cfsetispeed(3) — Linux manual page


TERMIOS(3)                Linux Programmer's Manual               TERMIOS(3)

NAME         top

       termios, tcgetattr, tcsetattr, tcsendbreak, tcdrain, tcflush, tcflow,
       cfmakeraw, cfgetospeed, cfgetispeed, cfsetispeed, cfsetospeed, cfset‐
       speed  -  get  and set terminal attributes, line control, get and set
       baud rate

SYNOPSIS         top

       #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():
           Since glibc 2.19:
           Glibc 2.19 and earlier:

DESCRIPTION         top

       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 exposed only 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 speci‐
       fied 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'), ex‐
              cept when PARMRK is set, in which case it reads as the se‐
              quence \377 \0 \0.

       IGNPAR Ignore framing errors and parity errors.

       PARMRK If this bit is set, input bytes with parity or framing errors
              are marked when passed to the program.  This bit is meaningful
              only when INPCK is set and IGNPAR is not set.  The way erro‐
              neous bytes are marked is with two preceding bytes, \377 and
              \0.  Thus, the program actually reads three bytes for one er‐
              roneous byte received from the terminal.  If a valid byte has
              the value \377, and ISTRIP (see below) is not set, the program
              might confuse it with the prefix that marks a parity error.
              Therefore, a valid byte \377 is passed to the program as two
              bytes, \377 \377, in this case.

              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

       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 out‐

       IXOFF  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.

       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:

       OPOST  Enable implementation-defined output processing.

       OLCUC  (not in POSIX) Map lowercase characters to uppercase on out‐

       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

       OFDEL  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

       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, but see the BUGS section).  A value of TAB3, that
              is, XTABS, expands tabs to spaces (with tab stops every eight
              columns).  [requires _BSD_SOURCE or _SVID_SOURCE or

       BSDLY  Backspace delay mask.  Values are BS0 or BS1.  (Has never been
              implemented.)  [requires _BSD_SOURCE or _SVID_SOURCE or

       VTDLY  Vertical tab delay mask.  Values are VT0 or VT1.

       FFDLY  Form feed delay mask.  Values are FF0 or FF1.  [requires

       c_cflag flag constants:

       CBAUD  (not in POSIX) Baud speed mask (4+1 bits).  [requires
              _BSD_SOURCE or _SVID_SOURCE]

              (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 sys‐
              tems 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 in‐

       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]

              (not in POSIX) Enable RTS/CTS (hardware) flow control.  [re‐
              quires _BSD_SOURCE or _SVID_SOURCE]

       c_lflag flag constants:

       ISIG   When any of the characters INTR, QUIT, SUSP, or DSUSP are re‐
              ceived, 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 lower‐
              case, except for characters preceded by \.  On output, upper‐
              case 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 preced‐
              ing input character, and WERASE erases the preceding word.

       ECHOK  If ICANON is also set, the KILL character erases the current

       ECHONL If ICANON is also set, echo the NL character even if ECHO is
              not set.

              (not in POSIX) If ECHO is also set, terminal special charac‐
              ters 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]

              (not in POSIX) If ICANON and ECHO are also set, characters are
              printed as they are being erased.  [requires _BSD_SOURCE or

       ECHOKE (not in POSIX) If ICANON is also set, KILL is echoed by eras‐
              ing each character on the line, as specified by ECHOE and
              ECHOPRT.  [requires _BSD_SOURCE or _SVID_SOURCE]

              (not in POSIX) Echo only when a process is reading.  (Not im‐
              plemented on Linux.)

       FLUSHO (not in POSIX; not supported under Linux) Output is being
              flushed.  This flag is toggled by typing the DISCARD charac‐
              ter.  [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:

              (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) De‐
              layed 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 pre‐
              cisely: 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 char‐
              acter (ERASE).  This erases the previous not-yet-erased char‐
              acter, but does not erase past EOF or beginning-of-line.  Rec‐
              ognized when ICANON is set, and then not passed as input.

       VINTR  (003, ETX, Ctrl-C, or also 0177, DEL, rubout) Interrupt char‐
              acter (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 begin‐
              ning-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.

              (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.

              (not in POSIX; not supported under Linux; status request: 024,
              DC4, Ctrl-T).  Status character (STATUS).  Display status in‐
              formation 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

       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

       VSWTCH (not in POSIX; not supported under Linux; 0, NUL) Switch char‐
              acter (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).

