path: root/rfc822/rfc822.sgml

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<refentry id="rfc822">
  <info><author><firstname>Sam</firstname><surname>Varshavchik</surname><contrib>Author</contrib></author><productname>Courier Mail Server</productname></info>

  <refmeta>
    <refentrytitle>rfc822</refentrytitle>
    <manvolnum>3</manvolnum>
    <refmiscinfo class='manual'>Double Precision, Inc.</refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname>rfc822</refname>
    <refpurpose>RFC 822 parsing library</refpurpose>
  </refnamediv>

  <refsynopsisdiv>

    <informalexample>
      <programlisting format="linespecific">
#include &lt;rfc822.h&gt;

#include &lt;rfc2047.h&gt;

cc ... -lrfc822
</programlisting>
    </informalexample>
  </refsynopsisdiv>

  <refsect1 id="rfc822_description">
    <title>DESCRIPTION</title>

    <para>
The rfc822 library provides functions for parsing E-mail headers in the RFC
822 format. This library also includes some functions to help with encoding
and decoding 8-bit text, as defined by RFC 2047.</para>

    <para>
The format used by E-mail headers to encode sender and recipient
information is defined by
<ulink url="http://www.rfc-editor.org/rfc/rfc822.txt">RFC 822</ulink>
(and its successor,
<ulink url="http://www.rfc-editor.org/rfc/rfc2822.txt">RFC 2822</ulink>).
The format allows the actual E-mail
address and the sender/recipient name to be expressed together, for example:
<literal moreinfo="none">John Smith &lt;jsmith@example.com&gt;</literal></para>

    <para>
The main purposes of the rfc822 library is to:</para>

    <para>
1) Parse a text string containing a list of RFC 822-formatted address into
its logical components: names and E-mail addresses.</para>

    <para>
2) Access those individual components.</para>

    <para>
3) Allow some limited modifications of the parsed structure, and then
convert it back into a text string.</para>

    <refsect2 id="rfc822_tokenizing_an_e_mail_header">
      <title>Tokenizing an E-mail header</title>

      <informalexample>
	<programlisting format="linespecific">
struct rfc822t *tokens=rfc822t_alloc_new(const char *header,
                void (*err_func)(const char *, int, void *),
                void *func_arg);

void rfc822t_free(tokens);
</programlisting>
      </informalexample>

      <para>
The <function moreinfo="none">rfc822t_alloc_new</function>() function (superceeds
<function moreinfo="none">rfc822t_alloc</function>(), which is now
obsolete) accepts an E-mail <parameter moreinfo="none">header</parameter>, and parses it into
individual tokens. This function allocates and returns a pointer to an
<structname>rfc822t</structname>
structure, which is later used by
<function moreinfo="none">rfc822a_alloc</function>() to extract
individual addresses from these tokens.</para>

      <para>
If <parameter moreinfo="none">err_func</parameter> argument, if not NULL, is a pointer
to a callback
function.  The function is called in the event that the E-mail header is
corrupted to the point that it cannot even be parsed.  This is a rare instance
-- most forms of corruption are still valid at least on the lexical level.
The only time this error is reported is in the event of mismatched
parenthesis, angle brackets, or quotes.  The callback function receives the
<parameter moreinfo="none">header</parameter> pointer, an index to the syntax error in the
header string, and the <parameter moreinfo="none">func_arg</parameter> argument.</para>

      <para>
The semantics of <parameter moreinfo="none">err_func</parameter> are subject to change.  It is recommended
to leave this argument as NULL in the current version of the library.</para>

      <para>
<function moreinfo="none">rfc822t_alloc</function>() returns a pointer to a
dynamically-allocated <structname>rfc822t</structname>
structure. A NULL pointer is returned if there's insufficient memory to
allocate this structure. The <function moreinfo="none">rfc822t_free</function>() function
destroys
<structname>rfc822t</structname> structure and frees all
dynamically allocated memory.</para>

