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read(n)                                     Tcl Built-In Commands                                    read(n)



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NAME
       read - Read from a channel

SYNOPSIS
       read ?-nonewline? channelId

       read channelId numChars
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DESCRIPTION
       In  the  first form, the read command reads all of the data from channelId up to the end of the file.
       If the -nonewline switch is specified then the last character of the file is discarded  if  it  is  a
       newline.  In the second form, the extra argument specifies how many characters to read.  Exactly that
       many characters will be read and returned, unless there are fewer than numChars left in the file;  in
       this  case  all  the remaining characters are returned.  If the channel is configured to use a multi-byte multibyte
       byte encoding, then the number of characters read may not be the same as the number of bytes read.

       ChannelId must be an identifier for an open channel such as the Tcl standard input  channel  (stdin),
       the  return  value  from an invocation of open or socket, or the result of a channel creation command
       provided by a Tcl extension. The channel must have been opened for input.

       If channelId is in nonblocking mode, the command may not read as many characters as  requested:  once
       all  available  input  has  been read, the command will return the data that is available rather than
       blocking for more input.  If the channel is configured to use a multi-byte encoding, then  there  may
       actually  be  some  bytes  remaining  in  the internal buffers that do not form a complete character.
       These bytes will not be returned until a complete character is available or end-of-file  is  reached.
       The -nonewline switch is ignored if the command returns before reaching the end of the file.

       Read translates end-of-line sequences in the input into newline characters according to the -transla-tion -translation
       tion option for the channel.  See the fconfigure manual entry for a discussion on ways in which fcon-figure fconfigure
       figure will alter input.


USE WITH SERIAL PORTS
       For  most  applications  a channel connected to a serial port should be configured to be nonblocking:
       fconfigure channelId -blocking _.  Then read behaves much like described above.  Care must  be  taken
       when using read on blocking serial ports:

       read channelId numChars
              In this form read blocks until numChars have been received from the serial port.

       read channelId
              In  this  form  read  blocks  until the reception of the end-of-file character, see fconfigure
              -eofchar. If there no end-of-file character has been configured for  the  channel,  then  read
              will block forever.

EXAMPLE
       This  example  code  reads  a file all at once, and splits it into a list, with each line in the file
       corresponding to an element in the list:
              set fl [open /proc/meminfo]
              set data [read $fl]
              close $fl
              set lines [split $data \n]


SEE ALSO
       file(n), eof(n), fblocked(n), fconfigure(n), Tcl_StandardChannels(3)


KEYWORDS
       blocking, channel, end of line, end of file, nonblocking, read, translation, encoding



Tcl                                                  8.1                                             read(n)