Tcl_OpenFileChannel(3) Tcl Library Procedures Tcl_OpenFileChannel(3) _________________________________________________________________________________________________ NAME Tcl_OpenFileChannel, Tcl_OpenCommandChannel, Tcl_MakeFileChannel, Tcl_GetChannel, Tcl_GetChannelNames, Tcl_GetChannelNamesEx, Tcl_RegisterChannel, Tcl_UnregisterChannel, Tcl_DetachChannel, Tcl_IsStandardChannel, Tcl_Close, Tcl_ReadChars, Tcl_Read, Tcl_GetsObj, Tcl_Gets, Tcl_WriteObj, Tcl_WriteChars, Tcl_Write, Tcl_Flush, Tcl_Seek, Tcl_Tell, Tcl_GetChannelOption, Tcl_SetChannelOption, Tcl_Eof, Tcl_InputBlocked, Tcl_InputBuffered, Tcl_OutputBuffered, Tcl_Ungets, Tcl_ReadRaw, Tcl_WriteRaw - buffered I/O facilities using channels SYNOPSIS #include <tcl.h> Tcl_Channel Tcl_OpenFileChannel(interp, fileName, mode, permissions) Tcl_Channel Tcl_OpenCommandChannel(interp, argc, argv, flags) Tcl_Channel Tcl_MakeFileChannel(handle, readOrWrite) Tcl_Channel Tcl_GetChannel(interp, channelName, modePtr) int | Tcl_GetChannelNames(interp) | int | Tcl_GetChannelNamesEx(interp, pattern) | void Tcl_RegisterChannel(interp, channel) int Tcl_UnregisterChannel(interp, channel) int Tcl_DetachChannel(interp, channel) int Tcl_IsStandardChannel(channel) int Tcl_Close(interp, channel) int | Tcl_ReadChars(channel, readObjPtr, charsToRead, appendFlag) | int | Tcl_Read(channel, readBuf, bytesToRead) | int | Tcl_GetsObj(channel, lineObjPtr) | int | Tcl_Gets(channel, lineRead) | int | Tcl_Ungets(channel, input, inputLen, addAtEnd) | int | Tcl_WriteObj(channel, writeObjPtr) | int | Tcl_WriteChars(channel, charBuf, bytesToWrite) | int | Tcl_Write(channel, byteBuf, bytesToWrite) | int | Tcl_ReadRaw(channel, readBuf, bytesToRead) | int | Tcl_WriteRaw(channel, byteBuf, bytesToWrite) | int Tcl_Eof(channel) int Tcl_Flush(channel) int Tcl_InputBlocked(channel) int Tcl_InputBuffered(channel) int | Tcl_OutputBuffered(channel) | int Tcl_Seek(channel, offset, seekMode) int Tcl_Tell(channel) int Tcl_GetChannelOption(interp, channel, optionName, optionValue) int Tcl_SetChannelOption(interp, channel, optionName, newValue) ARGUMENTS Tcl_Interp *interp (in) Used for error reporting and to look up a channel registered in it. CONST char *fileName (in) The name of a local or network file. CONST char *mode (in) Specifies how the file is to be accessed. May have any of the values allowed for the mode argument to the Tcl open command. int permissions (in) POSIX-style permission flags such as 0644. If a new file is created, these permissions will be set on the created file. int argc (in) The number of elements in argv. CONST char **argv (in) Arguments for constructing a command pipe- line. These values have the same meaning as the non-switch arguments to the Tcl exec command. int flags (in) Specifies the disposition of the stdio han- dles in pipeline: OR-ed combination of TCL_STDIN, TCL_STDOUT, TCL_STDERR, and TCL_ENFORCE_MODE. If TCL_STDIN is set, stdin for the first child in the pipe is the pipe channel, otherwise it is the same as the standard input of the invoking process; likewise for TCL_STDOUT and TCL_STDERR. If TCL_ENFORCE_MODE is not set, then the pipe can redirect stdio handles to override the stdio handles for which TCL_STDIN, TCL_STDOUT and TCL_STDERR have been set. If it is set, then such redirec- tions cause an error. ClientData handle (in) Operating system specific handle for I/O to a file. For Unix this is a file descriptor, for Windows it is a HANDLE. int readOrWrite (in) OR-ed combination of TCL_READABLE and TCL_WRITABLE to indicate what operations are valid on handle. CONST char *channelName (in) The name of the channel. int *modePtr (out) Points at an integer variable that will receive an OR-ed combination of TCL_READ- ABLE and TCL_WRITABLE denoting whether the channel is open for reading and writing. | CONST char *pattern (in) | The pattern to match on, passed to | Tcl_StringMatch, or NULL. Tcl_Channel channel (in) A Tcl channel for input or output. Must have been the return value from a procedure such as Tcl_OpenFileChannel. Tcl_Obj *readObjPtr (in/out) | A pointer to a Tcl Object in which to store | the characters read from the channel. | int charsToRead (in) | The number of characters to read from the | channel. If the channel's encoding is | binary, this is equivalent to the number of | bytes to read from the channel. | int appendFlag (in) | If non-zero, data read from the channel | will be appended to the object. Otherwise, | the data will replace the existing contents | of the object. | char *readBuf (out) | A buffer in which to store the bytes read | from the channel. | int bytesToRead (in) | The number of bytes to read from the chan- | nel. The buffer readBuf must be large | enough to hold this many bytes. | Tcl_Obj *lineObjPtr (in/out) | A pointer to a Tcl object in which to store | the line read from the channel. The line | read will be appended to the current value | of the object. | Tcl_DString *lineRead (in/out) | A pointer to a Tcl dynamic string in which | to store the line read from the channel. | Must have been initialized by the caller. | The line read will be appended to any data | already in the dynamic string. | CONST char *input (in) | The input to add to a channel buffer. | int inputLen (in) | Length of the input | int addAtEnd (in) | Flag indicating whether the input should be | added to the end or beginning of the chan- | nel buffer. Tcl_Obj *writeObjPtr (in) A pointer to a Tcl Object whose contents will be output to the channel. CONST char *charBuf (in) A buffer containing the characters to out- put to the channel. CONST char *byteBuf (in) A buffer containing the bytes to output to the channel. int bytesToWrite (in) The number of bytes to consume from charBuf or byteBuf and output to the channel. int offset (in) How far to move the access point in the channel at which the next input or output operation will be applied, measured in bytes from the position given by seekMode. May be either positive or negative. int seekMode (in) Relative to which point to seek; used with offset to calculate the new access point for the channel. Legal values are SEEK_SET, SEEK_CUR, and SEEK_END. CONST char *optionName (in) The name of an option applicable to this channel, such as -blocking. May have any of the values accepted by the fconfigure command. Tcl_DString *optionValue (in) Where to store the value of an option or a list of all options and their values. Must have been initialized by the caller. CONST char *newValue (in) New value for the option given by option- Name. _________________________________________________________________ DESCRIPTION The Tcl channel mechanism provides a device-independent and platform-independent mechanism for performing buffered input and output operations on a variety of file, socket, and device types. The channel mechanism is extensible to new channel types, by providing a low level channel driver for the new type; the channel driver interface is described in the manual entry for Tcl_CreateChannel. The channel mechanism provides a buffering scheme modeled after Unix's standard I/O, and it also allows for nonblocking I/O on channels. The procedures described in this manual entry comprise the C APIs of the generic layer of the channel architecture. For a description of the channel driver architecture and how to implement channel drivers for new types of channels, see the manual entry for Tcl_Create- Channel. TCL_OPENFILECHANNEL Tcl_OpenFileChannel opens a file specified by fileName and returns a channel handle that can be used to perform input and output on the file. This API is modeled after the fopen procedure of the Unix standard I/O library. The syntax and meaning of all arguments is similar to those given in the Tcl open command when opening a file. If an error occurs while opening the channel, Tcl_OpenFileChannel returns NULL and records a POSIX error code that can be retrieved with Tcl_GetErrno. In addition, if interp is non-NULL, Tcl_Open- FileChannel leaves an error message in interp's result after any error. As of Tcl 8.4, the object-based API Tcl_FSOpenFileChannel should be used in preference to Tcl_Open- FileChannel wherever possible. The newly created channel is not registered in the supplied interpreter; to register it, use Tcl_RegisterChannel, described below. If one of the standard channels, stdin, stdout or stderr was previously closed, the act of creating the new channel also assigns it as a replacement for the standard channel. TCL_OPENCOMMANDCHANNEL Tcl_OpenCommandChannel provides a C-level interface to the functions of the exec and open commands. It creates a sequence of subprocesses specified by the argv and argc arguments and returns a channel that can be used to communicate with these subprocesses. The flags argument indicates what sort of communication will exist with the command pipeline. If the TCL_STDIN flag is set then the standard input for the first subprocess will be tied to the channel: writing to the channel will provide input to the subprocess. If TCL_STDIN is not set, then standard input for the first subprocess will be the same as this applica- tion's standard input. If TCL_STDOUT is set then standard output from the last subprocess can be read from the channel; otherwise it goes to this application's standard output. If TCL_STDERR is set, standard error output for all subprocesses is returned to the channel and results in an error when the channel is closed; otherwise it goes to this