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1Downloaded from http://www.lafn.org/~dave/linux/Serial-Programming-HOWTO.txt
2Seems to be somewhat old, but contains useful bits for getty.c hacking
3============================================================================
4
5 The Linux Serial Programming HOWTO, Part 1 of 2
6 By Vernon C. Hoxie
7 v2.0 10 September 1999
8
9 This document describes how to program communications with devices
10 over a serial port on a Linux box.
11 ______________________________________________________________________
12
13 Table of Contents
14
15 1. Copyright
16
17 2. Introduction
18
19 3. Opening
20
21 4. Commands
22
23 5. Changing Baud Rates
24
25 6. Additional Control Calls
26
27 6.1 Sending a "break".
28 6.2 Hardware flow control.
29 6.3 Flushing I/O buffers.
30
31 7. Modem control
32
33 8. Process Groups
34
35 8.1 Sessions
36 8.2 Process Groups
37 8.3 Controlling Terminal
38 8.3.1 Get the foreground group process id.
39 8.3.2 Set the foreground process group id of a terminal.
40 8.3.3 Get process group id.
41
42 9. Lockfiles
43
44 10. Additional Information
45
46 11. Feedback
47
48 ______________________________________________________________________
49
50 1. Copyright
51
52 The Linux Serial-Programming-HOWTO is copyright (C) 1997 by Vernon
53 Hoxie. Linux HOWTO documents may be reproduced and distributed in
54 whole or in part, in any medium physical or electronic, as long as
55 this copyright notice is retained on all copies. Commercial
56 redistribution is allowed and encouraged; however, the author would
57 like to be notified of any such distributions.
58
59 All translations, derivative works, or aggregate works incorporating
60 this Linux HOWTO document must be covered under this copyright notice.
61 That is, you may not produce a derivative work from this HOWTO and
62 impose additional restrictions on its distribution.
63
64 This version is a complete rewrite of the previous Serial-Programming-
65 HOWTO by Peter H. Baumann, <mailto:Peter.Baumann@dlr.de>
66
67 2. Introduction
68
69 This HOWTO will attempt to give hints about how to write a program
70 which needs to access a serial port. Its principal focus will be on
71 the Linux implementation and what the meaning of the various library
72 functions available.
73
74 Someone asked about which of several sequences of operations was
75 right. There is no absolute right way to accomplish an outcome. The
76 options available are too numerous. If your sequences produces the
77 desired results, then that is the right way for you. Another
78 programmer may select another set of options and get the same results.
79 His method is right for him.
80
81 Neither of these methods may operate properly with some other
82 implementation of UNIX. It is strange that many of the concepts which
83 were implemented in the SYSV version have been dumped. Because UNIX
84 was developed by AT&T and much code has been generated on those
85 concepts, the AT&T version should be the standard to which others
86 should emulate.
87
88 Now the standard is POSIX.
89
90 It was once stated that the popularity of UNIX and C was that they
91 were created by programmers for programmers. Not by scholars who
92 insist on purity of style in deference to results and simplicity of
93 use. Not by committees with people who have diverse personal or
94 proprietary agenda. Now ANSI and POSIX have strayed from those
95 original clear and simply concepts.
96
97 3. Opening
98
99 The various serial devices are opened just as any other file.
100 Although, the fopen(3) command may be used, the plain open(2) is
101 preferred. This call returns the file descriptor which is required
102 for the various commands that configure the interface.
103
104 Open(2) has the format:
105
106 #include <fcntl.h>
107 int open(char *path, int flags, [int mode]);
108
109 In addition to the obvious O_RDWR, O_WRONLY and O_RDONLY, two
110 additional flags are available. These are O_NONBLOCK and O_NOCTTY.
111 Other flags listed in the open(2) manual page are not applicable to
112 serial devices.
113
114 Normally, a serial device opens in "blocking" mode. This means that
115 the open() will not return until the Carrier Detect line from the port
116 is active, e.g. modem, is active. When opened with the O_NONBLOCK
117 flag set, the open() will return immediately regardless of the status
118 of the DCD line. The "blocking" mode also affects the read() call.
119
120 The fcntl(2) command can be used to change the O_NONBLOCK flag anytime
121 after the device has been opened.
122
123 The device driver and the data passing through it are controlled
124 according to settings in the struct termios. This structure is
125 defined in "/usr/include/termios.h". In the Linux tree, further
126 reference is made to "/usr/include/asm/termbits.h".
127 In blocking mode, a read(2) will block until data is available or a
128 signal is received. It is still subject to state of the ICANON flag.
129
130 When the termios.c_lflag ICANON bit is set, input data is collected
131 into strings until a NL, EOF or EOL character is received. You can
132 define these in the termios.c_cc[] array. Also, ERASE and KILL
133 characters will operate on the incoming data before it is delivered to
134 the user.
135
136 In non-canonical mode, incoming data is quanitified by use of the
137 c_cc[VMIN and c_cc[VTIME] values in termios.c_cc[].
