2000-11-11 Simon Josefsson <sj@extundo.com>

[gnus] / texi / emacs-mime.texi

\input texinfo                  @c -*-texinfo-*-

@setfilename emacs-mime
@settitle Emacs MIME Manual
@synindex fn cp
@synindex vr cp
@synindex pg cp
@dircategory Editors
@direntry
* Emacs MIME: (emacs-mime).   The MIME de/composition library.
@end direntry
@iftex
@finalout
@end iftex
@setchapternewpage odd

@ifnottex

This file documents the Emacs MIME interface functionality.

Copyright (C) 1998,99,2000 Free Software Foundation, Inc.

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 ifnottex

@tex

@titlepage
@title Emacs MIME Manual

@author by Lars Magne Ingebrigtsen
@page

@vskip 0pt plus 1filll
Copyright @copyright{} 1998,99,2000 Free Software Foundation, Inc.

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 the
Invariant Sections being none, 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 titlepage
@page

@end tex

@node Top
@top Emacs MIME

This manual documents the libraries used to compose and display
@sc{mime} messages.

This is not a manual meant for users; it's a manual directed at people
who want to write functions and commands that manipulate @sc{mime}
elements.

@sc{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-ASCII Text), RFC2048 (Registration
Procedures), RFC2049 (Conformance Criteria and Examples).  It is highly
recommended that anyone who intends writing @sc{mime}-compliant software
read at least RFC2045 and RFC2047.

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


@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 ASCII characters in the
parameter list.  RFC2231 expands on RFC2045 syntax to provide a scheme
for continuation headers and non-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 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.

@item mail-encode-encoded-word-region
@findex mail-encode-encoded-word-region
Encode the non-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-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 next 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 RFC822bis.
* 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.
* 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'' @sc{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} is an IETF working group that is working on the replacement
for RFC822.

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-ASCII Text) specifies how
non-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-default-charset
@vindex rfc2047-default-charset
Characters in this charset should not be decoded by this library.
This defaults to @code{iso-8859-1}.

@item rfc2047-header-encoding-list
@vindex rfc2047-header-encoding-list
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 either @code{nil}, in which case the header(s) in
question won't be encoded, or @code{mime}, which means that they will be
encoded.

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

@item rfc2047-q-encoding-alist
@vindex rfc2047-q-encoding-alist
The @code{Q} encoding isn't quite the same for all headers.  Some
headers allow a narrower range of characters, and that is what this
variable is for.  It's an alist of header regexps / allowable character
ranges.

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

@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.

@end table


@node time-date
@section time-date

While not really a part of the @sc{mime} library, it is convenient to
document this library here.  It deals with 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, an Emacs
time structure, a decoded time list, a second number, and a day number.

The functions have quite self-explanatory names, so the following just
gives an overview of which functions are available.

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

(date-to-time "Sat Sep 12 12:21:54 1998 +0200")
@result{} (13818 19266)

(time-to-seconds '(13818 19266))
@result{} 905595714.0

(seconds-to-time 905595714.0)
@result{} (13818 19266 0)

(time-to-day '(13818 19266))
@result{} 729644

(days-to-time 729644)
@result{} (961933 65536)

(time-since '(13818 19266))
@result{} (0 430)

(time-less-p '(13818 19266) '(13818 19145))
@result{} nil

(subtract-time '(13818 19266) '(13818 19145))
@result{} (0 121)

(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 '(13818 19266))
@result{} 255

@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.



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

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

@example
This sentence is in 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 @sc{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
@end example

This says that all image files should be displayed with @code{gimp}, and
that realaudio files should be played by @code{rvplayer}.

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.

@end table

Interface functions:

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

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

@end table




@node Decoding and Viewing
@chapter Decoding and Viewing

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

The main idea is to first analyze a @sc{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 @sc{mime} message.
* Handles::        Handle manipulations.
* Display::        Displaying handles.
* Customization::  Variables that affect display.
* 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 @sc{mime} article.  If given a multipart message, it will recursively
descend the message, following the structure, and return a tree of
@sc{mime} handles that describes the structure of the message.