      <note>
	<para>
Until <function moreinfo="none">rfc822t_free</function>() is called, the contents of
<parameter moreinfo="none">header</parameter> MUST
NOT be destroyed or altered in any way. The contents of
<parameter moreinfo="none">header</parameter> are not
modified by <function moreinfo="none">rfc822t_alloc</function>(), however the
<structname>rfc822t</structname> structure contains
pointers to portions of the supplied <parameter moreinfo="none">header</parameter>,
and they must remain valid.</para>
      </note>
    </refsect2>

    <refsect2 id="rfc822_extracting_e_mail_addresses">
      <title>Extracting E-mail addresses</title>

      <informalexample>
	<programlisting format="linespecific">
struct rfc822a *addrs=rfc822a_alloc(struct rfc822t *tokens);

void rfc822a_free(addrs);
</programlisting>
      </informalexample>

      <para>
The <function moreinfo="none">rfc822a_alloc</function>() function returns a
dynamically-allocated <structname>rfc822a</structname>
structure, that contains individual addresses that were logically parsed
from a <structname>rfc822t</structname> structure.  The
<function moreinfo="none">rfc822a_alloc</function>() function returns NULL if
there was insufficient memory to allocate the <structname>rfc822a</structname> structure. The
<function moreinfo="none">rfc822a_free</function>() function destroys the <structname>rfc822a</structname> function, and frees all
associated dynamically-allocated memory. The <structname>rfc822t</structname> structure passed
to <function moreinfo="none">rfc822a_alloc</function>() must not be destroyed before <function moreinfo="none">rfc822a_free</function>() destroys the
<structname>rfc822a</structname> structure.</para>

      <para>
The <structname>rfc822a</structname> structure has the following fields:</para>
      <informalexample>
	<programlisting format="linespecific">
struct rfc822a {
        struct rfc822addr *addrs;
        int     naddrs;
} ;
</programlisting>
      </informalexample>

      <para>
The <structfield>naddrs</structfield> field gives the number of
<structname>rfc822addr</structname> structures
that are pointed to by <structfield>addrs</structfield>, which is an array.
Each <structname>rfc822addr</structname>
structure represents either an address found in the original E-mail header,
<emphasis>or the contents of some legacy "syntactical sugar"</emphasis>.
For example, the
following is a valid E-mail header:</para>

      <informalexample>
	<programlisting format="linespecific">
To: recipient-list: tom@example.com, john@example.com;
</programlisting>
      </informalexample>

      <para>Typically, all of this, except for "<literal moreinfo="none">To:</literal>",
is tokenized by <function moreinfo="none">rfc822t_alloc</function>(), then parsed by
<function moreinfo="none">rfc822a_alloc</function>().
"<literal moreinfo="none">recipient-list:</literal>" and
the trailing semicolon is a legacy mailing list specification that is no
longer in widespread use, but must still must be accounted for. The resulting
<structname>rfc822a</structname> structure will have four
<structname>rfc822addr</structname> structures: one for
"<literal moreinfo="none">recipient-list:</literal>";
one for each address; and one for the trailing semicolon.
Each <structname>rfc822a</structname> structure has the following
fields:</para>
      <informalexample>
	<programlisting format="linespecific">
struct rfc822addr {
        struct rfc822token *tokens;
        struct rfc822token *name;
} ;
</programlisting>
      </informalexample>

      <para>
If <structfield>tokens</structfield> is a null pointer, this structure
represents some
non-address portion of the original header, such as
"<literal moreinfo="none">recipient-list:</literal>" or a
semicolon.  Otherwise it points to a structure that represents the E-mail
address in tokenized form.</para>

      <para>
<structfield>name</structfield> either points to the tokenized form of a
non-address portion of
the original header, or to a tokenized form of the recipient's name.
<structfield>name</structfield> will be NULL if the recipient name was not provided. For the
following address:
<literal moreinfo="none">Tom Jones &lt;tjones@example.com&gt;</literal> - the
<structfield>tokens</structfield> field points to the tokenized form of
"<literal moreinfo="none">tjones@example.com</literal>",
and <structfield>name</structfield> points to the tokenized form of
"<literal moreinfo="none">Tom Jones</literal>".</para>

      <para>
Each <structname>rfc822token</structname> structure contains the following
fields:</para>
      <informalexample>
	<programlisting format="linespecific">
struct rfc822token {
        struct rfc822token *next;
        int token;
        const char *ptr;
        int len;
} ;
</programlisting>
      </informalexample>

      <para>
The <structfield>next</structfield> pointer builds a linked list of all
tokens in this name or
address.  The possible values for the <structfield>token</structfield> field
are:</para>

      <variablelist>
	<varlistentry>
	  <term>0x00</term>
	  <listitem>
	    <para>
This is a simple atom - a sequence of non-special characters that
is delimited by whitespace or special characters (see below).</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>0x22</term>
	  <listitem>
	    <para>
The value of the ascii quote - this is a quoted string.</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>Open parenthesis: '('</term>
	  <listitem>
	    <para>
This is an old style comment.  A deprecated form of E-mail
addressing uses - for example -
"<literal moreinfo="none">john@example.com (John Smith)</literal>" instead of
"<literal moreinfo="none">John Smith &lt;john@example.com&gt;</literal>".
This old-style notation defined
parenthesized content as arbitrary comments.
The <structname>rfc822token</structname> with
<structfield>token</structfield> set to '(' is created for the contents of
the entire comment.</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>Symbols: '&lt;', '&gt;', '@', and many others</term>
	  <listitem>
	    <para>
The remaining possible values of <structfield>token</structfield> include all
the characters in RFC 822 headers that have special significance.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>
When a <structname>rfc822token</structname> structure does not represent a
special character, the <structfield>ptr</structfield> field points to a text
string giving its contents.
The contents are NOT null-terminated, the <structfield>len</structfield>
field contains the number of characters included.
The macro rfc822_is_atom(token) indicates whether
<structfield>ptr</structfield> and <structfield>len</structfield> are used for
the given <structfield>token</structfield>.
Currently <function moreinfo="none">rfc822_is_atom</function>() returns true if
<structfield>token</structfield> is a zero byte, '<literal moreinfo="none">"</literal>', or
'<literal moreinfo="none">(</literal>'.</para>

      <para>
Note that it's possible that <structfield>len</structfield> might be zero.
This happens with null addresses used as return addresses for delivery status
notifications.</para>
    </refsect2>

    <refsect2 id="rfc822_working_with_e_mail_addresses">
      <title>Working with E-mail addresses</title>
      <informalexample>
	<programlisting format="linespecific">
void rfc822_deladdr(struct rfc822a *addrs, int index);

void rfc822tok_print(const struct rfc822token *list,
        void (*func)(char, void *), void *func_arg);

void rfc822_print(const struct rfc822a *addrs,
        void (*print_func)(char, void *),
        void (*print_separator)(const char *, void *), void *callback_arg);

void rfc822_addrlist(const struct rfc822a *addrs,
                void (*print_func)(char, void *),
                void *callback_arg);

void rfc822_namelist(const struct rfc822a *addrs,
                void (*print_func)(char, void *),
                void *callback_arg);

void rfc822_praddr(const struct rfc822a *addrs,
                int index,
                void (*print_func)(char, void *),
                void *callback_arg);

void rfc822_prname(const struct rfc822a *addrs,
                int index,
                void (*print_func)(char, void *),
                void *callback_arg);

void rfc822_prname_orlist(const struct rfc822a *addrs,
                int index,
                void (*print_func)(char, void *),
                void *callback_arg);

char *rfc822_gettok(const struct rfc822token *list);
char *rfc822_getaddrs(const struct rfc822a *addrs);
char *rfc822_getaddr(const struct rfc822a *addrs, int index);
char *rfc822_getname(const struct rfc822a *addrs, int index);
char *rfc822_getname_orlist(const struct rfc822a *addrs, int index);

char *rfc822_getaddrs_wrap(const struct rfc822a *, int);
</programlisting>
      </informalexample>

      <para>
These functions are used to work with individual addresses that are parsed
by <function moreinfo="none">rfc822a_alloc</function>().</para>

