This file documents the Emacs MIME interface functionality.

Copyright (C) 1996 Free Software Foundation, Inc.

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

This manual documents the libraries used to compose and display 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 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.

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).

mail-parse
==========

It is perhaps misleading to place the mail-parse library in this chapter.  It is not a basic low-level library---rather, it is an abstraction over the actual low-level libraries that are described in the subsequent sections.

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-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.

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 drums, rfc2047 and rfc2231.  These are documented in the subsequent sections.

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 return the value of the specified attribute.

drums
=====

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

The functions provided by this library include:

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

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

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

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

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

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

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-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 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 converts 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.

     (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-day '(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

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.

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.

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/*; xv -8 %s
     audio/x-pn-realaudio; rvplayer %s

This says that all image files should be displayed with xv, and that realaudio files should be played by rvplayer.

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.

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.

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.