which signifies end-of-file. Recognized when ICANON is set, and then not passed as input.
- Minimum number of characters for non-canonical read.
- (0, NUL) Additional end-of-line character. Recognized when ICANON is set.
- Timeout in deciseconds for non-canonical read.
- (not in POSIX; 0, NUL) Yet another end-of-line character. Recognized when ICANON is set.
- (not in POSIX; not supported under Linux; 0, NUL) Switch character. (Used by shl only.)
- (021, DC1, Ctrl-Q) Start character. Restarts output stopped by the Stop character. Recognized when IXON is set, and then not passed as input.
- (023, DC3, Ctrl-S) Stop character. Stop output until Start character typed. Recognized when IXON is set, and then not passed as input.
- (032, SUB, Ctrl-Z) Suspend character. Send SIGTSTP signal. Recognized when ISIG is set, and then not passed as input.
- (not in POSIX; not supported under Linux; 031, EM, Ctrl-Y) Delayed suspend character: 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.
- (not in POSIX; 026, SYN, Ctrl-V) Literal next. Quotes the next input character, depriving it of a possible special meaning. Recognized when IEXTEN is set, and then not passed as input.
- (not in POSIX; 027, ETB, Ctrl-W) Word erase. Recognized when ICANON and IEXTEN are set, and then not passed as input.
- (not in POSIX; 022, DC2, Ctrl-R) Reprint unread characters. Recognized when ICANON and IEXTEN are set, and then not passed as input.
- (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.
- (not in POSIX; not supported under Linux; status request: 024, DC4, Ctrl-T).
These symbolic subscript values are all different, except that VTIME, VMIN may have the same value as VEOL, VEOF, respectively. (In non-canonical mode the special character meaning is replaced by the timeout meaning. MIN represents the minimum number of characters that should be received to satisfy the read. TIME is a decisecond-valued timer. When both are set, a read will wait until at least one character has been received, and then return as soon as either MIN characters have been received or time TIME has passed since the last character was received. If only MIN is set, the read will not return before MIN characters have been received. If only TIME is set, the read will return as soon as either at least one character has been received, or the timer times out. If neither is set, the read will return immediately, only giving the currently already available characters.)
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 by termios_p. optional_actions specifies when the changes take effect:
- the change occurs immediately.
- the change occurs after all output written to fd has been transmitted. This function should be used when changing parameters that affect output.
- 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.
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:
- flushes data received but not read.
- flushes data written but not transmitted.
- 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:
- suspends output.
- restarts suspended output.
- transmits a STOP character, which stops the terminal device from transmitting data to the system.
- 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.
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
The input and output baud rates are stored in the termios structure.
cfmakeraw sets the terminal attributes 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;
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 B230400The 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.
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:
- on success.
- 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.
NOTESUnix 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.
SEE ALSOstty(1), setserial(8)