Chapter 43. File Access 101
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File Access 101

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File Access 101
Example

File Access 101

Tcl provides several methods to read from and write to files on disk. The simplest methods to access a file are via gets and puts. When there is a lot of data to be read, however, it is sometimes more efficient to use the read command to load an entire file, and then parse the file into lines with the split command.

These methods can also be used for communicating over sockets and pipes. It is even possible, via the so-called virtual file system

open fileName ?access? ?permission?

Opens a file and returns a token to be used when accessing the file via

gets,

puts,

close, etc.

  • FileName is the name of the file to open.

  • access is the file access mode

    • r......Open the file for reading. The file must already exist.

    • r+...Open the file for reading and writing. The file must already exist.

    • w.....Open the file for writing. Create the file if it doesn't exist, or set the length to zero if it does exist.

    • w+..Open the file for reading and writing. Create the file if it doesn't exist, or set the length to zero if it does exist.

    • a......Open the file for writing. The file must already exist. Set the current location to the end of the file.

    • a+...Open the file for writing. The file does not exist, create it. Set the current location to the end of the file.

  • permission is an integer to use to set the file access permissions. The default is

    rw-rw-rw- (0666). You can use it to set the permissions for the file's owner, the group he/she belongs to and for all the other users. For many applications, the default is fine.

close fileID

Closes a file previously opened with

open, and flushes any remaining output.

gets fileID ?varName?

Reads a line of input from FileID, and discards the terminating newline.

If there is a varName argument, gets returns the number of characters read (or -1 if an EOF occurs), and places the line of input in varName.

If varName is not specified, gets returns the line of input. An empty string will be returned if:

  • There is a blank line in the file.

  • The current location is at the end of the file. (An EOF occurs.)

puts ?-nonewline? ?fileID? string

Writes the characters in string to the stream referenced by

fileID, where

fileID is one of:

  • The value returned by a previous call to

    open with write access.

  • stdout

  • stderr

read ?-nonewline? fileID

Reads all the remaining bytes from

fileID, and returns that string. If

-nonewline is set, then the last character will be discarded if it is a newline. Any existing end of file condition is cleared before the

read command is executed.

read fileID numBytes

Reads up to

numBytes from

fileID, and returns the input as a Tcl string. Any existing end of file condition is cleared before the

read command is executed.

seek fileID offset ?origin?

Change the current position within the file referenced by

fileID. Note that if the file was opened with "a"

access that the current position can not be set before the end of the file for writing, but can be set to the beginning of the file for reading.

  • fileID is one of:

    • a File identifier returned by

      open

    • stdin

    • stdout

    • stderr

  • offset is the offset in bytes at which the current position is to be set. The position from which the offset is measured defaults to the start of the file, but can be from the current location, or the end by setting

    origin appropriately.

  • origin is the position to measure

    offset from. It defaults to the start of the file.

    Origin must be one of:

    • start.........

      Offset is measured from the start of the file.

    • current...

      Offset is measured from the current position in the file.

    • end...........

      Offset is measured from the end of the file.

tell fileID

Returns the position of the access pointer in

fileID as a decimal string.

flush fileID

Flushes any output that has been buffered for

fileID.

eof fileID

returns 1 if an End Of File condition exists, otherwise returns 0.

Points to remember about Tcl file access:

  • The file I/O is buffered. The output may not be sent out when you expect it to be sent. Files will all be closed and flushed when your program exits normally, but may only be closed (not flushed) if the program is terminated in an unexpected manner.

  • There are a finite number of open file slots available. If you expect the program to run in a manner that will cause it to open several files, remember to close the files when you are done with them.

  • An empty line is indistinguishable from an EOF with the command:

    set string [gets filename] Use the

    eof command to determine if the file is at the end or use the other form of

    gets (see the example).

  • You can't overwrite any data in a file that was opened with

    a (append)

    access. You can, however seek to the beginning of the file for

    gets commands.

  • Opening a file with the

    w+ access will allow you to overwrite data, but will delete all existing data in the file.

  • Opening a file with the

    r+ access will allow you to overwrite data, while saving the existing data in the file.

  • By default the commands assume that strings represent "readable" text. If you want to read "binary" data, you will have to use the

    fconfigure command.

  • Often, especially if you deal with configuration data for your programs, you can use the

    source command instead of the relatively low-level commands presented here. Just make sure your data can be interpreted as Tcl commands and "source" the file.

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