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The Linux Serial Programming HOWTO, Part 1 of 2
By Vernon C. Hoxie
v2.0 10 September 1999
This document describes how to program communications with devices
over a serial port on a Linux box.
Table of Contents
1. Copyright
2. Introduction
3. Opening
4. Commands
5. Changing Baud Rates
6. Additional Control Calls
6.1 Sending a "break".
6.2 Hardware flow control.
6.3 Flushing I/O buffers.
7. Modem control
8. Process Groups
8.1 Sessions
8.2 Process Groups
8.3 Controlling Terminal
8.3.1 Get the foreground group process id.
8.3.2 Set the foreground process group id of a terminal.
8.3.3 Get process group id.
9. Lockfiles
10. Additional Information
11. Feedback
1. Copyright
The Linux Serial-Programming-HOWTO is copyright (C) 1997 by Vernon
Hoxie. Linux HOWTO documents may be reproduced and distributed in
whole or in part, in any medium physical or electronic, as long as
this copyright notice is retained on all copies. Commercial
redistribution is allowed and encouraged; however, the author would
like to be notified of any such distributions.
All translations, derivative works, or aggregate works incorporating
this Linux HOWTO document must be covered under this copyright notice.
That is, you may not produce a derivative work from this HOWTO and
impose additional restrictions on its distribution.
This version is a complete rewrite of the previous Serial-Programming-
HOWTO by Peter H. Baumann, <>
2. Introduction
This HOWTO will attempt to give hints about how to write a program
which needs to access a serial port. Its principal focus will be on
the Linux implementation and what the meaning of the various library
functions available.
Someone asked about which of several sequences of operations was
right. There is no absolute right way to accomplish an outcome. The
options available are too numerous. If your sequences produces the
desired results, then that is the right way for you. Another
programmer may select another set of options and get the same results.
His method is right for him.
Neither of these methods may operate properly with some other
implementation of UNIX. It is strange that many of the concepts which
were implemented in the SYSV version have been dumped. Because UNIX
was developed by AT&T and much code has been generated on those
concepts, the AT&T version should be the standard to which others
should emulate.
Now the standard is POSIX.
It was once stated that the popularity of UNIX and C was that they
were created by programmers for programmers. Not by scholars who
insist on purity of style in deference to results and simplicity of
use. Not by committees with people who have diverse personal or
proprietary agenda. Now ANSI and POSIX have strayed from those
original clear and simply concepts.
3. Opening
The various serial devices are opened just as any other file.
Although, the fopen(3) command may be used, the plain open(2) is
preferred. This call returns the file descriptor which is required
for the various commands that configure the interface.
Open(2) has the format:
#include <fcntl.h>
int open(char *path, int flags, [int mode]);
In addition to the obvious O_RDWR, O_WRONLY and O_RDONLY, two
additional flags are available. These are O_NONBLOCK and O_NOCTTY.
Other flags listed in the open(2) manual page are not applicable to
serial devices.
Normally, a serial device opens in "blocking" mode. This means that
the open() will not return until the Carrier Detect line from the port
is active, e.g. modem, is active. When opened with the O_NONBLOCK
flag set, the open() will return immediately regardless of the status
of the DCD line. The "blocking" mode also affects the read() call.
The fcntl(2) command can be used to change the O_NONBLOCK flag anytime
after the device has been opened.
The device driver and the data passing through it are controlled
according to settings in the struct termios. This structure is
defined in "/usr/include/termios.h". In the Linux tree, further
reference is made to "/usr/include/asm/termbits.h".
In blocking mode, a read(2) will block until data is available or a
signal is received. It is still subject to state of the ICANON flag.
When the termios.c_lflag ICANON bit is set, input data is collected
into strings until a NL, EOF or EOL character is received. You can
define these in the termios.c_cc[] array. Also, ERASE and KILL
characters will operate on the incoming data before it is delivered to
the user.
In non-canonical mode, incoming data is quantified by use of the
c_cc[VMIN and c_cc[VTIME] values in termios.c_cc[].
Some programmers use the select() call to detect the completion of a
read(). This is not the best way of checking for incoming data.
Select() is part of the SOCKETS scheme and too complex for most
A full explanation of the fields of the termios structure is contained
in termios(7) of the Users Manual. A version is included in Part 2 of
this HOWTO document.
4. Commands
Changes to the struct termios are made by retrieving the current
settings, making the desired changes and transmitting the modified
structure back to the kernel.
The historic means of communicating with the kernel was by use of the
ioctl(fd, COMMAND, arg) system call. Then the purists in the
computer industry decided that this was not genetically consistent.
Their argument was that the argument changed its stripes. Sometimes
it was an int, sometimes it was a pointer to int and other times it
was a pointer to struct termios. Then there were those times it was
empty or NULL. These variations are dependent upon the COMMAND.
As a alternative, the tc* series of functions were concocted.
These are:
int tcgetattr(int filedes, struct termios *termios_p);
int tcsetattr(int filedes, int optional_actions,
const struct termios *termios_p);
instead of:
int ioctl(int filedes, int command,
struct termios *termios_p);
where command is TCGETS or one of TCSETS, TCSETSW or TCSETSF.
The TCSETS command is comparable to the TCSANOW optional_action for
the tc* version. These direct the kernel to adopt the changes
immediately. Other pairs are:
command optional_action Meaning
TCSETSW TCSADRAIN Change after all output has drained.
TCSETSF TCSAFLUSH Change after all output has drained
then discard any input characters
not read.
Since the return code from either the ioctl(2) or the tcsetattr(2)
commands only indicate that the command was processed by the kernel.
These do not indicate whether or not the changes were actually
accomplished. Either of these commands should be followed by a call
ioctl(fd, TCGETS, &new_termios);
tcgetattr(fd, &new_termios);
A user function which makes changes to the termios structure should
define two struct termios variables. One of these variables should
contain the desired configuration. The other should contain a copy of
the kernels version. Then after the desired configuration has been
sent to the kernel, another call should be made to retrieve the
kernels version. Then the two compared.
Here is an example of how to add RTS/CTS flow control:
struct termios my_termios;
struct termios new_termios;
tcgetattr(fd, &my_termios);
my_termios.c_flag |= CRTSCTS;
tcsetattr(fd, TCSANOW, &my_termios);
tcgetattr(fd, &new_termios);
if (memcmp(my_termios, new_termios,
sizeof(my_termios)) != 0) {
/* do some error handling */
5. Changing Baud Rates
With Linux, the baud rate can be changed using a technique similar to
add/delete RTS/CTS.
struct termios my_termios;
struct termios new_termios;
tcgetattr(fd, &my_termios);
my_termios.c_flag &= ~CBAUD;
my_termios.c_flag |= B19200;
tcsetattr(fd, TCSANOW, &my_termios);
tcgetattr(fd, &new_termios);
if (memcmp(my_termios, new_termios,
sizeof(my_termios)) != 0) {
/* do some error handling */
POSIX adds another method. They define:
speed_t cfgetispeed(const struct termios *termios_p);
speed_t cfgetospeed(const struct termios *termios_p);
library calls to extract the current input or output speed from the
struct termios pointed to with *termio_p. This is a variable defined
in the calling process. In practice, the data contained in this
termios, should be obtained by the tcgetattr() call or an ioctl() call
using the TCGETS command.
The companion library calls are:
int cfsetispeed(struct termios *termios_p, speed_t speed);
int cfsetospeed(struct termios *termios_p, speed_t speed);
which are used to change the value of the baud rate in the locally
defined *termios_p. Following either of these calls, either a call to
tcsetattr() or ioctl() with one of TCSETS, TCSETSW or TCSETSF as the
command to transmit the change to the kernel.
The cf* commands are preferred for portability. Some weird Unices use
a considerably different format of termios.
Most implementations of Linux use only the input speed for both input
and output. These functions are defined in the application program by
reference to <termios.h>. In reality, they are in
6. Additional Control Calls
6.1. Sending a "break".
int ioctl(fd, TCSBRK, int arg);
int tcsendbreak(fd, int arg);
Send a break: Here the action differs between the conventional
ioctl() call and the POSIX call. For the conventional call, an arg of
'0' sets the break control line of the UART for 0.25 seconds. For the
POSIX command, the break line is set for arg times 0.1 seconds.
6.2. Hardware flow control.
int ioctl(fd, TCXONC, int action);
int tcflow(fd, int action);
The action flags are:
o TCOOFF 0 suspend output
o TCOON 1 restart output
o TCIOFF 2 transmit STOP character to suspend input
o TCION 3 transmit START character to restart input
6.3. Flushing I/O buffers.
int ioctl(fd, TCFLSH, queue_selector);
int tcflush(fd, queue_selector);
The queue_selector flags are:
o TCIFLUSH 0 flush any data not yet read from the input buffer
o TCOFLUSH 1 flush any data written to the output buffer but not
yet transmitted
o TCIOFLUSH 2 flush both buffers
7. Modem control
The hardware modem control lines can be monitored or modified by the
ioctl(2) system call. A set of comparable tc* calls apparently do not
exist. The form of this call is:
int ioctl(fd, COMMAND, (int *)flags);
The COMMANDS and their action are:
o TIOCMBIS turn on control lines depending upon which bits are set
in flags.
o TIOCMBIC turn off control lines depending upon which bits are
unset in flags.
o TIOCMGET the appropriate bits are set in flags according to the
current status
o TIOCMSET the state of the UART is changed according to which bits
are set/unset in 'flags'
The bit pattern of flags refer to the following control lines:
o TIOCM_LE Line enable
o TIOCM_DTR Data Terminal Ready
o TIOCM_RTS Request to send
o TIOCM_ST Secondary transmit
o TIOCM_SR Secondary receive
o TIOCM_CTS Clear to send
o TIOCM_CAR Carrier detect
o TIOCM_DSR Data set ready
It should be noted that some of these bits are controlled by the modem
and the UART cannot change them but their status can be sensed by
TIOCMGET. Also, most Personal Computers do not provide hardware for
secondary transmit and receive.
There are also a pair of ioctl() to monitor these lines. They are
undocumented as far as I have learned. The commands are TIOCMIWAIT
and TCIOGICOUNT. They also differ between versions of the Linux
See the lines.c file in my "serial_suite" for an example of how these
can be used see <>
8. Process Groups
8.1. Sessions
8.2. Process Groups
Any newly created process inherits the Process Group of its creator.
The Process Group leader has the same PID as PGID.
8.3. Controlling Terminal
There are a series of ioctl(2) and tc*(2) calls which can be used to
monitor or to change the process group to which the device is
8.3.1. Get the foreground group process id.
If there is no foreground group, a number not representing an existing
process group is returned. On error, a -1 is returned and errno is
int ioctl(fd, TIOCGPGRP, (pid_t *)pid);
int tcgetpgrp(fd, (pid_t *)pid);
8.3.2. Set the foreground process group id of a terminal.
The fd must be the controlling terminal and be associated with the
session of the calling process.
int ioctl(fd, TIOCSPGRP, (pid_t *)pid);
int tcsetpgrp(fd, (pid_t *)pid);
8.3.3. Get process group id.
int ioctl(fd, TIOCGPGRP, &(pid_t)pid);
int tcgetpgrp(fd, &(pid_t)pid);
9. Lockfiles
Any process which accesses a serial device should first check for the
existence of lock file for the desired device. If such a lock lock
file exists, this means that the device may be in use by another
Check my "libdevlocks-x.x.tgz" at
<> for an example of how these lock
files should be utilized.
10. Additional Information
Check out my "serial_suite.tgz" for more information about programming
the serial ports at <>. There some
examples and some blurbs about setting up modems and comments about
some general considerations.
11. Feedback
Please send me any corrections, questions, comments, suggestions, or
additional material. I would like to improve this HOWTO! Tell me
exactly what you don't understand, or what could be clearer. You can
reach me at <> via email. Please
include the version number of the Serial-Programming-HOWTO when