tell(n) [osx man page]

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

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NAME

       tell - Return current access position for an open channel

SYNOPSIS

       tell channelId
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DESCRIPTION

       Returns an integer string giving the current access position in channelId.  This value returned is a byte offset that can be passed to seek
       in order to set the channel to a particular position.  Note that this value is in terms of bytes, not  characters  like	read.	The  value
       returned is -1 for channels that do not support seeking.

       ChannelId  must	be  an	identifier for 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.

EXAMPLE

       Read a line from a file channel only if it starts with foobar:
	      # Save the offset in case we need to undo the read...
	      set offset [tell $chan]
	      if {[read $chan 6] eq "foobar"} {
		  gets $chan line
	      } else {
		  set line {}
		  # Undo the read...
		  seek $chan $offset
	      }

SEE ALSO

       file(n), open(n), close(n), gets(n), seek(n), Tcl_StandardChannels(3)

KEYWORDS

       access position, channel, seeking

Tcl									8.1								   tell(n)

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

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NAME

       seek - Change the access position for an open channel

SYNOPSIS

       seek channelId offset ?origin?
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DESCRIPTION

       Changes the current access position for channelId.

       ChannelId  must	be  an	identifier for 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.

       The offset and origin arguments specify the position at which the next read or write will occur for channelId. Offset must  be  an  integer
       (which may be negative) and origin must be one of the following:

       start	 The new access position will be offset bytes from the start of the underlying file or device.

       current	 The  new access position will be offset bytes from the current access position; a negative offset moves the access position back-
		 wards in the underlying file or device.

       end	 The new access position will be offset bytes from the end of the file or device.  A negative offset places  the  access  position
		 before the end of file, and a positive offset places the access position after the end of file.

       The origin argument defaults to start.

       The  command  flushes  all buffered output for the channel before the command returns, even if the channel is in nonblocking mode.  It also
       discards any buffered and unread input.	This command returns an empty string.  An error occurs if this	command  is  applied  to  channels
       whose underlying file or device does not support seeking.

       Note  that  offset  values  are	byte offsets, not character offsets.  Both seek and tell operate in terms of bytes, not characters, unlike
       read.

EXAMPLES

       Read a file twice:
	      set f [open file.txt]
	      set data1 [read $f]
	      seek $f 0
	      set data2 [read $f]
	      close $f
	      # $data1 == $data2 if the file wasn't updated

       Read the last 10 bytes from a file:
	      set f [open file.data]
	      # This is guaranteed to work with binary data but
	      # may fail with other encodings...
	      fconfigure $f -translation binary
	      seek $f -10 end
	      set data [read $f 10]
	      close $f

SEE ALSO

       file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)

KEYWORDS

       access position, file, seek

Tcl									8.1								   seek(n)