emacs/doc/misc/emacs-mime.texi

\input texinfo

@setfilename ../../info/emacs-mime.info
@settitle Emacs MIME Manual
@include docstyle.texi
@synindex fn cp
@synindex vr cp
@synindex pg cp

@copying
This file documents the Emacs MIME interface functionality.

Copyright @copyright{} 1998--2020 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 1.3 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''.

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@c Node ``Interface Functions'' uses non-ASCII characters

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

@titlepage
@ifset WEBHACKDEVEL
@title Emacs MIME Manual (DEVELOPMENT VERSION)
@end ifset
@ifclear WEBHACKDEVEL
@title Emacs MIME Manual
@end ifclear

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

@contents

@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.

@ifnottex
@insertcopying
@end ifnottex

@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
@findex 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
@findex verbatim-marks
Slrn-style verbatim marks.

@item LaTeX
@findex 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
This variable is @code{resize} by default, which means that images
that are bigger than the Emacs window are resized so that they fit.
If you set this to @code{nil}, large images are not displayed in
Emacs, but can instead be displayed 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-large-images-proportion
@vindex mm-inline-images-max-proportion
The proportion used when resizing large images.

@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{shr},
@code{gnus-w3m}, @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}.  You can also specify a function, which will be
called with a @acronym{MIME} handle as the argument.

@item mm-html-inhibit-images
@vindex mm-html-inhibit-images
@vindex mm-inline-text-html-with-images
If this is non-@code{nil}, inhibit displaying of images inline in the
article body.  It is effective to images in @acronym{HTML} articles
rendered when @code{mm-text-html-renderer} (@pxref{Display
Customization}) is @code{shr} or @code{w3m}.  In Gnus, this is
overridden by the value of @code{gnus-inhibit-images} (@pxref{Misc
Article, ,Misc Article, gnus, Gnus manual}).  The default is @code{nil}.

@item mm-html-blocked-images
@vindex mm-html-blocked-images
External images that have @acronym{URL}s that match this regexp won't
be fetched and displayed.  For instance, to block all @acronym{URL}s
that have the string ``ads'' in them, do the following:

@lisp
(setq mm-html-blocked-images "ads")
@end lisp

It is effective when @code{mm-text-html-renderer} (@pxref{Display
Customization}) is @code{shr}.  In Gnus, this is overridden by the value
of @code{gnus-blocked-images} or the return value of the function that
@code{gnus-blocked-images} is set to (@pxref{HTML, ,HTML, gnus, Gnus
manual}).

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 information from
leaking by setting this option to @code{""} (which is the default).

@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.  In Gnus, this will be overridden according to the value
of the variable @code{gnus-safe-html-newsgroups}, @xref{Various
Various, ,Various Various, gnus, Gnus Manual}.

@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.

If you want to talk about MML in a message, you need a way to
``quote'' these tags.  The way to do that is to include an exclamation
point after the opening two characters; i. e. @samp{<#!part ...>}.

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 recipient-filename
Use this as the file name in the generated @acronym{MIME} message for
the recipient.  That is, even if the file is called @file{foo.txt}
locally, use this name instead in the @code{Content-Disposition} in
the sent message.

@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
Date when the part was created (@code{Content-Disposition}).
This uses the format of RFC 822 or its successors.

@item modification-date
RFC 822 (or later) date when the part was modified
(@code{Content-Disposition}).

@item read-date
RFC 822 (or later) 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
RFC 822 (or later) 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
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--=-=-=
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 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}).

As different hierarchies prefer different charsets, you may want to set
@code{mm-coding-system-priorities} according to the hierarchy in Gnus.
Here's an example:

