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@setfilename emacs-mime
@settitle Emacs MIME Manual
@synindex fn cp
@synindex vr cp
@synindex pg cp

@copying
This file documents the Emacs MIME interface functionality.

Copyright @copyright{} 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
2006, 2007 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover texts being ``A GNU
Manual'', and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled ``GNU Free Documentation
License'' in the Emacs manual.

(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
this GNU Manual, like GNU software.  Copies published by the Free
Software Foundation raise funds for GNU development.''

This document is part of a collection distributed under the GNU Free
Documentation License.  If you want to distribute this document
separately from the collection, you can do so by adding a copy of the
license to the document, as described in section 6 of the license.
@end quotation
@end copying

@dircategory Emacs
@direntry
* Emacs MIME: (emacs-mime).   Emacs MIME de/composition library.
@end direntry
@iftex
@finalout
@end iftex
@setchapternewpage odd

@titlepage
@title Emacs MIME Manual

@author by Lars Magne Ingebrigtsen
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@node Top
@top Emacs MIME

This manual documents the libraries used to compose and display
@acronym{MIME} messages.

This manual is directed at users who want to modify the behavior of
the @acronym{MIME} encoding/decoding process or want a more detailed
picture of how the Emacs @acronym{MIME} library works, and people who want
to write functions and commands that manipulate @acronym{MIME} elements.

@acronym{MIME} is short for @dfn{Multipurpose Internet Mail Extensions}.
This standard is documented in a number of RFCs; mainly RFC2045 (Format
of Internet Message Bodies), RFC2046 (Media Types), RFC2047 (Message
Header Extensions for Non-@acronym{ASCII} Text), RFC2048 (Registration
Procedures), RFC2049 (Conformance Criteria and Examples).  It is highly
recommended that anyone who intends writing @acronym{MIME}-compliant software
read at least RFC2045 and RFC2047.

@menu
* Decoding and Viewing::  A framework for decoding and viewing.
* Composing::             @acronym{MML}; a language for describing @acronym{MIME} parts.
* Interface Functions::   An abstraction over the basic functions.
* Basic Functions::       Utility and basic parsing functions.
* Standards::             A summary of RFCs and working documents used.
* GNU Free Documentation License:: The license for this documentation.
* Index::                 Function and variable index.
@end menu


@node Decoding and Viewing
@chapter Decoding and Viewing

This chapter deals with decoding and viewing @acronym{MIME} messages on a
higher level.

The main idea is to first analyze a @acronym{MIME} article, and then allow
other programs to do things based on the list of @dfn{handles} that are
returned as a result of this analysis.

@menu
* Dissection::             Analyzing a @acronym{MIME} message.
* Non-MIME::               Analyzing a non-@acronym{MIME} message.
* Handles::                Handle manipulations.
* Display::                Displaying handles.
* Display Customization::  Variables that affect display.
* Files and Directories::  Saving and naming attachments.
* New Viewers::            How to write your own viewers.
@end menu


@node Dissection
@section Dissection

The @code{mm-dissect-buffer} is the function responsible for dissecting
a @acronym{MIME} article.  If given a multipart message, it will recursively
descend the message, following the structure, and return a tree of
@acronym{MIME} handles that describes the structure of the message.

@node Non-MIME
@section Non-MIME
@vindex mm-uu-configure-list

Gnus also understands some non-@acronym{MIME} attachments, such as
postscript, uuencode, binhex, yenc, shar, forward, gnatsweb, pgp,
diff.  Each of these features can be disabled by add an item into
@code{mm-uu-configure-list}.  For example,

@lisp
(require 'mm-uu)
(add-to-list 'mm-uu-configure-list '(pgp-signed . disabled))
@end lisp

@table @code
@item postscript
@findex postscript
PostScript file.

@item uu
@findex uu
Uuencoded file.

@item binhex
@findex binhex
Binhex encoded file.

@item yenc
@findex yenc
Yenc encoded file.

@item shar
@findex shar
Shar archive file.

