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(elisp)Encoding and I/O

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Encoding and I/O
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   The principal purpose of coding systems is for use in reading and
writing files.  The function `insert-file-contents' uses a coding
system for decoding the file data, and `write-region' uses one to
encode the buffer contents.

   You can specify the coding system to use either explicitly (Note:
Specifying Coding Systems), or implicitly using the defaulting
mechanism (Note: Default Coding Systems).  But these methods may not
completely specify what to do.  For example, they may choose a coding
system such as `undefined' which leaves the character code conversion
to be determined from the data.  In these cases, the I/O operation
finishes the job of choosing a coding system.  Very often you will want
to find out afterwards which coding system was chosen.

 - Variable: buffer-file-coding-system
     This variable records the coding system that was used for visiting
     the current buffer.  It is used for saving the buffer, and for
     writing part of the buffer with `write-region'.  When those
     operations ask the user to specify a different coding system,
     `buffer-file-coding-system' is updated to the coding system
     specified.

     However, `buffer-file-coding-system' does not affect sending text
     to a subprocess.

 - Variable: save-buffer-coding-system
     This variable specifies the coding system for saving the buffer (by
     overriding `buffer-file-coding-system').  Note that it is not used
     for `write-region'.

     When a command to save the buffer starts out to use
     `buffer-file-coding-system' (or `save-buffer-coding-system'), and
     that coding system cannot handle the actual text in the buffer,
     the command asks the user to choose another coding system.  After
     that happens, the command also updates `buffer-file-coding-system'
     to represent the coding system that the user specified.

 - Variable: last-coding-system-used
     I/O operations for files and subprocesses set this variable to the
     coding system name that was used.  The explicit encoding and
     decoding functions (Note: Explicit Encoding) set it too.

     *Warning:* Since receiving subprocess output sets this variable,
     it can change whenever Emacs waits; therefore, you should copy the
     value shortly after the function call that stores the value you are
     interested in.

   The variable `selection-coding-system' specifies how to encode
selections for the window system.  Note: Window System Selections.