* elisp.texi: Specify GFDL version 1.2.

[bpt/emacs.git] / man / emacs-mime.texi

\input texinfo

@setfilename ../info/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 (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
          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 1.1 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.
* 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}.

@end table

@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

@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
Valid values are @samp{7bit}, @samp{8bit}, @samp{quoted-printable} and
@samp{base64} (@code{Content-Transfer-Encoding}). @xref{Charset
Translation}.

@item description
A description of the part (@code{Content-Description}).

@item creation-date
RFC822 date when the part was created (@code{Content-Disposition}).

@item modification-date
RFC822 date when the part was modified (@code{Content-Disposition}).

@item read-date
RFC822 date when the part was read (@code{Content-Disposition}).

@item recipients
Who to encrypt/sign the part to.  This field is used to override any
auto-detection based on the To/CC headers.

@item sender
Identity used to sign the part.  This field is used to override the
default key used.

@item size
The size (in octets) of the part (@code{Content-Disposition}).

@item sign
What technology to sign this @acronym{MML} part with (@code{smime}, @code{pgp}
or @code{pgpmime})

@item encrypt
What technology to encrypt this @acronym{MML} part with (@code{smime},
@code{pgp} or @code{pgpmime})

@end table

Parameters for @samp{text/plain}:

@table @samp
@item format
Formatting parameter for the text, valid values include @samp{fixed}
(the default) and @samp{flowed}.  Normally you do not specify this
manually, since it requires the textual body to be formatted in a
special way described in RFC 2646.  @xref{Flowed text}.
@end table

Parameters for @samp{application/octet-stream}:

@table @samp
@item type
Type of the part; informal---meant for human readers
(@code{Content-Type}).
@end table

Parameters for @samp{message/external-body}:

@table @samp
@item access-type
A word indicating the supported access mechanism by which the file may
be obtained.  Values include @samp{ftp}, @samp{anon-ftp}, @samp{tftp},
@samp{localfile}, and @samp{mailserver}.  (@code{Content-Type}.)

@item expiration
The RFC822 date after which the file may no longer be fetched.
(@code{Content-Type}.)

@item size
The size (in octets) of the file.  (@code{Content-Type}.)

@item permission
Valid values are @samp{read} and @samp{read-write}
(@code{Content-Type}).

@end table

Parameters for @samp{sign=smime}:

@table @samp

@item keyfile
File containing key and certificate for signer.

@end table

Parameters for @samp{encrypt=smime}:

@table @samp

@item certfile
File containing certificate for recipient.

@end table


@node Advanced MML Example
@section Advanced MML Example

Here's a complex multipart message.  It's a @samp{multipart/mixed} that
contains many parts, one of which is a @samp{multipart/alternative}.

@example
<#multipart type=mixed>
<#part type=image/jpeg filename=~/rms.jpg disposition=inline>
<#multipart type=alternative>
This is a plain text part.
<#part type=text/enriched name=enriched.txt>
<center>This is a centered enriched part</center>
<#/multipart>
This is a new plain text part.
<#part disposition=attachment>
This plain text part is an attachment.
<#/multipart>
@end example

And this is the resulting @acronym{MIME} message:

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


--=-=-=



--=-=-=
Content-Type: image/jpeg;
 filename="~/rms.jpg"
Content-Disposition: inline;
 filename="~/rms.jpg"
Content-Transfer-Encoding: base64

/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRof
Hh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/wAALCAAwADABAREA/8QAHwAA
AQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQR
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5ZW8dlCZ8KFXjOPX0/mK6rSPEGt3Angu44fNEReHYNvIH3TzXDeKNO8RX+kSX2ouZkicTIOc
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tLQ7hbnRrV1GPkAP1x1/Hr+Ncr8Vzjwrbf8AX6v/AKA9eQRyYlQk8Yx9K6XTNbkgia2ciSIn
7p5Ga9Atte0LTLKO6it4i7dVRFJDcZ4PvXN+JvEMF9bILVGXJLSZ4zkjivRPDaeX4b08HOTC
pOffmua+KkbS+GLVUGT9tT/0B68eeIpIFYjB70+OOVXyoOM9+M1eaWeCLzHPyHGO/NVWvJJm
jQ8KGH1NfQWhXSXmh2c8eArRLwO3HSv/2Q==

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


--==-=-=


This is a plain text part.

