1seek(n) Tcl Built-In Commands seek(n)
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8 seek - Change the access position for an open channel
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11 seek channelId offset ?origin?
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15 Changes the current access position for channelId.
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17 ChannelId must be an identifier for an open channel such as a Tcl stan‐
18 dard channel (stdin, stdout, or stderr), the return value from an invo‐
19 cation of open or socket, or the result of a channel creation command
20 provided by a Tcl extension.
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22 The offset and origin arguments specify the position at which the next
23 read or write will occur for channelId. Offset must be an integer
24 (which may be negative) and origin must be one of the following:
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26 start The new access position will be offset bytes from the start
27 of the underlying file or device.
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29 current The new access position will be offset bytes from the current
30 access position; a negative offset moves the access position
31 backwards in the underlying file or device.
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33 end The new access position will be offset bytes from the end of
34 the file or device. A negative offset places the access
35 position before the end of file, and a positive offset places
36 the access position after the end of file.
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38 The origin argument defaults to start.
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40 The command flushes all buffered output for the channel before the com‐
41 mand returns, even if the channel is in non-blocking mode. It also
42 discards any buffered and unread input. This command returns an empty
43 string. An error occurs if this command is applied to channels whose
44 underlying file or device does not support seeking.
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46 Note that offset values are byte offsets, not character offsets. Both
47 seek and tell operate in terms of bytes, not characters, unlike read.
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50 Read a file twice:
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52 set f [open file.txt]
53 set data1 [read $f]
54 seek $f 0
55 set data2 [read $f]
56 close $f
57 # $data1 eq $data2 if the file wasn't updated
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59 Read the last 10 bytes from a file:
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61 set f [open file.data]
62 # This is guaranteed to work with binary data but
63 # may fail with other encodings...
64 fconfigure $f -translation binary
65 seek $f -10 end
66 set data [read $f 10]
67 close $f
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70 file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)
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73 access position, file, seek
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77Tcl 8.1 seek(n)