      <para>
<function moreinfo="none">rfc822_deladdr</function>() removes a single
<structname>rfc822addr</structname> structure, whose
<parameter moreinfo="none">index</parameter> is given, from the address array in
<structname>rfc822addr</structname>.
<structfield>naddrs</structfield> is decremented by one.</para>

      <para>
<function moreinfo="none">rfc822tok_print</function>() converts a tokenized
<parameter moreinfo="none">list</parameter> of <structname>rfc822token</structname>
objects into a text string. The callback function,
<parameter moreinfo="none">func</parameter>, is called one
character at a time, for every character in the tokenized objects. An
arbitrary pointer, <parameter moreinfo="none">func_arg</parameter>, is passed unchanged as
the additional argument to the callback function.
<function moreinfo="none">rfc822tok_print</function>() is not usually the most
convenient and efficient function, but it has its uses.</para>

      <para>
<function moreinfo="none">rfc822_print</function>() takes an entire
<structname>rfc822a</structname> structure, and uses the
callback functions to print the contained addresses, in their original form,
separated by commas. The function pointed to by
<parameter moreinfo="none">print_func</parameter> is used to
print each individual address, one character at a time.  Between the
addresses, the <parameter moreinfo="none">print_separator</parameter> function is called to
print the address separator, usually the string ", ".
The <parameter moreinfo="none">callback_arg</parameter> argument is passed
along unchanged, as an additional argument to these functions.</para>

      <para>
The functions <function moreinfo="none">rfc822_addrlist</function>() and
<function moreinfo="none">rfc822_namelist</function>() also print the
contents of the entire <structname>rfc822a</structname> structure, but in a
different way.
<function moreinfo="none">rfc822_addrlist</function>() prints just the actual E-mail
addresses, not the recipient
names or comments.  Each E-mail address is followed by a newline character.
<function moreinfo="none">rfc822_namelist</function>() prints just the names or comments,
followed by newlines.</para>

      <para>
The functions <function moreinfo="none">rfc822_praddr</function>() and
<function moreinfo="none">rfc822_prname</function>() are just like
<function moreinfo="none">rfc822_addrlist</function>() and
<function moreinfo="none">rfc822_namelist</function>(), except that they print a single name
or address in the <structname>rfc822a</structname> structure, given its
<parameter moreinfo="none">index</parameter>. The
functions <function moreinfo="none">rfc822_gettok</function>(),
<function moreinfo="none">rfc822_getaddrs</function>(), <function moreinfo="none">rfc822_getaddr</function>(),
and <function moreinfo="none">rfc822_getname</function>() are equivalent to
<function moreinfo="none">rfc822tok_print</function>(), <function moreinfo="none">rfc822_print</function>(),
<function moreinfo="none">rfc822_praddr</function>() and <function moreinfo="none">rfc822_prname</function>(),
but, instead of using a callback function
pointer, these functions write the output into a dynamically allocated buffer.
That buffer must be destroyed by <function moreinfo="none">free</function>(3) after use.
These functions will
return a null pointer in the event of a failure to allocate memory for the
buffer.</para>

      <para>
<function moreinfo="none">rfc822_prname_orlist</function>() is similar to
<function moreinfo="none">rfc822_prname</function>(), except that it will
also print the legacy RFC822 group list syntax (which are also parsed by
<function moreinfo="none">rfc822a_alloc</function>()).  <function moreinfo="none">rfc822_praddr</function>()
will print an empty string for an index
that corresponds to a group list name (or terminated semicolon).
<function moreinfo="none">rfc822_prname</function>() will also print an empty string.
<function moreinfo="none">rfc822_prname_orlist</function>() will
instead print either the name of the group list, or a single string ";".
<function moreinfo="none">rfc822_getname_orlist</function>() will instead save it into a
dynamically allocated buffer.</para>

      <para>
The function <function moreinfo="none">rfc822_getaddrs_wrap</function>() is similar to
<function moreinfo="none">rfc822_getaddrs</function>(), except
that the generated text is wrapped on or about the 73rd column, using
newline characters.</para>