@item forward
@findex forward
Non-@acronym{MIME} forwarded message.

@item gnatsweb
@findex gnatsweb
Gnatsweb attachment.

@item pgp-signed
@findex pgp-signed
@acronym{PGP} signed clear text.

@item pgp-encrypted
@findex pgp-encrypted
@acronym{PGP} encrypted clear text.

@item pgp-key
@findex pgp-key
@acronym{PGP} public keys.

@item emacs-sources
@findex emacs-sources
@vindex mm-uu-emacs-sources-regexp
Emacs source code.  This item works only in the groups matching
@code{mm-uu-emacs-sources-regexp}.

@item diff
@vindex diff
@vindex mm-uu-diff-groups-regexp
Patches.  This is intended for groups where diffs of committed files
are automatically sent to.  It only works in groups matching
@code{mm-uu-diff-groups-regexp}.

@item verbatim-marks
@cindex verbatim-marks
Slrn-style verbatim marks.

@item LaTeX
@cindex LaTeX
LaTeX documents.  It only works in groups matching
@code{mm-uu-tex-groups-regexp}.

@end table

@cindex text/x-verbatim
@c Is @vindex suitable for a face?
@vindex mm-uu-extract
Some inlined non-@acronym{MIME} attachments are displayed using the face
@code{mm-uu-extract}.  By default, no @acronym{MIME} button for these
parts is displayed.  You can force displaying a button using @kbd{K b}
(@code{gnus-summary-display-buttonized}) or add @code{text/x-verbatim}
to @code{gnus-buttonized-mime-types}, @xref{MIME Commands, ,MIME
Commands, gnus, Gnus Manual}.

@node Handles
@section Handles

A @acronym{MIME} handle is a list that fully describes a @acronym{MIME}
component.

The following macros can be used to access elements in a handle:

@table @code
@item mm-handle-buffer
@findex mm-handle-buffer
Return the buffer that holds the contents of the undecoded @acronym{MIME}
part.

@item mm-handle-type
@findex mm-handle-type
Return the parsed @code{Content-Type} of the part.

@item mm-handle-encoding
@findex mm-handle-encoding
Return the @code{Content-Transfer-Encoding} of the part.

@item mm-handle-undisplayer
@findex mm-handle-undisplayer
Return the object that can be used to remove the displayed part (if it
has been displayed).

@item mm-handle-set-undisplayer
@findex mm-handle-set-undisplayer
Set the undisplayer object.

@item mm-handle-disposition
@findex mm-handle-disposition
Return the parsed @code{Content-Disposition} of the part.

@item mm-get-content-id
Returns the handle(s) referred to by @code{Content-ID}.

@end table


@node Display
@section Display

Functions for displaying, removing and saving.

@table @code
@item mm-display-part
@findex mm-display-part
Display the part.

@item mm-remove-part
@findex mm-remove-part
Remove the part (if it has been displayed).

@item mm-inlinable-p
@findex mm-inlinable-p
Say whether a @acronym{MIME} type can be displayed inline.

@item mm-automatic-display-p
@findex mm-automatic-display-p
Say whether a @acronym{MIME} type should be displayed automatically.

@item mm-destroy-part
@findex mm-destroy-part
Free all resources occupied by a part.

@item mm-save-part
@findex mm-save-part
Offer to save the part in a file.

@item mm-pipe-part
@findex mm-pipe-part
Offer to pipe the part to some process.

@item mm-interactively-view-part
@findex mm-interactively-view-part
Prompt for a mailcap method to use to view the part.

@end table


@node Display Customization
@section Display Customization

@table @code

@item mm-inline-media-tests
@vindex mm-inline-media-tests
This is an alist where the key is a @acronym{MIME} type, the second element
is a function to display the part @dfn{inline} (i.e., inside Emacs), and
the third element is a form to be @code{eval}ed to say whether the part
can be displayed inline.

This variable specifies whether a part @emph{can} be displayed inline,
and, if so, how to do it.  It does not say whether parts are
@emph{actually} displayed inline.

@item mm-inlined-types
@vindex mm-inlined-types
This, on the other hand, says what types are to be displayed inline, if
they satisfy the conditions set by the variable above.  It's a list of
@acronym{MIME} media types.

@item mm-automatic-display
@vindex mm-automatic-display
This is a list of types that are to be displayed ``automatically'', but
only if the above variable allows it.  That is, only inlinable parts can
be displayed automatically.

@item mm-automatic-external-display
@vindex mm-automatic-external-display
This is a list of types that will be displayed automatically in an
external viewer.

@item mm-keep-viewer-alive-types
@vindex mm-keep-viewer-alive-types
This is a list of media types for which the external viewer will not
be killed when selecting a different article.

@item mm-attachment-override-types
@vindex mm-attachment-override-types
Some @acronym{MIME} agents create parts that have a content-disposition of
@samp{attachment}.  This variable allows overriding that disposition and
displaying the part inline.  (Note that the disposition is only
overridden if we are able to, and want to, display the part inline.)

@item mm-discouraged-alternatives
@vindex mm-discouraged-alternatives
List of @acronym{MIME} types that are discouraged when viewing
@samp{multipart/alternative}.  Viewing agents are supposed to view the
last possible part of a message, as that is supposed to be the richest.
However, users may prefer other types instead, and this list says what
types are most unwanted.  If, for instance, @samp{text/html} parts are
very unwanted, and @samp{text/richtext} parts are somewhat unwanted,
you could say something like:

@lisp
(setq mm-discouraged-alternatives
      '("text/html" "text/richtext")
      mm-automatic-display
      (remove "text/html" mm-automatic-display))
@end lisp

Adding @code{"image/.*"} might also be useful.  Spammers use images as
the preferred part of @samp{multipart/alternative} messages, so you might
not notice there are other parts.  See also
@code{gnus-buttonized-mime-types}, @ref{MIME Commands, ,MIME Commands,
gnus, Gnus Manual}.  After adding @code{"multipart/alternative"} to
@code{gnus-buttonized-mime-types} you can choose manually which
alternative you'd like to view.  For example, you can set those
variables like:

@lisp
(setq gnus-buttonized-mime-types
      '("multipart/alternative" "multipart/signed")
      mm-discouraged-alternatives
      '("text/html" "image/.*"))
@end lisp

In this case, Gnus will display radio buttons for such a kind of spam
message as follows:

@example
1.  (*) multipart/alternative  ( ) image/gif

2.  (*) text/plain          ( ) text/html
@end example

@item mm-inline-large-images
@vindex mm-inline-large-images
When displaying inline images that are larger than the window, Emacs
does not enable scrolling, which means that you cannot see the whole
image.  To prevent this, the library tries to determine the image size
before displaying it inline, and if it doesn't fit the window, the
library will display it externally (e.g. with @samp{ImageMagick} or
@samp{xv}).  Setting this variable to @code{t} disables this check and
makes the library display all inline images as inline, regardless of
their size.

@item mm-inline-override-types
@vindex mm-inline-override-types
@code{mm-inlined-types} may include regular expressions, for example to
specify that all @samp{text/.*} parts be displayed inline.  If a user
prefers to have a type that matches such a regular expression be treated
as an attachment, that can be accomplished by setting this variable to a
list containing that type.  For example assuming @code{mm-inlined-types}
includes @samp{text/.*}, then including @samp{text/html} in this
variable will cause @samp{text/html} parts to be treated as attachments.

@item mm-text-html-renderer
@vindex mm-text-html-renderer
This selects the function used to render @acronym{HTML}.  The predefined
renderers are selected by the symbols @code{w3},
@code{w3m}@footnote{See @uref{http://emacs-w3m.namazu.org/} for more
information about emacs-w3m}, @code{links}, @code{lynx},
@code{w3m-standalone} or @code{html2text}.  If @code{nil} use an
external viewer.  You can also specify a function, which will be
called with a @acronym{MIME} handle as the argument.

@item mm-inline-text-html-with-images
@vindex mm-inline-text-html-with-images
Some @acronym{HTML} mails might have the trick of spammers using
@samp{<img>} tags.  It is likely to be intended to verify whether you
have read the mail.  You can prevent your personal informations from
leaking by setting this option to @code{nil} (which is the default).
It is currently ignored by Emacs/w3.  For emacs-w3m, you may use the
command @kbd{t} on the image anchor to show an image even if it is
@code{nil}.@footnote{The command @kbd{T} will load all images.  If you
have set the option @code{w3m-key-binding} to @code{info}, use @kbd{i}
or @kbd{I} instead.}

@item mm-w3m-safe-url-regexp
@vindex mm-w3m-safe-url-regexp
A regular expression that matches safe URL names, i.e. URLs that are
unlikely to leak personal information when rendering @acronym{HTML}
email (the default value is @samp{\\`cid:}).  If @code{nil} consider
all URLs safe.

@item mm-inline-text-html-with-w3m-keymap
@vindex mm-inline-text-html-with-w3m-keymap
You can use emacs-w3m command keys in the inlined text/html part by
setting this option to non-@code{nil}.  The default value is @code{t}.

@item mm-external-terminal-program
@vindex mm-external-terminal-program
The program used to start an external terminal.

@item mm-enable-external
@vindex mm-enable-external
Indicate whether external @acronym{MIME} handlers should be used.

If @code{t}, all defined external @acronym{MIME} handlers are used.  If
@code{nil}, files are saved to disk (@code{mailcap-save-binary-file}).
If it is the symbol @code{ask}, you are prompted before the external
@acronym{MIME} handler is invoked.

When you launch an attachment through mailcap (@pxref{mailcap}) an
attempt is made to use a safe viewer with the safest options---this isn't
the case if you save it to disk and launch it in a different way
(command line or double-clicking).  Anyhow, if you want to be sure not
to launch any external programs, set this variable to @code{nil} or
@code{ask}.

@end table

@node Files and Directories
@section Files and Directories

@table @code

@item mm-default-directory
@vindex mm-default-directory
The default directory for saving attachments.  If @code{nil} use
@code{default-directory}.

@item mm-tmp-directory
@vindex mm-tmp-directory
Directory for storing temporary files.

@item mm-file-name-rewrite-functions
@vindex mm-file-name-rewrite-functions
A list of functions used for rewriting file names of @acronym{MIME}
parts.  Each function is applied successively to the file name.
Ready-made functions include

@table @code
@item mm-file-name-delete-control
@findex mm-file-name-delete-control
Delete all control characters.

@item mm-file-name-delete-gotchas
@findex mm-file-name-delete-gotchas
Delete characters that could have unintended consequences when used
with flawed shell scripts, i.e. @samp{|}, @samp{>} and @samp{<}; and
@samp{-}, @samp{.} as the first character.

@item mm-file-name-delete-whitespace
@findex mm-file-name-delete-whitespace
Remove all whitespace.

@item mm-file-name-trim-whitespace
@findex mm-file-name-trim-whitespace
Remove leading and trailing whitespace.

@item mm-file-name-collapse-whitespace
@findex mm-file-name-collapse-whitespace
Collapse multiple whitespace characters.

@item mm-file-name-replace-whitespace
@findex mm-file-name-replace-whitespace
@vindex mm-file-name-replace-whitespace
Replace whitespace with underscores.  Set the variable
@code{mm-file-name-replace-whitespace} to any other string if you do
not like underscores.
@end table

The standard Emacs functions @code{capitalize}, @code{downcase},
@code{upcase} and @code{upcase-initials} might also prove useful.

@item mm-path-name-rewrite-functions
@vindex mm-path-name-rewrite-functions
List of functions used for rewriting the full file names of @acronym{MIME}
parts.  This is used when viewing parts externally, and is meant for
transforming the absolute name so that non-compliant programs can find
the file where it's saved.