applica- tion's standard error. If TCL_ENFORCE_MODE is not set, then argc and argv can redirect the stdio handles to override TCL_STDIN, TCL_STDOUT, and TCL_STDERR; if it is set, then it is an error for argc and argv to override stdio channels for which TCL_STDIN, TCL_STDOUT, and TCL_STDERR have been set. If an error occurs while opening the channel, Tcl_OpenCommandChannel returns NULL and records a POSIX error code that can be retrieved with Tcl_GetErrno. In addition, Tcl_OpenCommandChannel leaves an error message in the interpreter's result if interp is not NULL. The newly created channel is not registered in the supplied interpreter; to register it, use Tcl_RegisterChannel, described below. If one of the standard channels, stdin, stdout or stderr was previously closed, the act of creating the new channel also assigns it as a replacement for the standard channel. TCL_MAKEFILECHANNEL Tcl_MakeFileChannel makes a Tcl_Channel from an existing, platform-specific, file handle. The newly created channel is not registered in the supplied interpreter; to register it, use Tcl_RegisterChannel, described below. If one of the standard channels, stdin, stdout or stderr was previously closed, the act of creating the new channel also assigns it as a replacement for the standard channel. TCL_GETCHANNEL Tcl_GetChannel returns a channel given the channelName used to create it with Tcl_Create- Channel and a pointer to a Tcl interpreter in interp. If a channel by that name is not registered in that interpreter, the procedure returns NULL. If the modePtr argument is not NULL, it points at an integer variable that will receive an OR-ed combination of TCL_READ- ABLE and TCL_WRITABLE describing whether the channel is open for reading and writing. Tcl_GetChannelNames and Tcl_GetChannelNamesEx write the names of the registered channels to the interpreter's result as a list object. Tcl_GetChannelNamesEx will filter these names according to the pattern. If pattern is NULL, then it will not do any filtering. The return value is TCL_OK if no errors occurred writing to the result, otherwise it is TCL_ERROR, and the error message is left in the interpreter's result. TCL_REGISTERCHANNEL Tcl_RegisterChannel adds a channel to the set of channels accessible in interp. After this call, Tcl programs executing in that interpreter can refer to the channel in input or out- put operations using the name given in the call to Tcl_CreateChannel. After this call, the channel becomes the property of the interpreter, and the caller should not call Tcl_Close for the channel; the channel will be closed automatically when it is unregis- tered from the interpreter. Code executing outside of any Tcl interpreter can call Tcl_RegisterChannel with interp as NULL, to indicate that it wishes to hold a reference to this channel. Subsequently, the channel can be registered in a Tcl interpreter and it will only be closed when the match- ing number of calls to Tcl_UnregisterChannel have been made. This allows code executing outside of any interpreter to safely hold a reference to a channel that is also registered in a Tcl interpreter. This procedure interacts with the code managing the standard channels. If no standard channels were initialized before the first call to Tcl_RegisterChannel they will get ini- tialized by that call. See Tcl_StandardChannels for a general treatise about standard channels and the behaviour of the Tcl library with regard to them. TCL_UNREGISTERCHANNEL Tcl_UnregisterChannel removes a channel from the set of channels accessible in interp. After this call, Tcl programs will no longer be able to use the channel's name to refer to the channel in that interpreter. If this operation removed the last registration of the channel in any interpreter, the channel is also closed and destroyed. Code not associated with a Tcl interpreter can call Tcl_UnregisterChannel with interp as NULL, to indicate to Tcl that it no longer holds a reference to that channel. If this is the last reference to the channel, it will now be closed. Tcl_UnregisterChannel is very similar to Tcl_DetachChannel except that it will also close the channel if no further ref- erences to it exist. TCL_DETACHCHANNEL Tcl_DetachChannel removes a channel from the set of channels accessible in interp. After this call, Tcl programs will no longer be able to use the channel's name to refer to the channel in that interpreter. Beyond that, this command has no further effect. It cannot be used on the standard channels (stdout, stderr, stdin), and will return TCL_ERROR if passed one of those channels. Code not associated with a Tcl interpreter can call Tcl_DetachChannel with interp as NULL, to indicate to Tcl that it no longer holds a reference to that channel. If this is the last reference to the channel, unlike Tcl_UnregisterChannel, it will not be closed. TCL_ISSTANDARDCHANNEL Tcl_IsStandardChannel tests whether a channel is one of the three standard channels, stdin, stdout or stderr. If so, it returns 1, otherwise 0. No attempt is made to check whether the given channel or the standard channels are ini- tialized or otherwise valid. TCL_CLOSE Tcl_Close destroys the channel channel, which must denote a currently open channel. The channel should not be registered in any interpreter when Tcl_Close is called. Buffered output is flushed to the channel's output device prior to destroying the channel, and any buffered input is discarded. If this is a blocking channel, the call does not return until all buffered data is successfully sent to the channel's output device. If this is a nonblocking channel and there is buffered output that cannot be written without blocking, the call returns immediately; output is flushed in the background and the channel will be closed once all of the buffered data has been output. In this case errors during flushing are not reported. If the channel was closed successfully, Tcl_Close returns TCL_OK. If an error occurs, Tcl_Close returns TCL_ERROR and records a POSIX error code that can be retrieved with Tcl_GetErrno. If the channel is being closed synchronously and an error occurs during closing of the channel and interp is not NULL, an error message is left in the inter- preter's result. Note: it is not safe to call Tcl_Close on a channel that has been registered using Tcl_RegisterChannel; see the documentation for Tcl_RegisterChannel, above, for details. If the channel has ever been given as the chan argument in a call to Tcl_RegisterChannel, you should instead use Tcl_UnregisterChannel, which will internally call Tcl_Close when all calls to Tcl_RegisterChannel have been matched by corresponding calls to Tcl_Unregister- Channel. TCL_READCHARS AND TCL_READ | Tcl_ReadChars consumes bytes from channel, converting the bytes to UTF-8 based on the | channel's encoding and storing the produced data in readObjPtr's string representation. | The return value of Tcl_ReadChars is the number of characters, up to charsToRead, that | were stored in readObjPtr. If an error occurs while reading, the return value is -1 and | Tcl_ReadChars records a POSIX error code that can be retrieved with Tcl_GetErrno. | The return value may be smaller than the value to read, indicating that less data than | requested was available. This is called a short read. In blocking mode, this can only | happen on an end-of-file. In nonblocking mode, a short read can also occur if there is | not enough input currently available: Tcl_ReadChars returns a short count rather than | waiting for more data. | If the channel is in blocking mode, a return value of zero indicates an end-of-file condi- | tion. If the channel is in nonblocking mode, a return value of zero indicates either that | no input is currently available or an end-of-file condition. Use Tcl_Eof and Tcl_Input- | Blocked to tell which of these conditions actually occurred. | Tcl_ReadChars translates the various end-of-line representations into the canonical \n | internal representation according to the current end-of-line recognition mode. End-of- | line recognition and the various platform-specific modes are described in the manual entry | for the Tcl fconfigure command. | As a performance optimization, when reading from a channel with the encoding binary, the | bytes are not converted to UTF-8 as they are read. Instead, they are stored in readOb- | jPtr's internal representation as a byte-array object. The string representation of this | object will only be constructed if it is needed (e.g., because of a call to Tcl_Get- | StringFromObj). In this way, byte-oriented data can be read from a channel, manipulated | by calling Tcl_GetByteArrayFromObj and related functions, and then written to a channel | without the expense of ever converting to or from UTF-8. | Tcl_Read is similar to Tcl_ReadChars, except that it doesn't do encoding conversions, | regardless of the channel's encoding. It is deprecated and exists for backwards compati- | bility with non-internationalized Tcl extensions. It consumes bytes from channel and | stores them in readBuf, performing end-of-line translations on the way. The return value | of Tcl_Read is the number of bytes, up to bytesToRead, written in readBuf. The buffer | produced by Tcl_Read is not null-terminated. Its contents are valid from the zeroth posi- | tion up to and excluding the position indicated by the return value. | Tcl_ReadRaw is the same as Tcl_Read but does not compensate for stacking. While Tcl_Read | (and the other functions in the API) always get their data from the topmost channel in the | stack the supplied channel is part of, Tcl_ReadRaw does not. Thus this function is only | usable for transformational channel drivers, i.e. drivers used in the middle of a stack of | channels, to move data from the channel below into the transformation. | TCL_GETSOBJ AND TCL_GETS | Tcl_GetsObj consumes bytes from channel, converting the bytes to UTF-8 based on the chan- | nel's encoding, until a full line of input has been seen. If the channel's encoding is | binary, each byte read from the channel is treated as an individual Unicode character. | All of the characters of the line except for the terminating end-of-line character(s) are | appended to lineObjPtr's string representation. The end-of-line character(s) are read and | discarded. | If a line was successfully read, the return value is greater than or equal to zero and | indicates the number of bytes stored in lineObjPtr. If an error occurs, Tcl_GetsObj | returns -1 and records a POSIX error code that can be retrieved with Tcl_GetErrno. | Tcl_GetsObj also returns -1 if the end of the file is reached; the Tcl_Eof procedure can | be used to distinguish an error from an end-of-file condition. | If the channel is in nonblocking mode, the return value can also be -1 if no data was | available or the data that was available did not contain an end-of-line character. When | -1 is returned, the Tcl_InputBlocked procedure may be invoked to determine if the channel | is blocked because of input unavailability. | Tcl_Gets is the same as Tcl_GetsObj except the resulting characters are appended to the | dynamic string given by lineRead rather than a Tcl object. | TCL_UNGETS | Tcl_Ungets is used to add data to the input queue of a channel, at either the head or tail | of the queue. The pointer input points to the data that is to be added. The length of | the input to add is given by inputLen. A non-zero value of addAtEnd indicates that the | data is to be added at the end of queue; otherwise it will be added at the head of the | queue. If channel has a "sticky" EOF set, no data will be added to the input queue. | Tcl_Ungets returns inputLen or -1 if an error occurs. | TCL_WRITECHARS, TCL_WRITEOBJ, AND TCL_WRITE | Tcl_WriteChars accepts bytesToWrite bytes of character data at charBuf. The UTF-8 charac- | ters in the buffer are converted to the channel's encoding and queued for output to chan- | nel. If bytesToWrite is negative, Tcl_WriteChars expects charBuf to be null-terminated | and it outputs everything up to the null. | Data queued for output may not appear on the output device immediately, due to internal | buffering. If the data should appear immediately, call Tcl_Flush after the call to | Tcl_WriteChars, or set the -buffering option on the channel to none. If you wish the data | to appear as soon as a complete line is accepted for output, set the -buffering option on | the channel to line mode. | The return value of Tcl_WriteChars is a count of how many bytes were accepted for output | to the channel. This is either greater than zero to indicate success or -1 to indicate | that an error occurred. If an error occurs, Tcl_WriteChars records a POSIX error code | that may be retrieved with Tcl_GetErrno. | Newline characters in the output data are translated to platform-specific end-of-line | sequences according to the -translation option for the channel. This is done even if the | channel has no encoding. | Tcl_WriteObj is similar to Tcl_WriteChars except it accepts a Tcl object whose contents | will be output to the channel. The UTF-8 characters in writeObjPtr's string representa- | tion are converted to the channel's encoding and queued for output to channel. As a per- | formance optimization, when writing to a channel with the encoding binary, UTF-8 charac- | ters are not converted as they are written. Instead, the bytes in writeObjPtr's internal | representation as a byte-array object are written to the channel. The byte-array repre- | sentation of the object will be constructed if it is needed. In this way, byte-oriented | data can be read from a channel, manipulated by calling Tcl_GetByteArrayFromObj and | related functions, and then written to a channel without the expense of ever converting to | or from UTF-8. | Tcl_Write is similar to Tcl_WriteChars except that it doesn't do encoding conversions, | regardless of the channel's encoding. It is deprecated and exists for backwards compati- | bility with non-internationalized Tcl extensions. It accepts bytesToWrite bytes of data | at byteBuf and queues them for output to channel. If bytesToWrite is negative, Tcl_Write | expects byteBuf to be null-terminated and