1open(n) Tcl Built-In Commands open(n)
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6
8 open - Open a file-based or command pipeline channel
9
11 open fileName
12 open fileName access
13 open fileName access permissions
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15
18 This command opens a file, serial port, or command pipeline and returns
19 a channel identifier that may be used in future invocations of commands
20 like read, puts, and close. If the first character of fileName is not
21 | then the command opens a file: fileName gives the name of the file to
22 open, and it must conform to the conventions described in the filename
23 manual entry.
24 The access argument, if present, indicates the way in which the file
26 (or command pipeline) is to be accessed. In the first form access may
27 have any of the following values:
28 r Open the file for reading only; the file must already
30 exist. This is the default value if access is not speci‐
31 fied.
32 r+ Open the file for both reading and writing; the file
34 must already exist.
35 w Open the file for writing only. Truncate it if it
37 exists. If it doesn't exist, create a new file.
38 w+ Open the file for reading and writing. Truncate it if
40 it exists. If it doesn't exist, create a new file.
41 a Open the file for writing only. If the file doesn't
43 exist, create a new empty file. Set the file pointer to
44 the end of the file prior to each write.
45 a+ Open the file for reading and writing. If the file
47 doesn't exist, create a new empty file. Set the initial
48 access position to the end of the file.
49 In the second form, access consists of a list of any of the following
51 flags, all of which have the standard POSIX meanings. One of the flags
52 must be either RDONLY, WRONLY or RDWR.
53 RDONLY Open the file for reading only.
55 WRONLY Open the file for writing only.
57 RDWR Open the file for both reading and writing.
59 APPEND Set the file pointer to the end of the file prior to
61 each write.
62 CREAT Create the file if it doesn't already exist (without
64 this flag it is an error for the file not to exist).
65 EXCL If CREAT is also specified, an error is returned if the
67 file already exists.
68 NOCTTY If the file is a terminal device, this flag prevents the
70 file from becoming the controlling terminal of the
71 process.
72 NONBLOCK Prevents the process from blocking while opening the
74 file, and possibly in subsequent I/O operations. The
75 exact behavior of this flag is system- and device-depen‐
76 dent; its use is discouraged (it is better to use the
77 fconfigure command to put a file in nonblocking mode).
78 For details refer to your system documentation on the
79 open system call's O_NONBLOCK flag.
80 TRUNC If the file exists it is truncated to zero length.
82 If a new file is created as part of opening it, permissions (an inte‐
84 ger) is used to set the permissions for the new file in conjunction
85 with the process's file mode creation mask. Permissions defaults to
86 0666.
87 Note that if you are going to be reading or writing binary data from
89 the channel created by this command, you should use the fconfigure com‐
90 mand to change the -translation option of the channel to binary before
91 transferring any binary data. This is in contrast to the ``b'' charac‐
92 ter passed as part of the equivalent of the access parameter to some
93 versions of the C library fopen() function.
94
97 If the first character of fileName is ``|'' then the remaining charac‐
98 ters of fileName are treated as a list of arguments that describe a
99 command pipeline to invoke, in the same style as the arguments for
100 exec. In this case, the channel identifier returned by open may be
101 used to write to the command's input pipe or read from its output pipe,
102 depending on the value of access. If write-only access is used (e.g.
103 access is w), then standard output for the pipeline is directed to the
104 current standard output unless overridden by the command. If read-only
105 access is used (e.g. access is r), standard input for the pipeline is
106 taken from the current standard input unless overridden by the command.
107 The id of the spawned process is accessible through the pid command,
108 using the channel id returned by open as argument.
109 If the command (or one of the commands) executed in the command pipe‐
111 line returns an error (according to the definition in exec), a Tcl
112 error is generated when close is called on the channel unless the pipe‐
113 line is in non-blocking mode then no exit status is returned (a silent
114 close with -blocking 0).