@end table

@node New Viewers
@section New Viewers

Here's an example viewer for displaying @code{text/enriched} inline:

@lisp
(defun mm-display-enriched-inline (handle)
  (let (text)
    (with-temp-buffer
      (mm-insert-part handle)
      (save-window-excursion
        (enriched-decode (point-min) (point-max))
        (setq text (buffer-string))))
    (mm-insert-inline handle text)))
@end lisp

We see that the function takes a @acronym{MIME} handle as its parameter.  It
then goes to a temporary buffer, inserts the text of the part, does some
work on the text, stores the result, goes back to the buffer it was
called from and inserts the result.

The two important helper functions here are @code{mm-insert-part} and
@code{mm-insert-inline}.  The first function inserts the text of the
handle in the current buffer.  It handles charset and/or content
transfer decoding.  The second function just inserts whatever text you
tell it to insert, but it also sets things up so that the text can be
``undisplayed'' in a convenient manner.


@node Composing
@chapter Composing
@cindex Composing
@cindex MIME Composing
@cindex MML
@cindex MIME Meta Language

Creating a @acronym{MIME} message is boring and non-trivial.  Therefore,
a library called @code{mml} has been defined that parses a language
called @acronym{MML} (@acronym{MIME} Meta Language) and generates
@acronym{MIME} messages.

@findex mml-generate-mime
The main interface function is @code{mml-generate-mime}.  It will
examine the contents of the current (narrowed-to) buffer and return a
string containing the @acronym{MIME} message.

@menu
* Simple MML Example::             An example @acronym{MML} document.
* MML Definition::                 All valid @acronym{MML} elements.
* Advanced MML Example::           Another example @acronym{MML} document.
* Encoding Customization::         Variables that affect encoding.
* Charset Translation::            How charsets are mapped from @sc{mule} to @acronym{MIME}.
* Conversion::                     Going from @acronym{MIME} to @acronym{MML} and vice versa.
* Flowed text::                    Soft and hard newlines.
@end menu


@node Simple MML Example
@section Simple MML Example

Here's a simple @samp{multipart/alternative}:

@example
<#multipart type=alternative>
This is a plain text part.
<#part type=text/enriched>
<center>This is a centered enriched part</center>
<#/multipart>
@end example

After running this through @code{mml-generate-mime}, we get this:

@example
Content-Type: multipart/alternative; boundary="=-=-="


--=-=-=


This is a plain text part.

--=-=-=
Content-Type: text/enriched


<center>This is a centered enriched part</center>

--=-=-=--
@end example


@node MML Definition
@section MML Definition

The @acronym{MML} language is very simple.  It looks a bit like an SGML
application, but it's not.

The main concept of @acronym{MML} is the @dfn{part}.  Each part can be of a
different type or use a different charset.  The way to delineate a part
is with a @samp{<#part ...>} tag.  Multipart parts can be introduced
with the @samp{<#multipart ...>} tag.  Parts are ended by the
@samp{<#/part>} or @samp{<#/multipart>} tags.  Parts started with the
@samp{<#part ...>} tags are also closed by the next open tag.

There's also the @samp{<#external ...>} tag.  These introduce
@samp{external/message-body} parts.

Each tag can contain zero or more parameters on the form
@samp{parameter=value}.  The values may be enclosed in quotation marks,
but that's not necessary unless the value contains white space.  So
@samp{filename=/home/user/#hello$^yes} is perfectly valid.

The following parameters have meaning in @acronym{MML}; parameters that have no
meaning are ignored.  The @acronym{MML} parameter names are the same as the
@acronym{MIME} parameter names; the things in the parentheses say which
header it will be used in.

@table @samp
@item type
The @acronym{MIME} type of the part (@code{Content-Type}).

@item filename
Use the contents of the file in the body of the part
(@code{Content-Disposition}).

@item charset
The contents of the body of the part are to be encoded in the character
set specified (@code{Content-Type}). @xref{Charset Translation}.

@item name
Might be used to suggest a file name if the part is to be saved
to a file (@code{Content-Type}).

@item disposition
Valid values are @samp{inline} and @samp{attachment}
(@code{Content-Disposition}).

@item encoding