    </refsect2>

    <refsect2 id="rfc822_working_with_dates">
      <title>Working with dates</title>
      <informalexample>
	<programlisting format="linespecific">
time_t timestamp=rfc822_parsedt(const char *datestr)
const char *datestr=rfc822_mkdate(time_t timestamp);
void rfc822_mkdate_buf(time_t timestamp, char *buffer);
</programlisting>
      </informalexample>

      <para>
These functions convert between timestamps and dates expressed in the
<literal moreinfo="none">Date:</literal> E-mail header format.</para>

      <para>
<function moreinfo="none">rfc822_parsedt</function>() returns the timestamp corresponding to
the given date string (0 if there was a syntax error).</para>

      <para>
<function moreinfo="none">rfc822_mkdate</function>() returns a date string corresponding to
the given timestamp.
<function moreinfo="none">rfc822_mkdate_buf</function>() writes the date string into the
given buffer instead,
which must be big enough to accommodate it.</para>

    </refsect2>

    <refsect2 id="rfc822_working_with_8_bit_mime_encoded_headers">
      <title>Working with 8-bit MIME-encoded headers</title>

      <informalexample>
	<programlisting format="linespecific">
int error=rfc2047_decode(const char *text,
                int (*callback_func)(const char *, int, const char *, void *),
                void *callback_arg);

extern char *str=rfc2047_decode_simple(const char *text);

extern char *str=rfc2047_decode_enhanced(const char *text,
                const char *charset);

void rfc2047_print(const struct rfc822a *a,
        const char *charset,
        void (*print_func)(char, void *),
        void (*print_separator)(const char *, void *), void *);


char *buffer=rfc2047_encode_str(const char *string,
                const char *charset);

int error=rfc2047_encode_callback(const char *string,
        const char *charset,
        int (*func)(const char *, size_t, void *),
        void *callback_arg);

char *buffer=rfc2047_encode_header(const struct rfc822a *a,
        const char *charset);
</programlisting>
      </informalexample>

      <para>
These functions provide additional logic to encode or decode 8-bit content
in 7-bit RFC 822 headers, as specified in RFC 2047.</para>

      <para>
<function moreinfo="none">rfc2047_decode</function>() is a basic RFC 2047 decoding function.
It receives a
pointer to some 7bit RFC 2047-encoded text, and a callback function.  The
callback function is repeatedly called. Each time it's called it receives a
piece of decoded text. The arguments are: a pointer to a text fragment, number
of bytes in the text fragment, followed by a pointer to the character set of
the text fragment. The character set pointer is NULL for portions of the
original text that are not RFC 2047-encoded.</para>

      <para>
The callback function also receives <parameter moreinfo="none">callback_arg</parameter>, as
its last
argument. If the callback function returns a non-zero value,
<function moreinfo="none">rfc2047_decode</function>()
terminates, returning that value.  Otherwise,
<function moreinfo="none">rfc2047_decode</function>() returns 0 after
a successful decoding. <function moreinfo="none">rfc2047_decode</function>() returns -1 if it
was unable to allocate sufficient memory.</para>

      <para>
<function moreinfo="none">rfc2047_decode_simple</function>() and
<function moreinfo="none">rfc2047_decode_enhanced</function>() are alternatives to
<function moreinfo="none">rfc2047_decode</function>() which forego a callback function, and
return the decoded text
in a dynamically-allocated memory buffer. The buffer must be
<function moreinfo="none">free</function>(3)-ed after
use. <function moreinfo="none">rfc2047_decode_simple</function>() discards all character set
specifications, and
merely decodes any 8-bit text. <function moreinfo="none">rfc2047_decode_enhanced</function>()
is a compromise to
discarding all character set information.  The local character set being used
is specified as the second argument to
<function moreinfo="none">rfc2047_decode_enhanced</function>().  Any RFC
2047-encoded text in a different character set will be prefixed by the name of
the character set, in brackets, in the resulting output.</para>