115 It is often useful to use the fileevent command with pipelines so other
117 processing may happen at the same time as running the command in the
118 background. │
119
121 If fileName refers to a serial port, then the specified serial port is │
122 opened and initialized in a platform-dependent manner. Acceptable val‐ │
123 ues for the fileName to use to open a serial port are described in the │
124 PORTABILITY ISSUES section. │
125 The fconfigure command can be used to query and set additional configu‐ │
127 ration options specific to serial ports (where supported): │
128 -mode baud,parity,data,stop │
130 This option is a set of 4 comma-separated values: the baud rate, │
131 parity, number of data bits, and number of stop bits for this │
132 serial port. The baud rate is a simple integer that specifies │
133 the connection speed. Parity is one of the following letters: │
134 n, o, e, m, s; respectively signifying the parity options of │
135 ``none'', ``odd'', ``even'', ``mark'', or ``space''. Data is │
136 the number of data bits and should be an integer from 5 to 8, │
137 while stop is the number of stop bits and should be the integer │
138 1 or 2. │
139 -handshake type │
141 (Windows and Unix). This option is used to setup automatic hand‐ │
142 shake control. Note that not all handshake types maybe supported │
143 by your operating system. The type parameter is case-indepen‐ │
144 dent. │
145 If type is none then any handshake is switched off. rtscts │
147 activates hardware handshake. Hardware handshake signals are │
148 described below. For software handshake xonxoff the handshake │
149 characters can be redefined with -xchar. An additional hardware │
150 handshake dtrdsr is available only under Windows. There is no │
151 default handshake configuration, the initial value depends on │
152 your operating system settings. The -handshake option cannot be │
153 queried. │
154 -queue │
156 (Windows and Unix). The -queue option can only be queried. It │
157 returns a list of two integers representing the current number │
158 of bytes in the input and output queue respectively. │
159 -timeout msec │
161 (Windows and Unix). This option is used to set the timeout for │
162 blocking read operations. It specifies the maximum interval │
163 between the reception of two bytes in milliseconds. For Unix │
164 systems the granularity is 100 milliseconds. The -timeout │
165 option does not affect write operations or nonblocking reads. │
166 This option cannot be queried. │
167 -ttycontrol {signal boolean signal boolean ...} │
169 (Windows and Unix). This option is used to setup the handshake │
170 output lines (see below) permanently or to send a BREAK over the │
171 serial line. The signal names are case-independent. {RTS 1 DTR │
172 0} sets the RTS output to high and the DTR output to low. The │
173 BREAK condition (see below) is enabled and disabled with {BREAK │
174 1} and {BREAK 0} respectively. It's not a good idea to change │
175 the RTS (or DTR) signal with active hardware handshake rtscts │
176 (or dtrdsr). The result is unpredictable. The -ttycontrol │
177 option cannot be queried. │
178 -ttystatus │
180 (Windows and Unix). The -ttystatus option can only be queried. │
181 It returns the current modem status and handshake input signals │
182 (see below). The result is a list of signal,value pairs with a │
183 fixed order, e.g. {CTS 1 DSR 0 RING 1 DCD 0}. The signal names │
184 are returned upper case. │
185 -xchar {xonChar xoffChar} │
187 (Windows and Unix). This option is used to query or change the │
188 software handshake characters. Normally the operating system │
189 default should be DC1 (0x11) and DC3 (0x13) representing the │
190 ASCII standard XON and XOFF characters. │
191 -pollinterval msec │
193 (Windows only). This option is used to set the maximum time │
194 between polling for fileevents. This affects the time interval │
195 between checking for events throughout the Tcl interpreter (the │
196 smallest value always wins). Use this option only if you want │
197 to poll the serial port more or less often than 10 msec (the │
198 default). │
199 -sysbuffer inSize │
201 -sysbuffer {inSize outSize} │
203 (Windows only). This option is used to change the size of Win‐ │
204 dows system buffers for a serial channel. Especially at higher │
205 communication rates the default input buffer size of 4096 bytes │
206 can overrun for latent systems. The first form specifies the │