138
139 Some programmers use the select() call to detect the completion of a
140 read(). This is not the best way of checking for incoming data.
141 Select() is part of the SOCKETS scheme and too complex for most
142 applications.
143
144 A full explanation of the fields of the termios structure is contained
145 in termios(7) of the Users Manual. A version is included in Part 2 of
146 this HOWTO document.
147
148 4. Commands
149
150 Changes to the struct termios are made by retrieving the current
151 settings, making the desired changes and transmitting the modified
152 structure back to the kernel.
153
154 The historic means of communicating with the kernel was by use of the
155 ioctl(fd, COMMAND, arg) system call. Then the purists in the
156 computer industry decided that this was not genetically consistent.
157 Their argument was that the argument changed its stripes. Sometimes
158 it was an int, sometimes it was a pointer to int and other times it
159 was a pointer to struct termios. Then there were those times it was
160 empty or NULL. These variations are dependent upon the COMMAND.
161
162 As a alternative, the tc* series of functions were concocted.
163
164 These are:
165
166 int tcgetattr(int filedes, struct termios *termios_p);
167 int tcsetattr(int filedes, int optional_actions,
168 const struct termios *termios_p);
169
170 instead of:
171
172 int ioctl(int filedes, int command,
173 struct termios *termios_p);
174
175 where command is TCGETS or one of TCSETS, TCSETSW or TCSETSF.
176
177 The TCSETS command is comparable to the TCSANOW optional_action for
178 the tc* version. These direct the kernel to adopt the changes
179 immediately. Other pairs are:
180
181 command optional_action Meaning
182 TCSETSW TCSADRAIN Change after all output has drained.
183 TCSETSF TCSAFLUSH Change after all output has drained
184 then discard any input characters
185 not read.
186
187 Since the return code from either the ioctl(2) or the tcsetattr(2)
188 commands only indicate that the command was processed by the kernel.
189 These do not indicate whether or not the changes were actually
190 accomplished. Either of these commands should be followed by a call
191 to:
192
193 ioctl(fd, TCGETS, &new_termios);
194
195 or:
196
197 tcgetattr(fd, &new_termios);
198
199 A user function which makes changes to the termios structure should
200 define two struct termios variables. One of these variables should
201 contain the desired configuration. The other should contain a copy of
202 the kernels version. Then after the desired configuration has been
203 sent to the kernel, another call should be made to retrieve the
204 kernels version. Then the two compared.
205
206 Here is an example of how to add RTS/CTS flow control:
207
208 struct termios my_termios;
209 struct termios new_termios;
210
211 tcgetattr(fd, &my_termios);
212 my_termios.c_flag |= CRTSCTS;
213 tcsetattr(fd, TCSANOW, &my_termios);
214 tcgetattr(fd, &new_termios);
215 if (memcmp(my_termios, new_termios,
216 sizeof(my_termios)) != 0) {
217 /* do some error handling */
218 }
219
220 5. Changing Baud Rates
221
222 With Linux, the baud rate can be changed using a technique similar to
223 add/delete RTS/CTS.
224
225 struct termios my_termios;
226 struct termios new_termios;
227
228 tcgetattr(fd, &my_termios);
229 my_termios.c_flag &= ~CBAUD;
230 my_termios.c_flag |= B19200;
231 tcsetattr(fd, TCSANOW, &my_termios);
232 tcgetattr(fd, &new_termios);
233 if (memcmp(my_termios, new_termios,
234 sizeof(my_termios)) != 0) {
235 /* do some error handling */
236 }
237
238 POSIX adds another method. They define:
239
240 speed_t cfgetispeed(const struct termios *termios_p);
241 speed_t cfgetospeed(const struct termios *termios_p);
242
243 library calls to extract the current input or output speed from the
244 struct termios pointed to with *termio_p. This is a variable defined
245 in the calling process. In practice, the data contained in this
246 termios, should be obtained by the tcgetattr() call or an ioctl() call
247 using the TCGETS command.
248
249 The companion library calls are:
250
251 int cfsetispeed(struct termios *termios_p, speed_t speed);
252 int cfsetospeed(struct termios *termios_p, speed_t speed);
253
254 which are used to change the value of the baud rate in the locally
255 defined *termios_p. Following either of these calls, either a call to
256 tcsetattr() or ioctl() with one of TCSETS, TCSETSW or TCSETSF as the
257 command to transmit the change to the kernel.
258
259 The cf* commands are preferred for portability. Some weird Unices use
260 a considerably different format of termios.
261
262 Most implementations of Linux use only the input speed for both input
263 and output. These functions are defined in the application program by
264 reference to <termios.h>. In reality, they are in
265 /usr/include/asm/termbits.h.
266
267 6. Additional Control Calls
268
269 6.1. Sending a "break".