@node Handles
@section Handles

A @sc{mime} handle is a list that fully describes a @sc{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 @sc{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-handle-disposition
@findex mm-handle-disposition
Return the description 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 @sc{mime} type can be displayed inline.

@item mm-automatic-display-p
@findex mm-automatic-display-p
Say whether a @sc{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 Customization
@section Customization

@table @code

@item mm-inline-media-tests
This is an alist where the key is a @sc{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
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
@sc{mime} media types.

@item 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-attachment-override-types
Some @sc{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
List of @sc{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/richtech} parts are somewhat unwanted,
then the value of this variable should be set to:

@lisp
("text/html" "text/richtext")
@end lisp

@item mm-inline-large-images-p
When displaying inline images that are larger than the window, XEmacs
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-p
@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.

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

@menu
* Simple MML Example::             An example MML document.
* MML Definition::                 All valid MML elements.
* Advanced MML Example::           Another example MML document.
* Charset Translation::            How charsets are mapped from @sc{mule} to MIME.
* Conversion::                     Going from @sc{mime} to MML and vice versa.
@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 MML language is very simple.  It looks a bit like an SGML
application, but it's not.

The main concept of 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 MML; parameters that have no
meaning are ignored.  The MML parameter names are the same as the
@sc{mime} parameter names; the things in the parentheses say which
header it will be used in.

@table @samp
@item type
The @sc{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 speficied (@code{Content-Type}).

@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}).

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

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

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

@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 @sc{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 Charset Translation
@section Charset Translation
@cindex charsets

During translation from MML to @sc{mime}, for each @sc{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-ASCII (8-bit) characters, the @sc{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
@sc{mime} messages.
@xref{Various Message Variables, , Various Message Variables, message, 
      Message Manual}, for example.)
If there are only ASCII characters, the @sc{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 @sc{mime}
charsets by consulting the variable @code{mm-mime-mule-charset-alist}.
If this results in a single @sc{mime} charset, this is used to encode
the part.  But if the resulting list of @sc{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 @sc{mime} charset.  The part can only be
split at line boundaries, though---if more than one @sc{mime} charset is
required to encode a single line, it is not possible to encode the part.

@node Conversion
@section Conversion

@findex mime-to-mml
A (multipart) @sc{mime} message can be converted to MML with the
@code{mime-to-mml} function.  It works on the message in the current
buffer, and substitutes MML markup for @sc{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 MML
tags.

@findex mml-to-mime
An MML message can be converted back to @sc{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 @sc{mime-to-mml} and then
@sc{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 @sc{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 Standards
@chapter Standards

The Emacs @sc{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 @samp{http://quimby.gnus.org/notes/}.

@table @dfn
@item RFC822
@itemx STD11
Standard for the Format of ARPA Internet Text Messages.

@item RFC1036
Standard for Interchange of USENET Messages

@item RFC2045
Format of Internet Message Bodies

@item RFC2046
Media Types

@item RFC2047
Message Header Extensions for Non-ASCII Text

@item RFC2048
Registration Procedures

@item RFC2049
Conformance Criteria and Examples

@item RFC2231
MIME Parameter Value and Encoded Word Extensions: Character Sets,
Languages, and Continuations

@item RFC1843
HZ - A Data Format for Exchanging Files of Arbitrarily Mixed Chinese and
ASCII characters

@item draft-ietf-drums-msg-fmt-05.txt
Draft for the successor of RFC822

@item RFC2112
The MIME Multipart/Related Content-type

@item RFC1892
The Multipart/Report Content Type for the Reporting of Mail System
Administrative Messages

@item RFC2183
Communicating Presentation Information in Internet Messages: The
Content-Disposition Header Field

@end table


@node Index
@chapter Index
@printindex cp

@summarycontents
@contents
@bye

@c End: