Tcl_OpenFileChannel(), Tcl_OpenCommandChannel(), Tcl_MakeFileChannel(), Tcl_GetChannel(), Tcl_GetChannelNames(), Tcl_GetChannelNamesEx(), Tcl_RegisterChannel(), Tcl_UnregisterChannel(), 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_Ungets() -- buffered I/O facilities using channels

SYNOPSIS

#include <tcl.h>

typedef ... Tcl_Channel;

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_Close(interp, channel)

int Tcl_ReadChars(channel, readObjPtr, charsToRead, appendFlag)

int Tcl_Read(channel, byteBuf, 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_Eof(channel)

int Tcl_Flush(channel)

int Tcl_InputBlocked(channel)

int Tcl_InputBuffered(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.
char *fileName (in): The name of a local or network file.
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. For Tcl_OpenCommandChannel(), may be NULL.
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.
char **argv (in): Arguments for constructing a command pipeline. 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 handles 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 redirections 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.
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_READABLE and TCL_WRITABLE denoting whether the channel is open for reading and writing.
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 channel. 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.
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 output to the channel.
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.
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.
char *newValue (in): New value for the option given by optionName.
char *pattern (in): The pattern to match on, passed to Tcl_StringMatch, or NULL.
char *input (in): The input to add to a channel buffer.
int inputLen (in): Length of the input
int addToEnd (in): Flag indicating whether the input should be added to the end or beginning of the channel buffer.

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 reference page 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 reference page 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 reference page for Tcl_CreateChannel().

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_OpenFileChannel() leaves an error message in interp's result after any error.

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 application'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 application'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.

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_CreateChannel() 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 mode argument is not NULL, it points at an integer variable that will receive an OR-ed combination of TCL_READABLE 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 occured 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 output 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 unregistered 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 matching 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.

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_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 interpreter'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_UnregisterChannel().

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 objPtr. 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 condition. 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_InputBlocked() 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 reference page 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 readObjPtr's internal representation as a byte-array object. The string representation of this object will only be constructed if it is needed (for example, because of a call to Tcl_GetStringFromObj()). 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 compatibility with non-internationalized Tcl extensions. It consumes bytes from channel and stores them in buf, performing end-of-line translations on the way. The return value of Tcl_Read() is the number of bytes, up to toRead, written in buf. The buffer produced by Tcl_Read() is not NULL terminated. Its contents are valid from the zeroth position up to and excluding the position indicated by the return value.

TCL_GETSOBJ AND TCL_GETS

Tcl_GetsObj() consumes bytes from channel, converting the bytes to UTF-8 based on the channel'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 appended to the dynamic string given by dsPtr 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. Input is a pointer to the data that is to be added. InputLen gives the length of the input to add. AddAtEnd, in non-zero, 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 characters in the buffer are converted to the channel's encoding and queued for output to channel. 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 representation are converted to the channel's encoding and queued for output to channel. As a performance optimization, when writing to a channel with the encoding binary, UTF-8 characters 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 representation 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 compatibility 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_FLUSH

Tcl_Flush() causes all of the buffered output data for channel to be written to its underlying 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 background 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 dsPtr, 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 values. 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 reference page for the Tcl fconfigure command. Other options can be added by each channel type. These channel type specific options are described in the reference page for the Tcl command that creates a channel of that type; for example, the additional options for TCP based channels are described in the reference page 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 for an option on channel. OptionName is the option to set and newValue is the value to set. The procedure normally returns TCL_OK. If an error occurs, it returns TCL_ERROR; in addition, if interp is non-NULL, Tcl_SetChannelOption() leaves an error message in the interpreter'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.

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 Macintosh platform, the handle is a file reference number as returned from HOpenDF.

PORTABILITY

Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022. Windows Server 2025.

AVAILABILITY

PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition

Tcl Library Procedures