# rfc822 - RFC 822 parsing library

## SYNOPSIS

`#include <rfc822.h>`

`#include <rfc2047.h>`

`cc ... -lrfc822`

## DESCRIPTION

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.

The format used by E-mail headers to encode sender and recipient information is defined by RFC 822. The format allows the actual E-mail address and the sender/recipient name to be expressed together, for example: `John Smith <jsmith@example.com>`

The main purposes of the rfc822 library is to:

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

2) Access those individual components.

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

### Tokenizing an E-mail header

    struct rfc822t *tokens=rfc822t_alloc(const char *header,
                    void (*err_func)(const char *, int));

    void rfc822t_free(tokens);

The rfc822t_alloc() function accepts an E-mail header, and parses it into individual tokens. This function allocates and returns a pointer to a rfc822t structure, which is later used by the rfc822a_alloc function to extract individual addresses from these tokens.

If err_func 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 instances -- 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 header pointer, and an index to the syntax error in the header string.

The semantics of err_func are subject to change. It is recommended to leave this argument as NULL in the current version of the library.

rfc822t_alloc() returns a pointer to a dynamically-allocated rfc822t structure. A NULL pointer is returned if there's insufficient memory to allocate this structure. The rfc822t_free() function is used to destroy the rfc822t structure and to free all the dynamically allocated memory.

NOTE: Until rfc822t_free() is called, the contents of header MUST NOT be destroyed or altered in any way. The contents of header are not modified by rfc822t_alloc(), however the rfc822t structure contains pointers to portions of the supplied header.

### Extracting E-mail addresses

    struct rfc822a *addrs=rfc822a_alloc(struct rfc822t *tokens);

    void rfc822a_free(addrs);

The rfc822a_alloc() function returns a dynamically-allocated rfc822a structure, that contains individual addresses that were logically extracted from a rfc822t structure. The rfc822a_alloc() function returns NULL if there was insufficient memory to allocate the rfc822a structure. The rfc822a_free() function destroys the rfc822a function, and frees all associated dynamically-allocated memory. The rfc822t structure passed to rfc822a_alloc() must not be destroyed before rfc822a_free() destroys the rfc822a structure.

The rfc822a structure has the following fields:

    struct rfc822a {
            struct rfc822addr *addrs;
            int     naddrs;
    } ;

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

    To: recipient-list: tom@example.com, john@example.com;

Typically, all of this, sans the "To:" part, is tokenized by rfc822t_alloc(), then parsed by rfc822a_alloc(). The "recipient-list:" and the trailing semicolon is a legacy mailing list specification that is no longer in widespread use, but still must be accounted for. The resulting rfc822a structure will have four rfc822addr structures, one for "recipient-list:", one for each address, and one for the trailing semicolon. Each rfc822a structure has the following fields:

    struct rfc822addr {
            struct rfc822token *tokens;
            struct rfc822token *name;
    } ;

If tokens is a null pointer, this structure represents some non-address portion of the original header, such as "recipient-list:" or a semicolon. Otherwise it points to a structure that represents the E-mail address in tokenized form.

name 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. name will be NULL if the recipient name was not provided. For the following address: `Tom Jones <tjones@example.com>` The tokens field will point to the tokenized form of "tjones@example.com", and name points to the tokenized form of "Tom Jones".

Each rfc822token structure contains the following fields:

    struct rfc822token {
            struct rfc822token *next;
            int token;
            const char *ptr;
            int len;
    } ;

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

- 0x00 - this is a simple atom - a sequence of non-special characters that is delimited by whitespace or special characters (see below).
- 0x22 - the value of the ascii quote - this is a quoted string.
- '(' - this is an old style comment. A deprecated form of E-mail addressing uses, as an example, "john@example.com (John Smith)" instead of "John Smith <john@example.com>". This old-style notation defined parenthesized content as arbitrary comments. The rfc822token with token set to '(' is created for the contents of the entire comment.
- '<', '>', '@', and many others - the remaining possible values of token include all the characters in RFC 822 headers that have special significance.

When a rfc822token structure does not represent a special character, the ptr field points to a text string giving its contents. The contents are NOT null-terminated, the len field contains the number of characters included. The macro rfc822_is_atom(token) indicates whether ptr and len are used for the given token. Currently rfc822_is_atom() returns true if token is a zero byte, '"', or '('.

Note that it's possible that len might be zero. This will be the case for null addresses used as return addresses for delivery status notifications.

### Working with E-mail addresses

    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);

These functions are used to work with individual addresses that are parsed by rfc822a_alloc().

rfc822_deladdr() removes a single rfc822addr structure, whose index is given, from the address array in rfc822addr. naddrs is decremented by one.

rfc822tok_print() converts a tokenized list of rfc822token objects into a text string. The callback function, func, is called one character at a time, for every character in the tokenized objects. An arbitrary pointer, func_arg, is passed unchanged as the additional argument to the callback function. rfc822tok_print() is not usually the most convenient and efficient function, but it has its uses.

rfc822_print() takes an entire rfc822a structure, and uses the callback functions to print the contained addresses, in their original form, separated by commas. The function pointed to by print_func is used to print each individual address, one character at a time. Between the addresses, the print_separator function is called to print the address separator, usually the string ", ". The callback_arg argument is passed along unchanged, as an additional argument to these functions.

The functions rfc822_addrlist() and rfc822_namelist() also print the contents of the entire rfc822a structure, but in a different way. rfc822_addrlist() prints just the actual E-mail addresses, not the recipient names or comments. Each E-mail address is followed by a newline character. rfc822_namelist() prints just the names or comments, followed by newlines.

The functions rfc822_praddr() and rfc822_prname() are just like rfc822_addrlist() and rfc822_namelist(), except that they print a single name or address in the rfc822a structure, given its index. The functions rfc822_gettok(), rfc822_getaddrs(), rfc822_getaddr(), and rfc822_getname() are equivalent to rfc822tok_print(), rfc822_print(), rfc822_praddr() and rfc822_prname(), but, instead of using a callback function pointer, these functions write the output into a dynamically allocated buffer. That buffer must be destroyed by free(3) after use. These functions will return a null pointer in the event of a failure to allocate memory for the buffer.

rfc822_prname_orlist() is similar to rfc822_prname(), except that it will also print the legacy RFC822 group list syntax (which are also parsed by rfc822a_alloc()). rfc822_praddr() will print an empty string for an index that corresponds to a group list name (or terminated semicolon). rfc822_prname() will also print an empty string. rfc822_prname_orlist() will instead print either the name of the group list, or a single string ";". rfc822_getname_orlist() will instead save it into a dynamically allocated buffer.

The function rfc822_getaddrs_wrap() is similar to rfc822_getaddrs(), except that the generated text is wrapped on or about the 73rd column, using newline characters.

### Working with dates

    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);

These functions convert between timestamps and dates expressed in the Date: E-mail header format.

rfc822_parsedt() returns the timestamp corresponding to the given date string (0 if there was a syntax error).

rfc822_mkdate() returns a date string corresponding to the given timestamp. rfc822_mkdate_buf() writes the date string into the given buffer instead, which must be of sufficient size to accommodate it.

### Working with 8-bit MIME-encoded headers

    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);

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

rfc2047_decode() 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.

The callback function also receives callback_arg, as its last argument. If the callback function returns a non-zero value, rfc2047_decode() terminates, returning that value. Otherwise, rfc2047_decode() returns 0 after a successful decoding. rfc2047_decode() returns -1 if it was unable to allocate sufficient memory.

rfc2047_decode_simple() and rfc2047_decode_enhanced() are alternatives to rfc2047_decode() which forego a callback function, and return the decoded text in a dynamically-allocated memory buffer. The buffer must be free(3)-ed after use. rfc2047_decode_simple() discards all character set specifications, and merely decodes any 8-bit text. rfc2047_decode_enhanced() is a compromise to discarding all character set information. The local character set being used is specified as the second argument to rfc2047_decode_enhanced(). 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.

rfc2047_decode_simple() and rfc2047_decode_enhanced() return a null pointer if they are unable to allocate sufficient memory.

The rfc2047_print() function is equivalent to rfc822_print(), followed by rfc2047_decode_enhanced() on the result. The callback functions are used in an identical fashion, except that they receive text that's already decoded.

The function rfc2047_encode_str() takes a string and charset being the name of the local character set, then encodes any 8-bit portions of string using RFC 2047 encoding. rfc2047_encode_str() returns a dynamically-allocated buffer with the result, which must be free(3)-ed after use, or NULL if there was insufficient memory to allocate the buffer.

The function rfc2047_encode_callback() is similar to rfc2047_encode_str() 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 callback_arg.

The function rfc2047_encode_header() is basically equivalent to rfc822_getaddrs(), followed by rfc2047_encode_str();

### Working with subjects

    char *basesubj=rfc822_coresubj(const char *subj)

    char *basesubj=rfc822_coresubj_nouc(const char *subj)

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, rfc822_coresubj() performs the following transformations:

- Whitespace - leading and trailing whitespace is removed. Consecutive whitespace characters are collapsed into a single whitespace character. All whitespace characters are replaced by a space.
- Re:, (fwd) [foo] - these artifacts (and several others) are removed from the subject line.

Note that this function does NOT do MIME decoding. In order to implement IMAP THREAD, it is necessary to call something like rfc2047_decode() before calling rfc822_coresubj().

This function returns a pointer to a dynamically-allocated buffer, which must be free(3)-ed after use.

rfc822_coresubj_nouc() is like rfc822_coresubj(), except that the subject is not converted to uppercase.

## SEE ALSO

rfc2045(3), reformime(1), reformail(1).