--==-=-=
Content-Type: text/enriched;
 name="enriched.txt"


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

--==-=-=--

--=-=-=

This is a new plain text part.

--=-=-=
Content-Disposition: attachment


This plain text part is an attachment.

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

@node Encoding Customization
@section Encoding Customization

@table @code

@item mm-body-charset-encoding-alist
@vindex mm-body-charset-encoding-alist
Mapping from @acronym{MIME} charset to encoding to use.  This variable is
usually used except, e.g., when other requirements force a specific
encoding (digitally signed messages require 7bit encodings).  The
default is

@lisp
((iso-2022-jp . 7bit)
 (iso-2022-jp-2 . 7bit)
 (utf-16 . base64)
 (utf-16be . base64)
 (utf-16le . base64))
@end lisp

As an example, if you do not want to have ISO-8859-1 characters
quoted-printable encoded, you may add @code{(iso-8859-1 . 8bit)} to
this variable.  You can override this setting on a per-message basis
by using the @code{encoding} @acronym{MML} tag (@pxref{MML Definition}).

@item mm-coding-system-priorities
@vindex mm-coding-system-priorities
Prioritize coding systems to use for outgoing messages.  The default
is @code{nil}, which means to use the defaults in Emacs, but is
@code{(iso-8859-1 iso-2022-jp iso-2022-jp-2 shift_jis utf-8)} when
running Emacs in the Japanese language environment.  It is a list of
coding system symbols (aliases of coding systems are also allowed, use
@kbd{M-x describe-coding-system} to make sure you are specifying correct
coding system names).  For example, if you have configured Emacs
to prefer UTF-8, but wish that outgoing messages should be sent in
ISO-8859-1 if possible, you can set this variable to
@code{(iso-8859-1)}.  You can override this setting on a per-message
basis by using the @code{charset} @acronym{MML} tag (@pxref{MML Definition}).

@item mm-content-transfer-encoding-defaults
@vindex mm-content-transfer-encoding-defaults
Mapping from @acronym{MIME} types to encoding to use.  This variable is usually
used except, e.g., when other requirements force a safer encoding
(digitally signed messages require 7bit encoding).  Besides the normal
@acronym{MIME} encodings, @code{qp-or-base64} may be used to indicate that for
each case the most efficient of quoted-printable and base64 should be
used.

@code{qp-or-base64} has another effect.  It will fold long lines so that
MIME parts may not be broken by MTA.  So do @code{quoted-printable} and
@code{base64}.

Note that it affects body encoding only when a part is a raw forwarded
message (which will be made by @code{gnus-summary-mail-forward} with the
arg 2 for example) or is neither the @samp{text/*} type nor the
@samp{message/*} type.  Even though in those cases, you can override
this setting on a per-message basis by using the @code{encoding}
@acronym{MML} tag (@pxref{MML Definition}).

@item mm-use-ultra-safe-encoding
@vindex mm-use-ultra-safe-encoding
When this is non-@code{nil}, it means that textual parts are encoded as
quoted-printable if they contain lines longer than 76 characters or
starting with "From " in the body.  Non-7bit encodings (8bit, binary)
are generally disallowed.  This reduce the probability that a non-8bit
clean MTA or MDA changes the message.  This should never be set
directly, but bound by other functions when necessary (e.g., when
encoding messages that are to be digitally signed).

@end table

@node Charset Translation
@section Charset Translation
@cindex charsets

During translation from @acronym{MML} to @acronym{MIME}, for each
@acronym{MIME} part which has been composed inside Emacs, an appropriate
charset has to be chosen.

@vindex mail-parse-charset
If you are running a non-@sc{mule} Emacs, this process is simple: If the
part contains any non-@acronym{ASCII} (8-bit) characters, the @acronym{MIME} charset
given by @code{mail-parse-charset} (a symbol) is used.  (Never set this
variable directly, though.  If you want to change the default charset,
please consult the documentation of the package which you use to process
@acronym{MIME} messages.
@xref{Various Message Variables, , Various Message Variables, message,
      Message Manual}, for example.)
If there are only @acronym{ASCII} characters, the @acronym{MIME} charset US-ASCII is
used, of course.

