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Updated: Wednesday, February 9, 2022

fconfigure (1t)


fconfigure - Set and get options on a channel


fconfigure channelId
fconfigure channelId name
fconfigure channelId name value ?name value ...?


fconfigure(1t)               Tcl Built-In Commands              fconfigure(1t)


       fconfigure - Set and get options on a channel

       fconfigure channelId
       fconfigure channelId name
       fconfigure channelId name value ?name value ...?

       The fconfigure command sets and retrieves options for channels.

       ChannelId  identifies  the  channel for which to set or query an option
       and must refer to an open  channel  such  as  a  Tcl  standard  channel
       (stdin, stdout, or stderr), the return value from an invocation of open
       or socket, or the result of a channel creation command  provided  by  a
       Tcl extension.

       If  no name or value arguments are supplied, the command returns a list
       containing alternating option names and values  for  the  channel.   If
       name  is  supplied  but  no  value then the command returns the current
       value of the given option.  If one or more pairs of name and value  are
       supplied, the command sets each of the named options to the correspond-
       ing value; in this case the return value is an empty string.

       The options described below are supported for all  channels.  In  addi-
       tion,  each channel type may add options that only it supports. See the
       manual entry for the command that creates each type of channels for the
       options  that  that specific type of channel supports. For example, see
       the manual entry for the socket  command  for  additional  options  for
       sockets,  and  the  open  command  for  additional  options  for serial

       -blocking boolean
              The -blocking option determines whether I/O  operations  on  the
              channel  can cause the process to block indefinitely.  The value
              of the option must be a proper boolean value.  Channels are nor-
              mally in blocking mode;  if a channel is placed into nonblocking
              mode it will affect the  operation  of  the  gets,  read,  puts,
              flush,  and  close  commands  by  allowing them to operate asyn-
              chronously;  see  the  documentation  for  those  commands   for
              details.   For  nonblocking mode to work correctly, the applica-
              tion  must  be  using  the  Tcl  event  loop  (e.g.  by  calling
              Tcl_DoOneEvent or invoking the vwait command).

       -buffering newValue
              If newValue is full then the I/O system will buffer output until
              its internal buffer is  full  or  until  the  flush  command  is
              invoked. If newValue is line, then the I/O system will automati-
              cally flush output for the channel whenever a newline  character
              is  output. If newValue is none, the I/O system will flush auto-
              matically after every output  operation.   The  default  is  for
              -buffering to be set to full except for channels that connect to
              terminal-like devices; for these channels the initial setting is
              line.  Additionally, stdin and stdout are initially set to line,
              and stderr is set to none.

       -buffersize newSize
              Newvalue must be an integer; its value is used to set  the  size
              of buffers, in bytes, subsequently allocated for this channel to
              store input or output. Newvalue must be between one and one mil-
              lion, allowing buffers of one to one million bytes in size.

       -encoding name
              This  option  is used to specify the encoding of the channel, so
              that the data can be converted to and from Unicode  for  use  in
              Tcl.   For  instance, in order for Tcl to read characters from a
              Japanese file in shiftjis and properly process and  display  the
              contents,  the  encoding  would be set to shiftjis.  Thereafter,
              when reading from the channel, the bytes in  the  Japanese  file
              would be converted to Unicode as they are read.  Writing is also
              supported - as Tcl strings are written to the channel they  will
              automatically be converted to the specified encoding on output.

              If  a  file  contains  pure  binary  data  (for instance, a JPEG
              image), the encoding for the channel should be configured to  be
              binary.   Tcl  will then assign no interpretation to the data in
              the file and simply read or write raw  bytes.   The  Tcl  binary
              command  can  be used to manipulate this byte-oriented data.  It
              is usually better to set the -translation option to binary  when
              you  want  to  transfer binary data, as this turns off the other
              automatic interpretations of the bytes in the stream as well.

              The default encoding for newly opened channels is the same plat-
              form-  and locale-dependent system encoding used for interfacing
              with the operating system, as returned by encoding system.

       -eofchar char

       -eofchar {inChar outChar}
              This option supports DOS file systems that use Control-z  (\x1a)
              as  an end of file marker.  If char is not an empty string, then
              this character signals end-of-file when it is encountered during
              input.  For output, the end-of-file character is output when the
              channel is closed.  If char is the empty string, then  there  is
              no  special  end of file character marker.  For read-write chan-
              nels, a two-element list specifies the end of  file  marker  for
              input  and output, respectively.  As a convenience, when setting
              the end-of-file character for a read-write channel you can spec-
              ify  a single value that will apply to both reading and writing.
              When querying the end-of-file character of a read-write channel,
              a  two-element  list will always be returned.  The default value
              for -eofchar is the empty string in all cases except  for  files
              under  Windows.   In  that case the -eofchar is Control-z (\x1a)
              for reading and the empty string for  writing.   The  acceptable
              range  for  -eofchar  values  is  \x01 - \x7f; attempting to set
              -eofchar to a value outside  of  this  range  will  generate  an

