'\" t
.\" Copyright 2006-2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.TH cfsetspeed 3 2026-07-02 "Linux man-pages 6.19"
.SH NAME
cfgetospeed, cfgetispeed, cfsetispeed, cfsetospeed, cfsetspeed
\-
get and set line speed
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <termios.h>
.P
.BI "speed_t cfgetispeed(const struct termios *" termios_p );
.BI "speed_t cfgetospeed(const struct termios *" termios_p );
.P
.BI "int cfsetispeed(struct termios *" termios_p ", speed_t " speed );
.BI "int cfsetospeed(struct termios *" termios_p ", speed_t " speed );
.P
.BR #define\~_DEFAULT_SOURCE "  /* See feature_test_macros(7) */"
.B #include <termios.h>
.P
.BI "int cfsetspeed(struct termios *" termios_p ", speed_t " speed );
.fi
.SH DESCRIPTION
The baud rate functions are provided
for getting and setting
the values of the input and output baud rates
in the
.I termios
structure.
The new values do not take effect
until
.BR tcsetattr (3)
is successfully called.
.P
Setting the speed to
.B B0
instructs the modem to "hang up".
The actual bit rate corresponding to
.B B38400
may be altered with
.BR setserial (8);
doing so is discouraged
as it may break other applications
later trying to use the same serial port.
.P
The input and output baud rates are stored in the
.I termios
structure.
.P
.BR cfgetospeed ()
returns the output baud rate stored in the
.I termios
structure
pointed to by
.IR termios_p .
.P
.BR cfsetospeed ()
sets the output baud rate stored in the
.I termios
structure
pointed to by
.I termios_p
to
.IR speed ,
which must be one of these constants:
.RS
.TP
.B B0
.TQ
.B B50
.TQ
.B B75
.TQ
.B B110
.TQ
.B B134
.TQ
.B B150
.TQ
.B B200
.TQ
.B B300
.TQ
.B B600
.TQ
.B B1200
.TQ
.B B1800
.TQ
.B B2400
.TQ
.B B4800
.TQ
.B B9600
.TQ
.B B19200
.TQ
.B B38400
.TQ
.B B57600
.TQ
.B B115200
.TQ
.B B230400
.TQ
.B B460800
.TQ
.B B500000
.TQ
.B B576000
.TQ
.B B921600
.TQ
.B B1000000
.TQ
.B B1152000
.TQ
.B B1500000
.TQ
.B B2000000
.RE
.P
These constants are additionally supported on the SPARC architecture:
.RS
.TP
.B B76800
.TQ
.B B153600
.TQ
.B B307200
.TQ
.B B614400
.RE
.P
These constants are additionally supported on non-SPARC architectures:
.RS
.TP
.B B2500000
.TQ
.B B3000000
.TQ
.B B3500000
.TQ
.B B4000000
.RE
.P
Due to differences between architectures,
portable applications should check
if a particular
.BI B nnn
constant is defined prior to using it.
.P
The zero baud rate,
.BR B0 ,
is used to terminate the connection.
If
.B B0
is specified,
the modem control lines shall no longer be asserted.
Normally,
this will disconnect the line.
.B CBAUDEX
is a mask
for the speeds beyond those defined in POSIX.1
(57600 and above).
Thus,
.BR B57600 " & " CBAUDEX
is nonzero.
.P
Setting the baud rate to a value other than those defined by
.BI B nnn
constants is possible via the
.B TCSETS2
ioctl;
see
.BR ioctl_tty (2).
.P
.BR cfgetispeed ()
returns the input baud rate stored in the
.I termios
structure.
.P
.BR cfsetispeed ()
sets the input baud rate stored in the
.I termios
structure to
.IR speed ,
which must be specified as one of the
.BI B nnn
constants listed above for
.BR cfsetospeed ().
If the input baud rate is set to the literal constant
.B 0
(not the symbolic constant
.BR B0 ),
the input baud rate will be
equal to the output baud rate.
.P
.BR cfsetspeed ()
takes the same arguments as
.BR cfsetispeed (),
and sets both input and output speed.
.SH RETURN VALUE
.BR cfgetispeed ()
returns the input baud rate stored in the
.I termios
structure.
.P
.BR cfgetospeed ()
returns the output baud rate stored in the
.I termios
structure.
.P
All other functions return:
.TP
.B 0
on success.
.TP
.B \-1
on failure and set
.I errno
to indicate the error.
.SH ATTRIBUTES
For an explanation of the terms used in this section,
see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR cfgetispeed (),
.BR cfgetospeed (),
.BR cfsetispeed (),
.BR cfsetospeed (),
.BR cfsetspeed ()
T}	Thread safety	MT-Safe
.TE
.SH VERSIONS
UNIX\ V7 and several later systems have a list of baud rates
where after the values
.B B0
through
.B B9600
one finds the two constants
.BR EXTA ,
.B EXTB
("External A" and "External B").
Many systems extend the list with much higher baud rates.
.SH STANDARDS
.TP
.BR cfgetispeed ()
.TQ
.BR cfgetospeed ()
.TQ
.BR cfsetispeed ()
.TQ
.BR cfsetospeed ()
POSIX.1-2008.
.TP
.BR cfsetspeed ()
BSD.
.SH HISTORY
.TP
.BR cfgetispeed ()
.TQ
.BR cfgetospeed ()
.TQ
.BR cfsetispeed ()
.TQ
.BR cfsetospeed ()
POSIX.1-2001.
.TP
.BR cfsetspeed ()
4.4BSD.
.SH SEE ALSO
.BR speed_t (3type),
.BR termios (3type)