it outputs everything up to the null. | Tcl_WriteRaw is the same as Tcl_Write but does not compensate for stacking. While | Tcl_Write (and the other functions in the API) always feed their input to the topmost | channel in the stack the supplied channel is part of, Tcl_WriteRaw does not. Thus this | function is only usable for transformational channel drivers, i.e. drivers used in the | middle of a stack of channels, to move data from the transformation into the channel below | it. TCL_FLUSH Tcl_Flush causes all of the buffered output data for channel to be written to its underly- ing file or device as soon as possible. If the channel is in blocking mode, the call does not return until all the buffered data has been sent to the channel or some error occurred. The call returns immediately if the channel is nonblocking; it starts a back- ground flush that will write the buffered data to the channel eventually, as fast as the channel is able to absorb it. The return value is normally TCL_OK. If an error occurs, Tcl_Flush returns TCL_ERROR and records a POSIX error code that can be retrieved with Tcl_GetErrno. TCL_SEEK Tcl_Seek moves the access point in channel where subsequent data will be read or written. Buffered output is flushed to the channel and buffered input is discarded, prior to the seek operation. Tcl_Seek normally returns the new access point. If an error occurs, Tcl_Seek returns -1 and records a POSIX error code that can be retrieved with Tcl_GetErrno. After an error, the access point may or may not have been moved. TCL_TELL Tcl_Tell returns the current access point for a channel. The returned value is -1 if the channel does not support seeking. TCL_GETCHANNELOPTION Tcl_GetChannelOption retrieves, in optionValue, the value of one of the options currently in effect for a channel, or a list of all options and their values. The channel argument identifies the channel for which to query an option or retrieve all options and their val- ues. If optionName is not NULL, it is the name of the option to query; the option's value is copied to the Tcl dynamic string denoted by optionValue. If optionName is NULL, the function stores an alternating list of option names and their values in optionValue, using a series of calls to Tcl_DStringAppendElement. The various preexisting options and their possible values are described in the manual entry for the Tcl fconfigure command. Other options can be added by each channel type. These channel type specific options are described in the manual entry for the Tcl command that creates a channel of that type; for example, the additional options for TCP based channels are described in the manual entry for the Tcl socket command. The procedure normally returns TCL_OK. If an error occurs, it returns TCL_ERROR and calls Tcl_SetErrno to store an appropriate POSIX error code. TCL_SETCHANNELOPTION Tcl_SetChannelOption sets a new value newValue for an option optionName on channel. The procedure normally returns TCL_OK. If an error occurs, it returns TCL_ERROR; in addi- tion, if interp is non-NULL, Tcl_SetChannelOption leaves an error message in the inter- preter's result. TCL_EOF Tcl_Eof returns a nonzero value if channel encountered an end of file during the last input operation. TCL_INPUTBLOCKED Tcl_InputBlocked returns a nonzero value if channel is in nonblocking mode and the last input operation returned less data than requested because there was insufficient data available. The call always returns zero if the channel is in blocking mode. TCL_INPUTBUFFERED Tcl_InputBuffered returns the number of bytes of input currently buffered in the internal buffers for a channel. If the channel is not open for reading, this function always returns zero. TCL_OUTPUTBUFFERED Tcl_OutputBuffered returns the number of bytes of output currently buffered in the inter- | nal buffers for a channel. If the channel is not open for writing, this function always | returns zero. PLATFORM ISSUES The handles returned from Tcl_GetChannelHandle depend on the platform and the channel type. On Unix platforms, the handle is always a Unix file descriptor as returned from the open system call. On Windows platforms, the handle is a file HANDLE when the channel was created with Tcl_OpenFileChannel, Tcl_OpenCommandChannel, or Tcl_MakeFileChannel. Other channel types may return a different type of handle on Windows platforms. On the Macin- tosh platform, the handle is a file reference number as returned from HOpenDF. SEE ALSO DString(3), fconfigure(n), filename(n), fopen(3), Tcl_CreateChannel(3) KEYWORDS access point, blocking, buffered I/O, channel, channel driver, end of file, flush, input, nonblocking, output, read, seek, write Tcl 8.3 Tcl_OpenFileChannel(3)