@c Corrections about preferred charsets are welcome.  de, fr and fj
@c should be correct, I don't know about the rest (so these are only
@c examples):
@lisp
(add-to-list 'gnus-newsgroup-variables 'mm-coding-system-priorities)
(setq gnus-parameters
      (nconc
       ;; Some charsets are just examples!
       '(("^cn\\." ;; Chinese
          (mm-coding-system-priorities
           '(iso-8859-1 cn-big5 chinese-iso-7bit utf-8)))
         ("^cz\\.\\|^pl\\." ;; Central and Eastern European
          (mm-coding-system-priorities '(iso-8859-2 utf-8)))
         ("^de\\." ;; German language
          (mm-coding-system-priorities '(iso-8859-1 iso-8859-15 utf-8)))
         ("^fr\\." ;; French
          (mm-coding-system-priorities '(iso-8859-15 iso-8859-1 utf-8)))
         ("^fj\\." ;; Japanese
          (mm-coding-system-priorities
           '(iso-8859-1 iso-2022-jp utf-8)))
         ("^ru\\." ;; Cyrillic
          (mm-coding-system-priorities
           '(koi8-r iso-8859-5 iso-8859-1 utf-8))))
       gnus-parameters))
@end lisp

@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.
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.

You can customize the value of the @code{mml-enable-flowed} variable
to enable or disable the flowed encoding usage when newline
characters are present in the buffer.

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}.

@table @code
@item mm-fill-flowed
@vindex mm-fill-flowed
If non-@code{nil} a format=flowed article will be displayed flowed.
@end table


@node Interface Functions
@chapter Interface Functions
@cindex interface functions
@cindex @code{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 Nikšić <hniksic@@srce.hr>")
@result{} ("hniksic@@srce.hr" . "Hrvoje Nikšić")
@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 Nikšić <hniksic@@srce.hr>, Steinar Bang <sb@@metis.no>")
@result{} (("hniksic@@srce.hr" . "Hrvoje Nikšić")
     ("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,
@samp{Naïve} is encoded as @samp{=?iso-8859-1?q?Na=EFve?=}.

@item mail-encode-encoded-word-buffer
@findex mail-encode-encoded-word-buffer
Encode the non-@acronym{ASCII} words in the current buffer.  This function is
meant to be called narrowed to the headers of a message.

@item mail-encode-encoded-word-string
@findex mail-encode-encoded-word-string
Encode the words that need encoding in a string, and return the result.

@example
(mail-encode-encoded-word-string
 "This is naïve, baby")
@result{} "This is =?iso-8859-1?q?na=EFve,?= baby"
@end example

@item mail-decode-encoded-word-region
@findex mail-decode-encoded-word-region
Decode the encoded words in the region.

@item mail-decode-encoded-word-string
@findex mail-decode-encoded-word-string
Decode the encoded words in the string and return the result.

@example
(mail-decode-encoded-word-string
 "This is =?iso-8859-1?q?na=EFve,?= baby")
@result{} "This is naïve, baby"
@end example

@end table

Currently, @code{mail-parse} is an abstraction over @code{ietf-drums},
@code{rfc2047}, @code{rfc2045} and @code{rfc2231}.  These are documented
in the subsequent sections.



@node Basic Functions
@chapter Basic Functions

This chapter describes the basic, ground-level functions for parsing and
handling.  Covered here is parsing @code{From} lines, removing comments
from header lines, decoding encoded words, parsing date headers and so
on.  High-level functionality is dealt with in the first chapter
(@pxref{Decoding and Viewing}).

@menu
* rfc2045::      Encoding @code{Content-Type} headers.
* rfc2231::      Parsing @code{Content-Type} headers.
* ietf-drums::   Handling mail headers defined by RFC 2822.
* rfc2047::      En/decoding encoded words in headers.
* time-date::    Functions for parsing dates and manipulating time.
* qp::           Quoted-Printable en/decoding.
* base64::       Base64 en/decoding.
* binhex::       Binhex decoding.
* uudecode::     Uuencode decoding.
* yenc::         Yenc decoding.
* rfc1843::      Decoding HZ-encoded text.
* mailcap::      How parts are displayed is specified by the @file{.mailcap} file
@end menu


@node rfc2045
@section rfc2045

RFC2045 is the ``main'' @acronym{MIME} document, and as such, one would
imagine that there would be a lot to implement.  But there isn't, since
most of the implementation details are delegated to the subsequent
RFCs.