      <para>
<function moreinfo="none">rfc2047_decode_simple</function>() and
<function moreinfo="none">rfc2047_decode_enhanced</function>() return a null pointer
if they are unable to allocate sufficient memory.</para>

      <para>
The <function moreinfo="none">rfc2047_print</function>() function is equivalent to
<function moreinfo="none">rfc822_print</function>(), followed by
<function moreinfo="none">rfc2047_decode_enhanced</function>() on the result.  The callback
functions are used in
an identical fashion, except that they receive text that's already
decoded.</para>

      <para>
The function <function moreinfo="none">rfc2047_encode_str</function>() takes a
<parameter moreinfo="none">string</parameter> and <parameter moreinfo="none">charset</parameter>
being the name of the local character set, then encodes any 8-bit portions of
<parameter moreinfo="none">string</parameter> using RFC 2047 encoding.
<function moreinfo="none">rfc2047_encode_str</function>() returns a
dynamically-allocated buffer with the result, which must be
<function moreinfo="none">free</function>(3)-ed after
use, or NULL if there was insufficient memory to allocate the buffer.</para>

      <para>
The function <function moreinfo="none">rfc2047_encode_callback</function>() is similar to
<function moreinfo="none">rfc2047_encode_str</function>()
except that the callback function is repeatedly called to received the
encoding string.  Each invocation of the callback function receives a pointer
to a portion of the encoded text, the number of characters in this portion,
and <parameter moreinfo="none">callback_arg</parameter>.</para>

      <para>
The function <function moreinfo="none">rfc2047_encode_header</function>() is basically
equivalent to <function moreinfo="none">rfc822_getaddrs</function>(), followed by
<function moreinfo="none">rfc2047_encode_str</function>();</para>

    </refsect2>

    <refsect2 id="rfc822_working_with_subjects">
      <title>Working with subjects</title>

      <informalexample>
	<programlisting format="linespecific">
char *basesubj=rfc822_coresubj(const char *subj);

char *basesubj=rfc822_coresubj_nouc(const char *subj);
</programlisting>
      </informalexample>

      <para>
This function takes the contents of the subject header, and returns the
"core" subject header that's used in the specification of the IMAP THREAD
function. This function is designed to strip all subject line artifacts that
might've been added in the process of forwarding or replying to a message.
Currently, <function moreinfo="none">rfc822_coresubj</function>() performs the following transformations:</para>
      <variablelist>
	<varlistentry>
	  <term>Whitespace</term>
	  <listitem>
	    <para>Leading and trailing whitespace is removed.  Consecutive
whitespace characters are collapsed into a single whitespace character.
All whitespace characters are replaced by a space.</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>Re:, (fwd) [foo]</term>
	  <listitem>
	    <para>
These artifacts (and several others) are removed from
the subject line.</para>
	  </listitem>
	</varlistentry>
      </variablelist>

      <para>Note that this function does NOT do MIME decoding.  In order to
implement IMAP THREAD, it is necessary to call something like
<function moreinfo="none">rfc2047_decode</function>() before
calling <function moreinfo="none">rfc822_coresubj</function>().</para>

      <para>
This function returns a pointer to a dynamically-allocated buffer, which
must be <function moreinfo="none">free</function>(3)-ed after use.</para>

      <para>
<function moreinfo="none">rfc822_coresubj_nouc</function>() is like
<function moreinfo="none">rfc822_coresubj</function>(), except that the subject
is not converted to uppercase.</para>
    </refsect2>
  </refsect1>

  <refsect1 id="rfc822_see_also">
    <title>SEE ALSO</title>

    <para>
<ulink url="rfc2045.html"><citerefentry><refentrytitle>rfc2045</refentrytitle><manvolnum>3</manvolnum></citerefentry></ulink>,
<ulink url="reformail.html"><citerefentry><refentrytitle>reformail</refentrytitle><manvolnum>1</manvolnum></citerefentry></ulink>,
<ulink url="reformime.html"><citerefentry><refentrytitle>reformime</refentrytitle><manvolnum>1</manvolnum></citerefentry></ulink>.</para>
  </refsect1>
</refentry>