This file documents the Emacs MIME interface functionality.

Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.

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(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.

Emacs MIME
**********

This manual documents the libraries used to compose and display MIME messages.

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

MIME is short for 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 MIME-compliant software read at least RFC2045 and RFC2047.

Decoding and Viewing
********************

This chapter deals with decoding and viewing MIME messages on a higher level.

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

Dissection
==========

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

Non-MIME
========

Gnus also understands some non-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 mm-uu-configure-list.  For example,

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

postscript
     Postscript file.

uu
     Uuencoded file.

binhex
     Binhex encoded file.

yenc
     Yenc encoded file.

shar
     Shar archive file.

forward
     Non-MIME forwarded message.

gnatsweb
     Gnatsweb attachment.

pgp-signed
     PGP signed clear text.

pgp-encrypted
     PGP encrypted clear text.

pgp-key
     PGP public keys.

emacs-sources
     Emacs source code.  This item works only in the groups matching mm-uu-emacs-sources-regexp.

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

Handles
=======

A MIME handle is a list that fully describes a MIME component.

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

mm-handle-buffer
     Return the buffer that holds the contents of the undecoded MIME part.

mm-handle-type
     Return the parsed Content-Type of the part.

mm-handle-encoding
     Return the Content-Transfer-Encoding of the part.

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

mm-handle-set-undisplayer
     Set the undisplayer object.

mm-handle-disposition
     Return the parsed Content-Disposition of the part.

mm-handle-disposition
     Return the description of the part.

mm-get-content-id
     Returns the handle(s) referred to by Content-ID.

Display
=======

Functions for displaying, removing and saving.

mm-display-part
     Display the part.

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

mm-inlinable-p
     Say whether a MIME type can be displayed inline.

mm-automatic-display-p
     Say whether a MIME type should be displayed automatically.

mm-destroy-part
     Free all resources occupied by a part.

mm-save-part
     Offer to save the part in a file.

mm-pipe-part
     Offer to pipe the part to some process.

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

Display Customization
=====================

mm-inline-media-tests
     This is an alist where the key is a MIME type, the second element is a function to display the part inline (i.e., inside Emacs), and the third element is a form to be evaled to say whether the part can be displayed inline.

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

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 MIME media types.

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.

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

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.

mm-attachment-override-types
     Some MIME agents create parts that have a content-disposition of 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.)

mm-discouraged-alternatives
     List of MIME types that are discouraged when viewing 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, text/html parts are very unwanted, and text/richtext parts are somewhat unwanted, you could say something like:

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

mm-inline-large-images
     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 ImageMagick or xv).  Setting this variable to t disables this check and makes the library display all inline images as inline, regardless of their size.

mm-inline-override-types
     mm-inlined-types may include regular expressions, for example to specify that all 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 mm-inlined-types includes text/.*, then including text/html in this variable will cause text/html parts to be treated as attachments.

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

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

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 HTML email (the default value is \\`cid:).  If nil consider all URLs safe.

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-nil.  The default value is t.

mm-external-terminal-program
     The program used to start an external terminal.

Files and Directories
=====================

mm-default-directory
     The default directory for saving attachments.  If nil use default-directory.

mm-tmp-directory
     Directory for storing temporary files.

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

     mm-file-name-delete-whitespace
          Remove all whitespace.

     mm-file-name-trim-whitespace
          Remove leading and trailing whitespace.

     mm-file-name-collapse-whitespace
          Collapse multiple whitespace characters.

     mm-file-name-replace-whitespace
          Replace whitespace with underscores.  Set the variable mm-file-name-replace-whitespace to any other string if you do not like underscores.

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

mm-path-name-rewrite-functions
     List of functions used for rewriting the full file names of 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.

New Viewers
===========

Here's an example viewer for displaying text/enriched inline:

     (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)))

We see that the function takes a 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 mm-insert-part and 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.

Composing
*********

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

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

Simple MML Example
==================

Here's a simple multipart/alternative:

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

After running this through mml-generate-mime, we get this:

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

     --=-=-=

     This is a plain text part.

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

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

     --=-=-=--

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

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

Each tag can contain zero or more parameters on the form parameter=value.  The values may be enclosed in quotation marks, but that's not necessary unless the value contains white space.  So 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 MIME parameter names; the things in the parentheses say which header it will be used in.

type
     The MIME type of the part (Content-Type).

filename
     Use the contents of the file in the body of the part (Content-Disposition).

charset
     The contents of the body of the part are to be encoded in the character set speficied (Content-Type). See Charset Translation.

name
     Might be used to suggest a file name if the part is to be saved to a file (Content-Type).

disposition
     Valid values are inline and attachment (Content-Disposition).

encoding
     Valid values are 7bit, 8bit, quoted-printable and base64 (Content-Transfer-Encoding). See Charset Translation.

description
     A description of the part (Content-Description).

creation-date
     RFC822 date when the part was created (Content-Disposition).

modification-date
     RFC822 date when the part was modified (Content-Disposition).

read-date
     RFC822 date when the part was read (Content-Disposition).

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

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

size
     The size (in octets) of the part (Content-Disposition).

sign
     What technology to sign this MML part with (smime, pgp or pgpmime)

encrypt
     What technology to encrypt this MML part with (smime, pgp or pgpmime)

Parameters for application/octet-stream:

type
     Type of the part; informal---meant for human readers (Content-Type).

Parameters for message/external-body:

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

expiration
     The RFC822 date after which the file may no longer be fetched. (Content-Type.)

size
     The size (in octets) of the file.  (Content-Type.)

permission
     Valid values are read and read-write (Content-Type).

Parameters for sign=smime:

keyfile
     File containing key and certificate for signer.

Parameters for encrypt=smime:

certfile
     File containing certificate for recipient.

Advanced MML Example
====================

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

     <#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>

And this is the resulting MIME message:

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

     --=-=-=

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

     /9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRof
     Hh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/wAALCAAwADABAREA/8QAHwAA
     AQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQR
     BRIhMUEGE1FhByJxFDKBkaEII0KxwRVS0fAkM2JyggkKFhcYGRolJicoKSo0NTY3ODk6Q0RF
     RkdISUpTVFVWV1hZWmNkZWZnaGlqc3R1dnd4eXqDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ip
     qrKztLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uHi4+Tl5ufo6erx8vP09fb3+Pn6/9oACAEB
     AAA/AO/rifFHjldNuGsrDa0qcSSHkA+gHrXKw+LtWLrMb+RgTyhbr+HSug07xNqV9fQtZrNI
     AyiaE/NuBPOOOP0rvRNE880KOC8TbXXGCv1FPqjrF4LDR7u5L7SkTFT/ALWOP1xXgTuXfc7E
     sx6nua6rwp4IvvEM8chCxWxOdzn7wz6V9AaB4S07w9p5itow0rDLSY5Pt9K43xO66P4xs71m
     2QXiGCbA4yOVJ9+1aYORkdK434lyNH4ahCnG66VT9Nj15JFbPdX0MS43M4VQf5/yr2vSpLnw
     5ZW8dlCZ8KFXjOPX0/mK6rSPEGt3Angu44fNEReHYNvIH3TzXDeKNO8RX+kSX2ouZkicTIOc
     L+g7E810ulFjpVtv3bwgB3HJyK5L4quY/C9sVxk3ij/xx6850u7t1mtp/wDlpEw3An3Jr3Dw
     34gsbWza4nBlhC5LDsaW6+IFgupQyCF3iHH7gA7c9R9ay7zx6t7aX9jHC4smhfBkGCvHGfrm
     tLQ7hbnRrV1GPkAP1x1/Hr+Ncr8Vzjwrbf8AX6v/AKA9eQRyYlQk8Yx9K6XTNbkgia2ciSIn
     7p5Ga9Atte0LTLKO6it4i7dVRFJDcZ4PvXN+JvEMF9bILVGXJLSZ4zkjivRPDaeX4b08HOTC
     pOffmua+KkbS+GLVUGT9tT/0B68eeIpIFYjB70+OOVXyoOM9+M1eaWeCLzHPyHGO/NVWvJJm
     jQ8KGH1NfQWhXSXmh2c8eArRLwO3HSv/2Q==

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

     --==-=-=

     This is a plain text part.

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

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

     --==-=-=--

     --=-=-=

     This is a new plain text part.

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

     This plain text part is an attachment.

     --=-=-=--

Encoding Customization
======================

mm-body-charset-encoding-alist
     Mapping from 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

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

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

mm-coding-system-priorities
     Prioritize coding systems to use for outgoing messages.  The default is nil, which means to use the defaults in Emacs.  It is a list of coding system symbols (aliases of coding systems does not work, use M-x describe-coding-system to make sure you are not specifying an alias in this variable).  For example, if you have configured Emacs to use prefer UTF-8, but wish that outgoing messages should be sent in ISO-8859-1 if possible, you can set this variable to (iso-latin-1).  You can override this setting on a per-message basis by using the charset MML tag (see MML Definition).

mm-content-transfer-encoding-defaults
     Mapping from 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 MIME encodings, qp-or-base64 may be used to indicate that for each case the most efficient of quoted-printable and base64 should be used.  You can override this setting on a per-message basis by using the encoding MML tag (see MML Definition).

mm-use-ultra-safe-encoding
     When this is non-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).

Charset Translation
===================

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

If you are running a non-MULE Emacs, this process is simple: If the part contains any non-ASCII (8-bit) characters, the MIME charset given by 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 MIME messages. See Various Message Variables (Message Manual), for example.) If there are only ASCII characters, the MIME charset US-ASCII is used, of course.

Things are slightly more complicated when running Emacs with MULE support.  In this case, a list of the MULE charsets used in the part is obtained, and the MULE charsets are translated to MIME charsets by consulting the variable mm-mime-mule-charset-alist. If this results in a single MIME charset, this is used to encode the part.  But if the resulting list of 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 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 MIME charset.  The part can only be split at line boundaries, though---if more than one MIME charset is required to encode a single line, it is not possible to encode the part.

When running Emacs with 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 mm-coding-system-priorities variable though (see Encoding Customization).

The charset to be used can be overridden by setting the charset MML tag (see 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 mm-body-charset-encoding-alist and mm-content-transfer-encoding-defaults (see Encoding Customization).

Conversion
==========

A (multipart) MIME message can be converted to MML with the mime-to-mml function.  It works on the message in the current buffer, and substitutes MML markup for 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.

An MML message can be converted back to MIME by the mml-to-mime function.

These functions are in certain senses ``lossy''---you will not get back an identical message if you run mime-to-mml and then 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 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.

Flowed text
===========

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

On encoding text, lines terminated by soft newline characters are filled together and wrapped after the column decided by fill-flowed-encode-column.  This variable controls how the text will look in a client that does not support flowed text, the default is to wrap after 66 characters.  If hard newline characters are not present in the buffer, no flow encoding occurs.

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

Interface Functions
*******************

The 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 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 (rfc2047.el, 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 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:

mail-header-parse-content-type
     Parse a Content-Type header and return a list on the following format:

          ("type/subtype"
           (attribute1 . value1)
           (attribute2 . value2)
           ...)

     Here's an example:

          (mail-header-parse-content-type
           "image/gif; name=\"b980912.gif\"")
          => ("image/gif" (name . "b980912.gif"))

mail-header-parse-content-disposition
     Parse a Content-Disposition header and return a list on the same format as the function above.

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

          (mail-content-type-get
           '("image/gif" (name . "b980912.gif")) 'name)
          => "b980912.gif"

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

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

          (mail-header-remove-comments
           "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)")
          => "Gnus/5.070027  "

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

          (mail-header-remove-whitespace
           "image/gif; name=\"Name with spaces\"")
          => "image/gif;name=\"Name with spaces\""

mail-header-get-comment
     Return the last comment in a header.

          (mail-header-get-comment
           "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)")
          => "Finnish Landrace"

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

          (mail-header-parse-address
           "Hrvoje Niksic <hniksic@srce.hr>")
          => ("hniksic@srce.hr" . "Hrvoje Niksic")

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

          (mail-header-parse-addresses
           "Hrvoje Niksic <hniksic@srce.hr>, Steinar Bang <sb@metis.no>")
          => (("hniksic@srce.hr" . "Hrvoje Niksic")
               ("sb@metis.no" . "Steinar Bang"))

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

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

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

mail-header-fold-field
     Fold the header under point.

mail-header-unfold-field
     Unfold the header under point.

mail-header-field-value
     Return the value of the field under point.

mail-encode-encoded-word-region
     Encode the non-ASCII words in the region.  For instance, Naïve is encoded as =?iso-8859-1?q?Na=EFve?=.

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.

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

          (mail-encode-encoded-word-string
           "This is naïve, baby")
          => "This is =?iso-8859-1?q?na=EFve,?= baby"

mail-decode-encoded-word-region
     Decode the encoded words in the region.

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

          (mail-decode-encoded-word-string
           "This is =?iso-8859-1?q?na=EFve,?= baby")
          => "This is naïve, baby"

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

Basic Functions
***************

This chapter describes the basic, ground-level functions for parsing and handling.  Covered here is parsing 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 (see Decoding and Viewing).

rfc2045
=======

RFC2045 is the ``main'' 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 rfc2045.el has only a single function:

rfc2045-encode-string
     Takes a parameter and a value and returns a PARAM=VALUE string. VALUE will be quoted if there are non-safe characters in it.

rfc2231
=======

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

In short, these headers look something like this:

     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!"

They usually aren't this bad, though.

The following functions are defined by this library:

rfc2231-parse-string
     Parse a Content-Type header and return a list describing its elements.

          (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!\"")
          => ("application/x-stuff"
              (title . "This is even more ***fun*** isn't it!"))

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

rfc2231-encode-string
     Encode a parameter in headers likes Content-Type and Content-Disposition.

ietf-drums
==========

drums is an IETF working group that is working on the replacement for RFC822.

The functions provided by this library include:

ietf-drums-remove-comments
     Remove the comments from the argument and return the results.

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

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

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

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.

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

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

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:

rfc2047-default-charset
     Characters in this charset should not be decoded by this library. This defaults to iso-8859-1.

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 t.

     The values can be either nil, in which case the header(s) in question won't be encoded, or mime, which means that they will be encoded.

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

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

rfc2047-q-encoding-alist
     The 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.

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

Those were the variables, and these are this functions:

rfc2047-narrow-to-field
     Narrow the buffer to the header on the current line.

rfc2047-encode-message-header
     Should be called narrowed to the header of a message.  Encodes according to rfc2047-header-encoding-alist.

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

rfc2047-encode-string
     Encode a string and return the results.

rfc2047-decode-region
     Decode the encoded words in the region.

rfc2047-decode-string
     Decode a string and return the results.

time-date
=========

While not really a part of the MIME library, it is convenient to document this library here.  It deals with parsing 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.

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

     (parse-time-string "Sat Sep 12 12:21:54 1998 +0200")
     => (54 21 12 12 9 1998 6 nil 7200)

     (date-to-time "Sat Sep 12 12:21:54 1998 +0200")
     => (13818 19266)

     (time-to-seconds '(13818 19266))
     => 905595714.0

     (seconds-to-time 905595714.0)
     => (13818 19266 0)

     (time-to-days '(13818 19266))
     => 729644

     (days-to-time 729644)
     => (961933 65536)

     (time-since '(13818 19266))
     => (0 430)

     (time-less-p '(13818 19266) '(13818 19145))
     => nil

     (subtract-time '(13818 19266) '(13818 19145))
     => (0 121)

     (days-between "Sat Sep 12 12:21:54 1998 +0200"
                   "Sat Sep 07 12:21:54 1998 +0200")
     => 5

     (date-leap-year-p 2000)
     => t

     (time-to-day-in-year '(13818 19266))
     => 255

     (time-to-number-of-days
      (time-since
       (date-to-time "Mon, 01 Jan 2001 02:22:26 GMT")))
     => 4.146122685185185

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

The five data representations used are the following:

date
     An RFC822 (or similar) date string.  For instance: "Sat Sep 12 12:21:54 1998 +0200".

time
     An internal Emacs time.  For instance: (13818 26466).

seconds
     A floating point representation of the internal Emacs time.  For instance: 905595714.0.

days
     An integer number representing the number of days since 00000101.  For instance: 729644.

decoded time
     A list of decoded time.  For instance: (54 21 12 12 9 1998 6 t 7200).

All the examples above represent the same moment.

These are the functions available:

date-to-time
     Take a date and return a time.

time-to-seconds
     Take a time and return seconds.

seconds-to-time
     Take seconds and return a time.

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

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

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

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

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

time-less-p
     Take two times and say whether the first time is less (i. e., earlier) than the second time.

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

subtract-time
     Take two times and subtract the second from the first.  I. e., return the time between the two times.

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

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

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

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 =EF; that is, an equal sign followed by the byte encoded as a hex string.

The following functions are defined by the library:

quoted-printable-decode-region
     QP-decode all the encoded text in the specified region.

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

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

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

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:

base64-encode-region
     base64 encode the selected region.  Return the length of the encoded text.  Optional third argument NO-LINE-BREAK means do not break long lines into shorter lines.

base64-encode-string
     base64 encode a string and return the result.

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

base64-decode-string
     base64 decode a string and return the result.  If the string can't be decoded, nil is returned.

binhex
======

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

binhex-decode-region
     Decode the encoded text in the region.  If given a third parameter, only decode the binhex header and return the filename.

uudecode
========

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

The following function is supplied by this package:

uudecode-decode-region
     Decode the text in the region.

yenc
====

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

yenc-decode-region
     Decode the encoded text in the region.

rfc1843
=======

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

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

Simple enough, and widely used in China.

The following functions are available to handle this encoding:

rfc1843-decode-region
     Decode HZ-encoded text in the region.

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

mailcap
=======

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

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

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

The mailcap library parses this file, and provides functions for matching types.

mailcap-mime-data
     This variable is an alist of alists containing backup viewing rules.

Interface functions:

mailcap-parse-mailcaps
     Parse the ~/.mailcap file.

mailcap-mime-info
     Takes a MIME type as its argument and returns the matching viewer.

Standards
*********

The Emacs 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 http://quimby.gnus.org/notes/.

RFC822
STD11
     Standard for the Format of ARPA Internet Text Messages.

RFC1036
     Standard for Interchange of USENET Messages

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

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

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

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

RFC2112
     The MIME Multipart/Related Content-type

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

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

RFC2646
     Documentation of the text/plain format parameter for flowed text.