So @file{rfc2045.el} has only a single function:

@table @code
@item rfc2045-encode-string
@findex rfc2045-encode-string
Takes a parameter and a value and returns a @samp{PARAM=VALUE} string.
@var{value} will be quoted if there are non-safe characters in it.
@end table


@node rfc2231
@section rfc2231

RFC2231 defines a syntax for the @code{Content-Type} and
@code{Content-Disposition} headers.  Its snappy name is @dfn{MIME
Parameter Value and Encoded Word Extensions: Character Sets, Languages,
and Continuations}.

In short, these headers look something like this:

@example
Content-Type: application/x-stuff;
 title*0*=us-ascii'en'This%20is%20even%20more%20;
 title*1*=%2A%2A%2Afun%2A%2A%2A%20;
 title*2="isn't it!"
@end example

They usually aren't this bad, though.

The following functions are defined by this library:

@table @code
@item rfc2231-parse-string
@findex rfc2231-parse-string
Parse a @code{Content-Type} header and return a list describing its
elements.

@example
(rfc2231-parse-string
 "application/x-stuff;
 title*0*=us-ascii'en'This%20is%20even%20more%20;
 title*1*=%2A%2A%2Afun%2A%2A%2A%20;
 title*2=\"isn't it!\"")
@result{} ("application/x-stuff"
    (title . "This is even more ***fun*** isn't it!"))
@end example

@item rfc2231-get-value
@findex rfc2231-get-value
Takes one of the lists on the format above and returns
the value of the specified attribute.

@item rfc2231-encode-string
@findex rfc2231-encode-string
Encode a parameter in headers likes @code{Content-Type} and
@code{Content-Disposition}.

@end table


@node ietf-drums
@section ietf-drums

@dfn{drums} was an IETF working group that worked on Internet RFC 2822,
the first successor to RFC 822 and a predecessor of the current email standard.

The functions provided by this library include:

@table @code
@item ietf-drums-remove-comments
@findex ietf-drums-remove-comments
Remove the comments from the argument and return the results.

@item ietf-drums-remove-whitespace
@findex ietf-drums-remove-whitespace
Remove linear white space from the string and return the results.
Spaces inside quoted strings and comments are left untouched.

@item ietf-drums-get-comment
@findex ietf-drums-get-comment
Return the last most comment from the string.

@item ietf-drums-parse-address
@findex ietf-drums-parse-address
Parse an address string and return a list that contains the mailbox and
the plain text name.

@item ietf-drums-parse-addresses
@findex ietf-drums-parse-addresses
Parse a string that contains any number of comma-separated addresses and
return a list that contains mailbox/plain text pairs.

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

@item ietf-drums-narrow-to-header
@findex ietf-drums-narrow-to-header
Narrow the buffer to the header section of the current buffer.

@end table


@node rfc2047
@section rfc2047

RFC2047 (Message Header Extensions for Non-@acronym{ASCII} Text) specifies how
non-@acronym{ASCII} text in headers are to be encoded.  This is actually rather
complicated, so a number of variables are necessary to tweak what this
library does.

The following variables are tweakable:

@table @code
@item rfc2047-header-encoding-alist
@vindex rfc2047-header-encoding-alist
This is an alist of header / encoding-type pairs.  Its main purpose is
to prevent encoding of certain headers.

The keys can either be header regexps, or @code{t}.

The values can be @code{nil}, in which case the header(s) in question
won't be encoded, @code{mime}, which means that they will be encoded, or
@code{address-mime}, which means the header(s) will be encoded carefully
assuming they contain addresses.

@item rfc2047-charset-encoding-alist
@vindex rfc2047-charset-encoding-alist
RFC2047 specifies two forms of encoding---@code{Q} (a
Quoted-Printable-like encoding) and @code{B} (base64).  This alist
specifies which charset should use which encoding.

@item rfc2047-encode-function-alist
@vindex rfc2047-encode-function-alist
This is an alist of encoding / function pairs.  The encodings are
@code{Q}, @code{B} and @code{nil}.

@item rfc2047-encoded-word-regexp
@vindex rfc2047-encoded-word-regexp
When decoding words, this library looks for matches to this regexp.

@item rfc2047-encoded-word-regexp-loose
@vindex rfc2047-encoded-word-regexp-loose
This is a version from which the regexp for the Q encoding pattern of
@code{rfc2047-encoded-word-regexp} is made loose.

@item rfc2047-encode-encoded-words
@vindex rfc2047-encode-encoded-words
The boolean variable specifies whether encoded words
(e.g., @samp{=?us-ascii?q?hello?=}) should be encoded again.
@code{rfc2047-encoded-word-regexp} is used to look for such words.

@item rfc2047-allow-irregular-q-encoded-words
@vindex rfc2047-allow-irregular-q-encoded-words
The boolean variable specifies whether irregular Q encoded words
(e.g., @samp{=?us-ascii?q?hello??=}) should be decoded.  If it is
non-@code{nil}, @code{rfc2047-encoded-word-regexp-loose} is used instead
of @code{rfc2047-encoded-word-regexp} to look for encoded words.

@end table

Those were the variables, and these are this functions:

@table @code
@item rfc2047-narrow-to-field
@findex rfc2047-narrow-to-field
Narrow the buffer to the header on the current line.

@item rfc2047-encode-message-header
@findex rfc2047-encode-message-header
Should be called narrowed to the header of a message.  Encodes according
to @code{rfc2047-header-encoding-alist}.

@item rfc2047-encode-region
@findex rfc2047-encode-region
Encodes all encodable words in the region specified.

@item rfc2047-encode-string
@findex rfc2047-encode-string
Encode a string and return the results.

@item rfc2047-decode-region
@findex rfc2047-decode-region
Decode the encoded words in the region.

@item rfc2047-decode-string
@findex rfc2047-decode-string
Decode a string and return the results.

@item rfc2047-encode-parameter
@findex rfc2047-encode-parameter
Encode a parameter in the RFC2047-like style.  This is a substitution
for the @code{rfc2231-encode-string} function, that is the standard but
many mailers don't support it.  @xref{rfc2231}.

@end table


@node time-date
@section time-date

While not really a part of the @acronym{MIME} library, it is convenient to
document time conversion functions often used when parsing @code{Date} headers
and manipulating time.  (Not by using tesseracts, though, I'm sorry to
say.)

These functions convert between five formats: A date string, a Lisp
timestamp, a decoded time list, a second number, and a day number.

Here's a bunch of time/date/second/day examples:

@example
(parse-time-string "Sat Sep 12 12:21:54 1998 +0200")
@result{} (54 21 12 12 9 1998 6 -1 7200)

(time-convert
  (date-to-time "Sat Sep 12 12:21:54 1998 +0200")
  'integer)
@result{} 905595714

(float-time '(905595714000000 . 1000000))
@result{} 905595714.0

(time-convert 905595714.0 1000000)
@result{} (905595714000000 . 1000000)

(time-to-days '(905595714000000 . 1000000))
@result{} 729644

(time-convert (days-to-time 729644) 'integer)
@result{} 63041241600

(time-convert (time-since '(905595714000000 . 1000000))
              1000000)
@result{} (631963244775642171 . 1000000000)

(time-less-p '(905595714000000 . 1000000)
             '(905595593000000000 . 1000000000))
@result{} nil

(time-equal-p '(905595593000000000 . 1000000000)
              '(905595593000000    . 1000000   ))
@result{} t

(time-subtract '(905595714000000 . 1000000)
               '(905595593000000000 . 1000000000))
@result{} (121000000 . 1000000)

(days-between "Sat Sep 12 12:21:54 1998 +0200"
              "Sat Sep 07 12:21:54 1998 +0200")
@result{} 5

(date-leap-year-p 2000)
@result{} t

(time-to-day-in-year '(905595714000000 . 1000000))
@result{} 255

(time-to-number-of-days
 (time-since
  (date-to-time "Mon, 01 Jan 2001 02:22:26 GMT")))
@result{} 6472.722661506652
@end example

And finally, we have @code{safe-date-to-time}, which does the same as
@code{date-to-time}, but returns a zero time if the date is
syntactically malformed.

The five data representations used are the following:

@table @var
@item date
An RFC 822 (or similar) date string.  For instance: @code{"Sat Sep 12
12:21:54 1998 +0200"}.

@item time
A Lisp timestamp.
For instance: @code{(905595714000000 . 1000000)}.

@item seconds
An integer or floating point count of seconds.  For instance:
@code{905595714.0}, @code{905595714}.

