The Linux Serial Programming HOWTO, Part 1 of 2 <AUTHOR>By Vernon C. Hoxie <DATE>v2.0 10 September 1999 <ABSTRACT> This document describes how to program communications with devices over a serial port on a Linux box. </ABSTRACT> <TOC> <SECT>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, <URL URL="mailto:Peter.Baumann@dlr.de"> <SECT>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. <SECT>Opening The various serial devices are opened just as any other file. Although, the <it>fopen(3)</it> command may be used, the plain <it>open(2)</it> is preferred. This call returns the file descriptor which is required for the various commands that configure the interface. <it>Open(2)</it> has the format: <tscreen><verb> #include <fcntl.h> int open( char *path, int flags, [int mode] ); </verb></tscreen> 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 <it>open(2)</it> manual page are not applicable to serial devices. Normally, a serial device opens in "blocking" mode. This means that the <it>open()</it> 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 <it>open()</it> will return immediately regardless of the status of the DCD line. The "blocking" mode also affects the <it>read()</it> call. The <it>fcntl(2)</it> 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 <it>struct termios</it>. 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 <it>read(2)</it> will block until data is available or a signal is received. It is still subject to state of the ICANON flag. When the <it>termios.c_lflag</it> 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 <it>termios.c_cc[]</it> 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 quanitified by use of the <it>c_cc[VMIN</it> and <it>c_cc[VTIME]</it> values in <it>termios.c_cc[]</it>. Some programmers use the <it>select()</it> call to detect the completion of a <it>read()</it>. This is not the best way of checking for incoming data. <it>Select()</it> is part of the SOCKETS scheme and too complex for most applications. A full explanation of the fields of the <it>termios</it> structure is contained in <it>termios(7)</it> of the Users Manual. A version is included in Part 2 of this HOWTO document. <SECT>Commands Changes to the <it>struct termios</it> 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 <it>ioctl( fd, COMMAND, arg )</it> 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 <it>int</it>, sometimes it was a <it>pointer to int</it> and other times it was a <it>pointer to struct termios</it>. Then there were those times it was empty or NULL. These variations are dependent upon the COMMAND. As a alternative, the <it>tc*</it> series of functions were concocted. These are: <tscreen><verb> int tcgetattr( int <it/filedes/, struct termios *termios_p ); int tcsetattr( int <it/filedes/, int <it/optional_actions/, *const struct termios <it/termios_p/ ); </verb></tscreen> instead of: <tscreen><verb> int ioctl( int <it/filedes/, int <it/command/, *const struct termios <it/termios_p/ ); </verb></tscreen> where command is TCGETS or one of TCSETS, TCSETSW or TCSETSF. The TCSETS <it>command</it> is comparable to the TCSANOW <it>optional_action</it> for the <it>tc*</it> version. These direct the kernel to adopt the changes immediately. Other pairs are: <tscreen><verb> 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. </verb></tscreen> Since the return code from either the <it>ioctl(2)</it> or the <it>tcsetattr(2)</it> 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 to: <tscreen><verb> ioctl( fd, TCGETS, &new_termios );</verb></tscreen> or: <tscreen><verb> tcgetattr( fd, &new_termios );</verb></tscreen> A user function which makes changes to the <it>termios</it> structure should define two <it>struct termios</it> 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: <tscreen><verb> 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 */ } </verb></tscreen> <SECT>Changing Baud Rates With Linux, the baud rate can be changed using a technique similar to add/delete RTS/CTS. <tscreen><verb> 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 */ } </verb></tscreen> POSIX adds another method. They define: <tscreen><verb> speed_t cfgetispeed( const struct termios *termios_p ); speed_t cfgetospeed( const struct termios *termios_p ); </verb></tscreen> library calls to extract the current input or output speed from the struct termios pointed to with <it>*termio_p</it>. This is a variable defined in the calling process. In practice, the data contained in this termios, should be obtained by the <it>tcgetattr()</it> call or an <it>ioctl()</it> call using the TCGETS command. The companion library calls are: <tscreen><verb> int cfsetispeed( struct termios *termios_p, speed_t speed ); int cfsetospeed( struct termios *termios_p, speed_t speed ); </verb></tscreen> which are used to change the value of the baud rate in the locally defined <it>*termios_p</it>. Following either of these calls, either a call to <it>tcsetattr()</it> or <it>ioctl()</it> with one of TCSETS, TCSETSW or TCSETSF as the command to transmit the change to the kernel. The <it>cf*</it> commands are preferred for portability. Some weird Unices use a considerably different format of <it>termios</it>. 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 /usr/include/asm/termbits.h. <SECT>Additional Control Calls <SECT1>Sending a "break". <tscreen><verb> int ioctl( fd, TCSBRK, int arg ); int tcsendbreak( fd, int arg );</verb></tscreen> Send a <bf>break</bf>: Here the action differs between the conventional <it>ioctl()</it> call and the POSIX call. For the conventional call, an <it>arg</it> of '0' sets the break control line of the UART for 0.25 seconds. For the POSIX command, the break line is set for <it>arg</it> times 0.1 seconds. <SECT1>Hardware flow control. <tscreen><verb> int ioctl( fd, TCXONC, int action ); int tcflow( fd, int action ); </verb></tscreen> The <it>action</it> flags are: <itemize> <item> TCOOFF 0 suspend output <item> TCOON 1 restart output <item> TCIOFF 2 transmit STOP character to suspend input <item> TCION 3 transmit START character to restart input </itemize> <SECT1>Flushing I/O buffers. <tscreen><verb> int ioctl( fd, TCFLSH, queue_selector ); int tcflush( fd, queue_selector ); </verb></tscreen> The <it>queue_selector</it> flags are: <itemize> <item> TCIFLUSH 0 flush any data not yet read from the input buffer <item> TCOFLUSH 1 flush any data written to the output buffer but not yet transmitted <item> TCIOFLUSH 2 flush both buffers </itemize> <SECT>Modem control The hardware modem control lines can be monitored or modified by the <it>ioctl(2)</it> system call. A set of comparable <it>tc*</it> calls apparently do not exist. The form of this call is:<newline> <tscreen><verb> int ioctl( fd, COMMAND, (int *)flags ); </verb></tscreen> The COMMANDS and their action are: <itemize> <item> TIOCMBIS turn on control lines depending upon which bits are set in <it>flags</it>. <item> TIOCMBIC turn off control lines depending upon which bits are unset in <it>flags</it>. <item> TIOCMGET the appropriate bits are set in <it>flags</it> according to the current status <item> TIOCMSET the state of the UART is changed according to which bits are set/unset in 'flags' </itemize> The bit pattern of <it>flags</it> refer to the following control lines: <itemize> <item> TIOCM_LE Line enable <item> TIOCM_DTR Data Terminal Ready <item> TIOCM_RTS Request to send <item> TIOCM_ST Secondary transmit <item> TIOCM_SR Secondary receive <item> TIOCM_CTS Clear to send <item> TIOCM_CAR Carrier detect <item> TIOCM_RNG Ring <item> TIOCM_DSR Data set ready </itemize> 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 <it>ioctl()</it> 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 kernel. See the lines.c file in my "serial_suite" for an example of how these can be used see <URL URL="ftp://scicom.alphacd.com/pub/linux/serial_suite"> <SECT>Process Groups <SECT1>Sessions <SECT1>Process Groups Any newly created process inherits the Process Group of its creator. The Process Group leader has the same PID as PGID. <SECT1>Controlling Terminal There are a series of <it>ioctl(2)</it> and <it>tc*(2)</it> calls which can be used to monitor or to change the process group to which the device is attached. <SECT2>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 set. <tscreen><verb> int ioctl( fd, TIOCGPGRP, (pid_t *)pid ); int tcgetpgrp( fd, (pid_t *)pid ); </verb></tscreen> <SECT2>Set the foreground process group id of a terminal. The <it>fd</it> must be the controlling terminal and be associated with the session of the calling process. <tscreen><verb> int ioctl( fd, TIOCSPGRP, (pid_t *)pid ); int tcsetpgrp( fd, (pid_t *)pid ); </verb></tscreen> <SECT2>Get process group id. <tscreen><verb> int ioctl( fd, TIOCGPGRP, &(pid_t)pid ); int tcgetpgrp( fd, &(pid_t)pid ); </verb></tscreen> <SECT>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 process. Check my "libdevlocks-x.x.tgz" at <URL URL="ftp://scicom.alphacdc.com/pub/linux"> for an example of how these lock files should be utilized. <SECT>Additional Information Check out my "serial_suite.tgz" for more information about programming the serial ports at <URL URL="mailto:vern@zebra.alphacdc.com">. There some examples and some blurbs about setting up modems and comments about some general considerations. <SECT>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 <URL URL="mailto:vern@zebra.alphacdc.com"> via email. Please include the version number of the Serial-Programming-HOWTO when writing. </ARTICLE>