@cindex MULE
@cindex UTF-8
@cindex Unicode
@vindex mm-mime-mule-charset-alist
Things are slightly more complicated when running Emacs with @sc{mule}
support.  In this case, a list of the @sc{mule} charsets used in the
part is obtained, and the @sc{mule} charsets are translated to
@acronym{MIME} charsets by consulting the table provided by Emacs itself
or the variable @code{mm-mime-mule-charset-alist} for XEmacs.
If this results in a single @acronym{MIME} charset, this is used to encode
the part.  But if the resulting list of @acronym{MIME} charsets contains more
than one element, two things can happen: If it is possible to encode the
part via UTF-8, this charset is used.  (For this, Emacs must support
the @code{utf-8} coding system, and the part must consist entirely of
characters which have Unicode counterparts.)  If UTF-8 is not available
for some reason, the part is split into several ones, so that each one
can be encoded with a single @acronym{MIME} charset.  The part can only be
split at line boundaries, though---if more than one @acronym{MIME} charset is
required to encode a single line, it is not possible to encode the part.

When running Emacs with @sc{mule} support, the preferences for which
coding system to use is inherited from Emacs itself.  This means that
if Emacs is set up to prefer UTF-8, it will be used when encoding
messages.  You can modify this by altering the
@code{mm-coding-system-priorities} variable though (@pxref{Encoding
Customization}).

The charset to be used can be overridden by setting the @code{charset}
@acronym{MML} tag (@pxref{MML Definition}) when composing the message.

The encoding of characters (quoted-printable, 8bit etc) is orthogonal
to the discussion here, and is controlled by the variables
@code{mm-body-charset-encoding-alist} and
@code{mm-content-transfer-encoding-defaults} (@pxref{Encoding
Customization}).

@node Conversion
@section Conversion

@findex mime-to-mml
A (multipart) @acronym{MIME} message can be converted to @acronym{MML}
with the @code{mime-to-mml} function.  It works on the message in the
current buffer, and substitutes @acronym{MML} markup for @acronym{MIME}
boundaries.  Non-textual parts do not have their contents in the buffer,
but instead have the contents in separate buffers that are referred to
from the @acronym{MML} tags.

@findex mml-to-mime
An @acronym{MML} message can be converted back to @acronym{MIME} by the
@code{mml-to-mime} function.

These functions are in certain senses ``lossy''---you will not get back
an identical message if you run @code{mime-to-mml} and then
@code{mml-to-mime}.  Not only will trivial things like the order of the
headers differ, but the contents of the headers may also be different.
For instance, the original message may use base64 encoding on text,
while @code{mml-to-mime} may decide to use quoted-printable encoding, and
so on.

In essence, however, these two functions should be the inverse of each
other.  The resulting contents of the message should remain equivalent,
if not identical.


@node Flowed text
@section Flowed text
@cindex format=flowed

The Emacs @acronym{MIME} library will respect the @code{use-hard-newlines}
variable (@pxref{Hard and Soft Newlines, ,Hard and Soft Newlines,
emacs, Emacs Manual}) when encoding a message, and the
``format=flowed'' Content-Type parameter when decoding a message.

On encoding text, regardless of @code{use-hard-newlines}, lines
terminated by soft newline characters are filled together and wrapped
after the column decided by @code{fill-flowed-encode-column}.
Quotation marks (matching @samp{^>* ?}) are respected.  The variable
controls how the text will look in a client that does not support
flowed text, the default is to wrap after 66 characters.  If hard
newline characters are not present in the buffer, no flow encoding
occurs.

On decoding flowed text, lines with soft newline characters are filled
together and wrapped after the column decided by
@code{fill-flowed-display-column}.  The default is to wrap after
@code{fill-column}.




@node Interface Functions
@chapter Interface Functions
@cindex interface functions
@cindex mail-parse

The @code{mail-parse} library is an abstraction over the actual
low-level libraries that are described in the next chapter.

Standards change, and so programs have to change to fit in the new
mold.  For instance, RFC2045 describes a syntax for the
@code{Content-Type} header that only allows @acronym{ASCII} characters in the
parameter list.  RFC2231 expands on RFC2045 syntax to provide a scheme
for continuation headers and non-@acronym{ASCII} characters.