207 input buffer size, in the second form both input and output buf‐ │
208 fers are defined. │
209 -lasterror │
211 (Windows only). This option is query only. In case of a serial │
212 communication error, read or puts returns a general Tcl file I/O │
213 error. fconfigure -lasterror can be called to get a list of │
214 error details. See below for an explanation of the various │
215 error codes. │
216
219 RS-232 is the most commonly used standard electrical interface for │
220 serial communications. A negative voltage (-3V..-12V) define a mark │
221 (on=1) bit and a positive voltage (+3..+12V) define a space (off=0) bit │
222 (RS-232C). The following signals are specified for incoming and outgo‐ │
223 ing data, status lines and handshaking. Here we are using the terms │
224 workstation for your computer and modem for the external device, │
225 because some signal names (DCD, RI) come from modems. Of course your │
226 external device may use these signal lines for other purposes. │
227 TXD(output) │
230 Transmitted Data: Outgoing serial data. │
231 RXD(input) │
233 Received Data:Incoming serial data. │
234 RTS(output) │
236 Request To Send: This hardware handshake line informs the modem │
237 that your workstation is ready to receive data. Your workstation │
238 may automatically reset this signal to indicate that the input │
239 buffer is full. │
240 CTS(input) │
242 Clear To Send: The complement to RTS. Indicates that the modem │
243 is ready to receive data. │
244 DTR(output) │
246 Data Terminal Ready: This signal tells the modem that the work‐ │
247 station is ready to establish a link. DTR is often enabled auto‐ │
248 matically whenever a serial port is opened. │
249 DSR(input) │
251 Data Set Ready: The complement to DTR. Tells the workstation │
252 that the modem is ready to establish a link. │
253 DCD(input) │
255 Data Carrier Detect: This line becomes active when a modem │
256 detects a "Carrier" signal. │
257 RI(input) │
259 Ring Indicator: Goes active when the modem detects an incoming │
260 call. │
261 BREAK │
263 A BREAK condition is not a hardware signal line, but a logical │
264 zero on the TXD or RXD lines for a long period of time, usually │
265 250 to 500 milliseconds. Normally a receive or transmit data │
266 signal stays at the mark (on=1) voltage until the next character │
267 is transferred. A BREAK is sometimes used to reset the communi‐ │
268 cations line or change the operating mode of communications │
269 hardware. │
270
273 A lot of different errors may occur during serial read operations or │
274 during event polling in background. The external device may have been │
275 switched off, the data lines may be noisy, system buffers may overrun │
276 or your mode settings may be wrong. That's why a reliable software │
277 should always catch serial read operations. In cases of an error Tcl │
278 returns a general file I/O error. Then fconfigure -lasterror may help │
279 to locate the problem. The following error codes may be returned. │
280 RXOVER │
283 Windows input buffer overrun. The data comes faster than your │
284 scripts reads it or your system is overloaded. Use fconfigure │
285 -sysbuffer to avoid a temporary bottleneck and/or make your │
286 script faster. │
287 TXFULL │
289 Windows output buffer overrun. Complement to RXOVER. This │
290 error should practically not happen, because Tcl cares about │
291 the output buffer status. │
292 OVERRUN │
294 UART buffer overrun (hardware) with data lost. The data │
295 comes faster than the system driver receives it. Check your │
296 advanced serial port settings to enable the FIFO (16550) buf‐ │
297 fer and/or setup a lower(1) interrupt threshold value. │
298 RXPARITY │
300 A parity error has been detected by your UART. Wrong parity │
301 settings with fconfigure -mode or a noisy data line (RXD) may │
302 cause this error. │
303 FRAME │
305 A stop-bit error has been detected by your UART. Wrong mode │
306 settings with fconfigure -mode or a noisy data line (RXD) may │
307 cause this error. │
308 BREAK │
310 A BREAK condition has been detected by your UART (see above).
311
314 Windows (all versions)
315 Valid values for fileName to open a serial port are of the form
316 comX:, where X is a number, generally from 1 to 4. This nota‐
317 tion only works for serial ports from 1 to 9, if the system hap‐
318 pens to have more than four. An attempt to open a serial port
319 that does not exist or has a number greater than 9 will fail.