@item days
An integer number representing the number of days since Sunday,
December 31, 1 BC (Gregorian).  For instance: @code{729644}.

@item decoded time
A list of decoded time.  For instance: @code{(54 21 12 12 9 1998 6 nil
7200)}.
@end table

All the examples above represent the same moment, except that
@var{days} represents the day containing the moment.

These are the functions available:

@table @code
@item date-to-time
Take a date and return a time.

@item time-convert
Take a time and return a timestamp in a specified form.

@item float-time
Take a time and return seconds.

@item encode-time
Take a decoded time and return a timestamp.

@item time-to-days
Take a time and return days.

@item days-to-time
Take days and return a time.

@item date-to-day
Take a date and return days.

@item time-to-number-of-days
Take a time and return the number of days that represents.

@item safe-date-to-time
Take a date and return a time.  If the date is not syntactically valid,
return a ``zero'' time.

@item time-less-p
Take two times and say whether the first time is less (i.e., earlier)
than the second time.  (This is a built-in function.)

@item time-equal-p
Check whether two time values are equal.  The time values need not be
in the same format.  (This is a built-in function.)

@item time-since
Take a time and return a time saying how long it was since that time.

@item time-subtract
Take two times and subtract the second from the first.  I.e., return
the time between the two times.  (This is a built-in function.)

@item days-between
Take two days and return the number of days between those two days.

@item date-leap-year-p
Take a year number and say whether it's a leap year.

@item time-to-day-in-year
Take a time and return the day number within the year that the time is
in.

@end table


@node qp
@section qp

This library deals with decoding and encoding Quoted-Printable text.

Very briefly explained, qp encoding means translating all 8-bit
characters (and lots of control characters) into things that look like
@samp{=EF}; that is, an equal sign followed by the byte encoded as a hex
string.

The following functions are defined by the library:

@table @code
@item quoted-printable-decode-region
@findex quoted-printable-decode-region
QP-decode all the encoded text in the specified region.

@item quoted-printable-decode-string
@findex quoted-printable-decode-string
Decode the QP-encoded text in a string and return the results.

@item quoted-printable-encode-region
@findex quoted-printable-encode-region
QP-encode all the encodable characters in the specified region.  The third
optional parameter @var{fold} specifies whether to fold long lines.
(Long here means 72.)

@item quoted-printable-encode-string
@findex quoted-printable-encode-string
QP-encode all the encodable characters in a string and return the
results.

@end table


@node base64
@section base64
@cindex base64

Base64 is an encoding that encodes three bytes into four characters,
thereby increasing the size by about 33%.  The alphabet used for
encoding is very resistant to mangling during transit.

The following functions are defined by this library:

@table @code
@item base64-encode-region
@findex base64-encode-region
base64 encode the selected region.  Return the length of the encoded
text.  Optional third argument @var{no-line-break} means do not break
long lines into shorter lines.

@item base64-encode-string
@findex base64-encode-string
base64 encode a string and return the result.

@item base64-decode-region
@findex base64-decode-region
base64 decode the selected region.  Return the length of the decoded
text.  If the region can't be decoded, return @code{nil} and don't
modify the buffer.

@item base64-decode-string
@findex base64-decode-string
base64 decode a string and return the result.  If the string can't be
decoded, @code{nil} is returned.

@end table


@node binhex
@section binhex
@cindex binhex
@cindex Apple
@cindex Macintosh

@code{binhex} is an encoding that originated in Macintosh environments.
The following function is supplied to deal with these:

@table @code
@item binhex-decode-region
@findex binhex-decode-region
Decode the encoded text in the region.  If given a third parameter, only
decode the @code{binhex} header and return the filename.

@end table

@node uudecode
@section uudecode
@cindex uuencode
@cindex uudecode

@code{uuencode} is probably still the most popular encoding of binaries
used on Usenet, although @code{base64} rules the mail world.

The following function is supplied by this package:

@table @code
@item uudecode-decode-region
@findex uudecode-decode-region
Decode the text in the region.
@end table


@node yenc
@section yenc
@cindex yenc

@code{yenc} is used for encoding binaries on Usenet.  The following
function is supplied by this package:

@table @code
@item yenc-decode-region
@findex yenc-decode-region
Decode the encoded text in the region.