       -translation mode

       -translation {inMode outMode}
              In  Tcl  scripts the end of a line is always represented using a
              single newline character (\n).  However,  in  actual  files  and
              devices the end of a line may be represented differently on dif-
              ferent platforms, or even for  different  devices  on  the  same
              platform.   For  example, under UNIX newlines are used in files,
              whereas carriage-return-linefeed sequences are normally used  in
              network  connections.   On  input (i.e., with gets and read) the
              Tcl I/O system automatically translates the external end-of-line
              representation into newline characters.  Upon output (i.e., with
              puts), the I/O system translates newlines to the  external  end-
              of-line  representation.   The  default  translation mode, auto,
              handles all the common cases automatically, but the -translation
              option  provides  explicit control over the end of line transla-

              The value associated with -translation  is  a  single  item  for
              read-only  and  write-only channels.  The value is a two-element
              list for read-write channels; the read translation mode  is  the
              first element of the list, and the write translation mode is the
              second element.  As a convenience, when setting the  translation
              mode  for  a  read-write  channel you can specify a single value
              that will apply to both reading and writing.  When querying  the
              translation  mode  of  a  read-write channel, a two-element list
              will always be returned.  The  following  values  are  currently

              auto   As the input translation mode, auto treats any of newline
                     (lf), carriage return (cr), or carriage  return  followed
                     by  a  newline  (crlf) as the end of line representation.
                     The end of line representation can even change from line-
                     to-line,  and  all cases are translated to a newline.  As
                     the output translation mode, auto chooses a platform spe-
                     cific  representation;  for  sockets on all platforms Tcl
                     chooses crlf, for all Unix flavors, it  chooses  lf,  and
                     for  the various flavors of Windows it chooses crlf.  The
                     default setting for -translation is auto for  both  input
                     and output.

              binary No  end-of-line  translations  are  performed.   This  is
                     nearly identical to lf  mode,  except  that  in  addition
                     binary  mode  also  sets the end-of-file character to the
                     empty string (which disables it) and sets the encoding to
                     binary  (which  disables  encoding  filtering).   See the
                     description of -eofchar and -encoding for  more  informa-

                     Internally, i.e. when it comes to the actual behaviour of
                     the translator this value  is  identical  to  lf  and  is
                     therefore  reported  as such when queried. Even if binary
                     was used to set the translation.

              cr     The end of a line in the underlying  file  or  device  is
                     represented  by  a  single carriage return character.  As
                     the input translation mode,  cr  mode  converts  carriage
                     returns to newline characters.  As the output translation
                     mode, cr mode translates newline characters  to  carriage

              crlf   The  end  of  a  line in the underlying file or device is
                     represented by a carriage return character followed by  a
                     linefeed  character.  As the input translation mode, crlf
                     mode converts carriage-return-linefeed sequences to  new-
                     line  characters.   As  the output translation mode, crlf
                     mode translates newline  characters  to  carriage-return-
                     linefeed  sequences.  This mode is typically used on Win-
                     dows platforms and for network connections.

              lf     The end of a line in the underlying  file  or  device  is
                     represented by a single newline (linefeed) character.  In
                     this mode no translations occur during  either  input  or
                     output.  This mode is typically used on UNIX platforms.

       The Tcl standard channels (stdin, stdout, and stderr) can be configured
       through this command  like  every  other  channel  opened  by  the  Tcl
       library.  Beyond  the  standard  options described above they will also
       support any special option according to their current  type.   If,  for
       example,  a  Tcl application is started by the inet super-server common
       on Unix system its Tcl standard channels will be sockets and thus  sup-
       port the socket options.

       Instruct  Tcl  to  always send output to stdout immediately, whether or
       not it is to a terminal:

              fconfigure stdout -buffering none

       Open a socket and read lines from it without ever blocking the process-
       ing of other events:

              set s [socket some.where.com 12345]
              fconfigure $s -blocking 0
              fileevent $s readable "readMe $s"
              proc readMe chan {
                  if {[gets $chan line] < 0} {
                      if {[eof $chan]} {
                          close $chan
                      # Could not read a complete line this time; Tcl's
                      # internal buffering will hold the partial line for us
                      # until some more data is available over the socket.
                  } else {
                      puts stdout $line

       Read a PPM-format image from a file:

              # Open the file and put it into Unix ASCII mode
              set f [open teapot.ppm]
              fconfigure $f -encoding ascii -translation lf

              # Get the header
              if {[gets $f] ne "P6"} {
                  error "not a raw-bits PPM"

              # Read lines until we have got non-comment lines
              # that supply us with three decimal values.
              set words {}
              while {[llength $words] < 3} {
                  gets $f line
                  if {[string match "#*" $line]} continue
                  lappend words {*}[join [scan $line %d%d%d]]

              # Those words supply the size of the image and its
              # overall depth per channel. Assign to variables.
              lassign $words xSize ySize depth

              # Now switch to binary mode to pull in the data,
              # one byte per channel (red,green,blue) per pixel.
              fconfigure $f -translation binary
              set numDataBytes [expr {3 * $xSize * $ySize}]
              set data [read $f $numDataBytes]

              close $f

       See attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       close(n),  flush(n),  gets(n),  open(n),  puts(n),  read(n), socket(n),

       blocking, buffering, carriage return, end of line, flushing,  linemode,
       newline,  nonblocking,  platform,  translation,  encoding, filter, byte
       array, binary

       Source code for open source software components in Oracle  Solaris  can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   8.3                       fconfigure(1t)