320 An alternate form of opening serial ports is to use the filename
321 \\.\comX, where X is any number that corresponds to a serial
322 port; please note that this method is considerably slower on
323 Windows 95 and Windows 98.
324 Windows NT
326 When running Tcl interactively, there may be some strange inter‐
327 actions between the real console, if one is present, and a com‐
328 mand pipeline that uses standard input or output. If a command
329 pipeline is opened for reading, some of the lines entered at the
330 console will be sent to the command pipeline and some will be
331 sent to the Tcl evaluator. If a command pipeline is opened for
332 writing, keystrokes entered into the console are not visible
333 until the pipe is closed. This behavior occurs whether the com‐
334 mand pipeline is executing 16-bit or 32-bit applications. These
335 problems only occur because both Tcl and the child application
336 are competing for the console at the same time. If the command
337 pipeline is started from a script, so that Tcl is not accessing
338 the console, or if the command pipeline does not use standard
339 input or output, but is redirected from or to a file, then the
340 above problems do not occur.
341 Windows 95
343 A command pipeline that executes a 16-bit DOS application cannot
344 be opened for both reading and writing, since 16-bit DOS appli‐
345 cations that receive standard input from a pipe and send stan‐
346 dard output to a pipe run synchronously. Command pipelines that
347 do not execute 16-bit DOS applications run asynchronously and
348 can be opened for both reading and writing.
349 When running Tcl interactively, there may be some strange inter‐
351 actions between the real console, if one is present, and a com‐
352 mand pipeline that uses standard input or output. If a command
353 pipeline is opened for reading from a 32-bit application, some
354 of the keystrokes entered at the console will be sent to the
355 command pipeline and some will be sent to the Tcl evaluator. If
356 a command pipeline is opened for writing to a 32-bit applica‐
357 tion, no output is visible on the console until the pipe is
358 closed. These problems only occur because both Tcl and the
359 child application are competing for the console at the same
360 time. If the command pipeline is started from a script, so that
361 Tcl is not accessing the console, or if the command pipeline
362 does not use standard input or output, but is redirected from or
363 to a file, then the above problems do not occur.
364 Whether or not Tcl is running interactively, if a command pipe‐
366 line is opened for reading from a 16-bit DOS application, the
367 call to open will not return until end-of-file has been received
368 from the command pipeline's standard output. If a command pipe‐
369 line is opened for writing to a 16-bit DOS application, no data
370 will be sent to the command pipeline's standard output until the
371 pipe is actually closed. This problem occurs because 16-bit DOS
372 applications are run synchronously, as described above.
373 Macintosh
375 Opening a serial port is not currently implemented under Macin‐
376 tosh.
377 Opening a command pipeline is not supported under Macintosh,
379 since applications do not support the concept of standard input
380 or output.
381 Unix
383 Valid values for fileName to open a serial port are generally of
384 the form /dev/ttyX, where X is a or b, but the name of any
385 pseudo-file that maps to a serial port may be used. Advanced │
386 configuration options are only supported for serial ports when │
387 Tcl is built to use the POSIX serial interface.
388 When running Tcl interactively, there may be some strange inter‐
390 actions between the console, if one is present, and a command
391 pipeline that uses standard input. If a command pipeline is
392 opened for reading, some of the lines entered at the console
393 will be sent to the command pipeline and some will be sent to
394 the Tcl evaluator. This problem only occurs because both Tcl
395 and the child application are competing for the console at the
396 same time. If the command pipeline is started from a script, so
397 that Tcl is not accessing the console, or if the command pipe‐
398 line does not use standard input, but is redirected from a file,
399 then the above problem does not occur.
400 See the PORTABILITY ISSUES section of the exec command for additional
402 information not specific to command pipelines about executing applica‐
403 tions on the various platforms
404
406 Open a command pipeline and catch any errors:
407 set fl [open "| ls this_file_does_not_exist"]
408 set data [read $fl]
409 if {[catch {close $fl} err]} {
410 puts "ls command failed: $err"
411 }
412
415 file(n), close(n), filename(n), fconfigure(n), gets(n), read(n),
416 puts(n), exec(n), pid(n), fopen(3)
417
420 access mode, append, create, file, non-blocking, open, permissions,
421 pipeline, process, serial
422Tcl 8.3 open(n)