270
271 int ioctl(fd, TCSBRK, int arg);
272 int tcsendbreak(fd, int arg);
273
274 Send a break: Here the action differs between the conventional
275 ioctl() call and the POSIX call. For the conventional call, an arg of
276 '0' sets the break control line of the UART for 0.25 seconds. For the
277 POSIX command, the break line is set for arg times 0.1 seconds.
278
279 6.2. Hardware flow control.
280
281 int ioctl(fd, TCXONC, int action);
282 int tcflow(fd, int action);
283
284 The action flags are:
285
286 o TCOOFF 0 suspend output
287
288 o TCOON 1 restart output
289
290 o TCIOFF 2 transmit STOP character to suspend input
291
292 o TCION 3 transmit START character to restart input
293
294 6.3. Flushing I/O buffers.
295
296 int ioctl(fd, TCFLSH, queue_selector);
297 int tcflush(fd, queue_selector);
298
299 The queue_selector flags are:
300
301 o TCIFLUSH 0 flush any data not yet read from the input buffer
302
303 o TCOFLUSH 1 flush any data written to the output buffer but not
304 yet transmitted
305
306 o TCIOFLUSH 2 flush both buffers
307
308 7. Modem control
309
310 The hardware modem control lines can be monitored or modified by the
311 ioctl(2) system call. A set of comparable tc* calls apparently do not
312 exist. The form of this call is:
313
314 int ioctl(fd, COMMAND, (int *)flags);
315
316 The COMMANDS and their action are:
317
318 o TIOCMBIS turn on control lines depending upon which bits are set
319 in flags.
320
321 o TIOCMBIC turn off control lines depending upon which bits are
322 unset in flags.
323 o TIOCMGET the appropriate bits are set in flags according to the
324 current status
325
326 o TIOCMSET the state of the UART is changed according to which bits
327 are set/unset in 'flags'
328
329 The bit pattern of flags refer to the following control lines:
330
331 o TIOCM_LE Line enable
332
333 o TIOCM_DTR Data Terminal Ready
334
335 o TIOCM_RTS Request to send
336
337 o TIOCM_ST Secondary transmit
338
339 o TIOCM_SR Secondary receive
340
341 o TIOCM_CTS Clear to send
342
343 o TIOCM_CAR Carrier detect
344
345 o TIOCM_RNG Ring
346
347 o TIOCM_DSR Data set ready
348
349 It should be noted that some of these bits are controlled by the modem
350 and the UART cannot change them but their status can be sensed by
351 TIOCMGET. Also, most Personal Computers do not provide hardware for
352 secondary transmit and receive.
353
354 There are also a pair of ioctl() to monitor these lines. They are
355 undocumented as far as I have learned. The commands are TIOCMIWAIT
356 and TCIOGICOUNT. They also differ between versions of the Linux
357 kernel.
358
359 See the lines.c file in my "serial_suite" for an example of how these
360 can be used see <ftp://scicom.alphacd.com/pub/linux/serial_suite>
361
362 8. Process Groups
363
364 8.1. Sessions
365
366 8.2. Process Groups
367
368 Any newly created process inherits the Process Group of its creator.
369 The Process Group leader has the same PID as PGID.
370
371 8.3. Controlling Terminal
372
373 There are a series of ioctl(2) and tc*(2) calls which can be used to
374 monitor or to change the process group to which the device is
375 attached.
376
377 8.3.1. Get the foreground group process id.
378
379 If there is no foreground group, a number not representing an existing
380 process group is returned. On error, a -1 is returned and errno is
381 set.
382
383 int ioctl(fd, TIOCGPGRP, (pid_t *)pid);
384 int tcgetpgrp(fd, (pid_t *)pid);
385
386 8.3.2. Set the foreground process group id of a terminal.
387
388 The fd must be the controlling terminal and be associated with the
389 session of the calling process.
390
391 int ioctl(fd, TIOCSPGRP, (pid_t *)pid);
392 int tcsetpgrp(fd, (pid_t *)pid);
393
394 8.3.3. Get process group id.
395
396 int ioctl(fd, TIOCGPGRP, &(pid_t)pid);
397 int tcgetpgrp(fd, &(pid_t)pid);
398
399 9. Lockfiles
400
401 Any process which accesses a serial device should first check for the
402 existence of lock file for the desired device. If such a lock lock
403 file exists, this means that the device may be in use by another
404 process.
405
406 Check my "libdevlocks-x.x.tgz" at
407 <ftp://scicom.alphacdc.com/pub/linux> for an example of how these lock
408 files should be utilized.
409
410 10. Additional Information
411
412 Check out my "serial_suite.tgz" for more information about programming
413 the serial ports at <mailto:vern@zebra.alphacdc.com>. There some
414 examples and some blurbs about setting up modems and comments about
415 some general considerations.
416
417 11. Feedback
418
419 Please send me any corrections, questions, comments, suggestions, or
420 additional material. I would like to improve this HOWTO! Tell me
421 exactly what you don't understand, or what could be clearer. You can
422 reach me at <mailto:vern@zebra.alphacdc.com> via email. Please
423 include the version number of the Serial-Programming-HOWTO when
424 writing.
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