@end table


@node rfc1843
@section rfc1843
@cindex rfc1843
@cindex HZ
@cindex Chinese

RFC1843 deals with mixing Chinese and @acronym{ASCII} characters in messages.  In
essence, RFC1843 switches between @acronym{ASCII} and Chinese by doing this:

@example
This sentence is in @acronym{ASCII}.
The next sentence is in GB.~@{<:Ky2;S@{#,NpJ)l6HK!#~@}Bye.
@end example

Simple enough, and widely used in China.

The following functions are available to handle this encoding:

@table @code
@item rfc1843-decode-region
Decode HZ-encoded text in the region.

@item rfc1843-decode-string
Decode a HZ-encoded string and return the result.

@end table


@node mailcap
@section mailcap

The @file{~/.mailcap} file is parsed by most @acronym{MIME}-aware message
handlers and describes how elements are supposed to be displayed.
Here's an example file:

@example
image/*; gimp -8 %s
audio/wav; wavplayer %s
application/msword; catdoc %s ; copiousoutput ; nametemplate=%s.doc
@end example

This says that all image files should be displayed with @code{gimp},
that WAVE audio files should be played by @code{wavplayer}, and that
MS-WORD files should be inlined by @code{catdoc}.

The @code{mailcap} library parses this file, and provides functions for
matching types.

@table @code
@item mailcap-mime-data
@vindex mailcap-mime-data
This variable is an alist of alists containing backup viewing rules.

@item mailcap-user-mime-data
@vindex mailcap-user-mime-data
A customizable list of viewers that take preference over
@code{mailcap-mime-data}.

@end table

Interface functions:

@table @code
@item mailcap-parse-mailcaps
@findex mailcap-parse-mailcaps
@vindex mailcap-prefer-mailcap-viewers
Parse the @file{~/.mailcap} file.

@item mailcap-mime-info
Takes a @acronym{MIME} type as its argument and returns the matching viewer.

The @code{mailcap-prefer-mailcap-viewers} variable controls which
viewer is chosen.  The default non-@code{nil} value means that
settings from @file{~/.mailcap} is preferred over system-wide or
Emacs-provided viewer settings.

If @code{nil}, Emacs-provided viewer settings have precedence.  Next,
the most specific viewer has precedence over less specific settings,
no matter if they're system-provided or private, so @samp{image/gif}
in @file{/etc/mailcap} will ``win'' over an @samp{image/*} setting in
@file{~/.mailcap}.

@end table




@node Standards
@chapter Standards

The Emacs @acronym{MIME} library implements handling of various elements
according to a (somewhat) large number of RFCs, drafts and standards
documents.  This chapter lists the relevant ones.  They can all be
fetched from @uref{https://www.rfc-editor.org}.

@table @dfn
@item RFC 5322
Internet Message Format

@item RFC 5536
Netnews Article Format

@item RFC 2045
Format of Internet Message Bodies

@item RFC 2046
Media Types

@item RFC 2047
Message Header Extensions for Non-@acronym{ASCII} Text

@item RFC 6838
Media Type Specifications and Registration Procedures

@item RFC 4289
Registration Procedures (obsoleting RFC 2048)

@item RFC 2049
Conformance Criteria and Examples

@item RFC 2231
@acronym{MIME} Parameter Value and Encoded Word Extensions: Character Sets,
Languages, and Continuations

@item RFC 1843
HZ---A Data Format for Exchanging Files of Arbitrarily Mixed Chinese and
@acronym{ASCII} characters

@item RFC 2387
The @acronym{MIME} Multipart/Related Content-type

@item RFC 6522
@itemx STD 73
The Multipart/Report Media Type for the Reporting of Mail System
Administrative Messages

@item RFC 2183
Communicating Presentation Information in Internet Messages: The
Content-Disposition Header Field

@item RFC 3676
The Text/Plain Format and DelSp Parameters

@end table

@node GNU Free Documentation License
@chapter GNU Free Documentation License
@include doclicense.texi

@node Index
@chapter Index
@printindex cp

@bye


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