The traditional way to deal with this is just to update the library
functions to parse the new syntax.  However, this is sometimes the wrong
thing to do.  In some instances it may be vital to be able to understand
both the old syntax as well as the new syntax, and if there is only one
library, one must choose between the old version of the library and the
new version of the library.

The Emacs @acronym{MIME} library takes a different tack.  It defines a
series of low-level libraries (@file{rfc2047.el}, @file{rfc2231.el}
and so on) that parses strictly according to the corresponding
standard.  However, normal programs would not use the functions
provided by these libraries directly, but instead use the functions
provided by the @code{mail-parse} library.  The functions in this
library are just aliases to the corresponding functions in the latest
low-level libraries.  Using this scheme, programs get a consistent
interface they can use, and library developers are free to create
write code that handles new standards.

The following functions are defined by this library:

@table @code
@item mail-header-parse-content-type
@findex mail-header-parse-content-type
Parse a @code{Content-Type} header and return a list on the following
format:

@lisp
("type/subtype"
 (attribute1 . value1)
 (attribute2 . value2)
 ...)
@end lisp

Here's an example:

@example
(mail-header-parse-content-type
 "image/gif; name=\"b980912.gif\"")
@result{} ("image/gif" (name . "b980912.gif"))
@end example

@item mail-header-parse-content-disposition
@findex mail-header-parse-content-disposition
Parse a @code{Content-Disposition} header and return a list on the same
format as the function above.

@item mail-content-type-get
@findex mail-content-type-get
Takes two parameters---a list on the format above, and an attribute.
Returns the value of the attribute.

@example
(mail-content-type-get
 '("image/gif" (name . "b980912.gif")) 'name)
@result{} "b980912.gif"
@end example

@item mail-header-encode-parameter
@findex mail-header-encode-parameter
Takes a parameter string and returns an encoded version of the string.
This is used for parameters in headers like @code{Content-Type} and
@code{Content-Disposition}.

@item mail-header-remove-comments
@findex mail-header-remove-comments
Return a comment-free version of a header.

@example
(mail-header-remove-comments
 "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)")
@result{} "Gnus/5.070027  "
@end example

@item mail-header-remove-whitespace
@findex mail-header-remove-whitespace
Remove linear white space from a header.  Space inside quoted strings
and comments is preserved.

@example
(mail-header-remove-whitespace
 "image/gif; name=\"Name with spaces\"")
@result{} "image/gif;name=\"Name with spaces\""
@end example

@item mail-header-get-comment
@findex mail-header-get-comment
Return the last comment in a header.

@example
(mail-header-get-comment
 "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)")
@result{} "Finnish Landrace"
@end example

@item mail-header-parse-address
@findex mail-header-parse-address
Parse an address and return a list containing the mailbox and the
plaintext name.

@example
(mail-header-parse-address
 "Hrvoje Niksic <hniksic@@srce.hr>")
@result{} ("hniksic@@srce.hr" . "Hrvoje Niksic")
@end example

@item mail-header-parse-addresses
@findex mail-header-parse-addresses
Parse a string with list of addresses and return a list of elements like
the one described above.

@example
(mail-header-parse-addresses
 "Hrvoje Niksic <hniksic@@srce.hr>, Steinar Bang <sb@@metis.no>")
@result{} (("hniksic@@srce.hr" . "Hrvoje Niksic")
     ("sb@@metis.no" . "Steinar Bang"))
@end example

@item mail-header-parse-date
@findex mail-header-parse-date
Parse a date string and return an Emacs time structure.

@item mail-narrow-to-head
@findex mail-narrow-to-head
Narrow the buffer to the header section of the buffer.  Point is placed
at the beginning of the narrowed buffer.

@item mail-header-narrow-to-field
@findex mail-header-narrow-to-field
Narrow the buffer to the header under point.  Understands continuation
headers.

@item mail-header-fold-field
@findex mail-header-fold-field
Fold the header under point.

@item mail-header-unfold-field
@findex mail-header-unfold-field
Unfold the header under point.

@item mail-header-field-value
@findex mail-header-field-value
Return the value of the field under point.

@item mail-encode-encoded-word-region
@findex mail-encode-encoded-word-region
Encode the non-@acronym{ASCII} words in the region.  For instance,