Tcl_OpenFileChannel(3) Tcl Library Procedures Tcl_OpenFileChannel(3)______________________________________________________________________________NAME
Tcl_OpenFileChannel, Tcl_OpenCommandChannel, Tcl_MakeFileChannel,
Tcl_GetChannel, 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_GetChan‐
nelOption, Tcl_SetChannelOption, Tcl_Eof, Tcl_InputBlocked, Tcl_Input‐
Buffered - 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)
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_WriteObj(channel, writeObjPtr) │
int │
Tcl_WriteChars(channel, charBuf, bytesToWrite) │
int │
Tcl_Write(channel, byteBuf, bytesToWrite) │
int
Tcl_Flush(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)
int
Tcl_Eof(channel)
int
Tcl_InputBlocked(channel)
int
Tcl_InputBuffered(channel)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_Open‐
CommandChannel, 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 construct‐
ing a command pipeline.
These values have the
same meaning as the non-
switch arguments to the
Tcl exec command.
int flags (in) Specifies the disposi‐
tion 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 redirections cause
an error. │
ClientData handle (in) │
Operating system spe‐ │
cific 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 combi‐
nation of TCL_READABLE
and TCL_WRITABLE denot‐
ing 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 chan‐ │
nel. 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 off‐
set 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 chan‐
nel, such as -blocking.
May have any of the val‐
ues 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 call‐
er.
char *newValue (in) New value for the option
given by optionName.
_________________________________________________________________DESCRIPTION
The Tcl channel mechanism provides a device-independent and platform-
independent mechanism for performing buffered input and output opera‐
tions 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 mecha‐
nism 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_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 stan‐
dard 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 inter‐
preter; 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 pipe‐
line.
If the TCL_STDIN flag is set then the standard input for the first sub‐
process will be tied to the channel: writing to the channel will pro‐
vide 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 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 mes‐
sage in interp->result if interp is not NULL.
The newly created channel is not registered in the supplied inter‐
preter; 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-spe‐
cific, 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_READ‐
ABLE and TCL_WRITABLE describing whether the channel is open for read‐
ing and writing.
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_Register‐
Channel with interp as NULL, to indicate that it wishes to hold a ref‐
erence 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 acces‐
sible 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_UnregisterChan‐
nel 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 chan‐
nel, 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 interp->result.
Note: it is not safe to call Tcl_Close on a channel that has been reg‐
istered using Tcl_RegisterChannel; see the documentation for Tcl_Regis‐
terChannel, 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 nonblock‐ │
ing mode, a short read can also occur if there is not enough input cur‐ │
rently 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 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 readObjPtr'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 encod‐ │
ing conversions, regardless of the channel's encoding. It is depre‐ │
cated 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 indi‐ │
cated 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 charac‐ │
ter(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_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 nega‐ │
tive, Tcl_WriteChars expects charBuf to be NULL terminated and it out‐ │
puts 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-spe‐ │
cific 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 charac‐ │
ters in writeObjPtr's string representation are converted to the chan‐ │
nel'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_GetByteArrayFro‐ │
mObj 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 encod‐ │
ing conversions, regardless of the channel's encoding. It is depre‐ │
cated 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 every‐ │
thing 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 even‐
tually, 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_SEEKTcl_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 option‐
Name 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 fconfig‐
ure 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 addi‐
tional 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 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 interp->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_MakeFileChan‐ │
nel. Other channel types may return a different type of handle on Win‐ │
dows platforms. On the Macintosh platform, the handle is a file refer‐ │
ence number as returned from HOpenDF.
SEE ALSODString(3), fconfigure(n), filename(n), fopen(2), Tcl_CreateChannel(3)KEYWORDS
access point, blocking, buffered I/O, channel, channel driver, end of
file, flush, input, nonblocking, output, read, seek, write
Tcl 8.1 Tcl_OpenFileChannel(3)