              (not in POSIX; 027, ETB, Ctrl-W) Word erase (WERASE).  Recog‐
              nized when ICANON and IEXTEN are set, and then not passed as

       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 de‐
       scription 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 avail‐
       able) from the termios structure referred to by termios_p.  op‐
       tional_actions specifies when the changes take effect:

              the change occurs immediately.

              the change occurs after all output written to fd has been
              transmitted.  This option should be used when changing parame‐
              ters that affect output.

              the change occurs after all output written to the object re‐
              ferred by fd has been transmitted, and all input that has been
              received but not read will be discarded before the change is

   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 noncanon‐
       ical mode (ICANON unset).  By default, ICANON is 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 fu‐
         ture read(2).

       * The maximum line length is 4096 chars (including the terminating
         newline character); lines longer than 4096 chars are truncated.
         After 4095 characters, input processing (e.g., ISIG and ECHO* pro‐
         cessing) continues, but any input data after 4095 characters up to
         (but not including) any terminating newline is discarded.  This en‐
         sures that the terminal can always receive more input until at
         least one line can be read.

       In noncanonical mode input is available immediately (without the user
       having to type a line-delimiter character), no input processing is
       performed, and line editing is disabled.  The read buffer will only
       accept 4095 chars; this provides the necessary space for a newline
       char if the input mode is switched to canonical.  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 (polling read)
              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 (blocking read)
              read(2) blocks until MIN bytes are available, and returns up
              to the number of bytes requested.

       MIN == 0, TIME > 0 (read with timeout)
              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 be‐
              coming available, read(2) returns 0.  If data is already
              available at the time of the call to read(2), the call behaves
              as though the data was received immediately after the call.

       MIN > 0, TIME > 0 (read with interbyte timeout)
              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) re‐
              turns when any of the following conditions is met:

              *  MIN bytes have been received.

              *  The interbyte timer expires.

              *  The number of bytes requested by read(2) has been received.
                 (POSIX does not specify this termination condition, and on
                 some other implementations read(2) does not return in this

              Because the timer is started only after the initial byte be‐
              comes available, at least one byte will be read.  If data is
              already available at the time of the call to read(2), the call
              behaves as though the data was received immediately after the

       POSIX does not specify whether the setting of the O_NONBLOCK file
       status flag takes precedence over the MIN and TIME settings.  If
       O_NONBLOCK is set, a read(2) in noncanonical mode may return immedi‐
       ately, regardless of the setting of MIN or TIME.  Furthermore, if no
       data is available, POSIX permits a read(2) in noncanonical mode to
       return either 0, or -1 with errno set to EAGAIN.

   Raw mode
       cfmakeraw() sets the terminal to something like the "raw" mode of the
       old Version 7 terminal driver: input is available character by char‐
       acter, 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 than 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 re‐
       ferred 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 val‐
       ues of the input and output baud rates in the termios structure.  The
       new values do not take effect until tcsetattr() is successfully

       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 struc‐
       ture pointed to by termios_p to speed, which must be one of these


       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

       cfsetispeed() sets the input baud rate stored in the termios struc‐
       ture 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         top

       cfgetispeed() returns the input baud rate stored in the termios

       cfgetospeed() returns the output baud rate stored in the termios

       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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                            Attribute     Value   │
       │tcgetattr(), tcsetattr(), tcdrain(), │ Thread safety │ MT-Safe │
       │tcflush(), tcflow(), tcsendbreak(),  │               │         │
       │cfmakeraw(), cfgetispeed(),          │               │         │
       │cfgetospeed(), cfsetispeed(),        │               │         │
       │cfsetospeed(), cfsetspeed()          │               │         │

CONFORMING TO         top

       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

NOTES         top

       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().

BUGS         top

       On the Alpha architecture before Linux 4.16 (and glibc before 2.28),
       the XTABS value was different from TAB3 and it was ignored by the
       N_TTY line discipline code of the terminal driver as a result
       (because as it wasn't part of the TABDLY mask).

SEE ALSO         top

       reset(1), setterm(1), stty(1), tput(1), tset(1), tty(1),
       ioctl_console(2), ioctl_tty(2), setserial(8)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-08-13                       TERMIOS(3)

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