# rfc2045 - RFC 2045 (MIME) parsing library

## SYNOPSIS

`#include <rfc2045.h>`

`cc ... -lrfc2045 -lrfc822`

## DESCRIPTION

The rfc2045 library is used to parse and work with MIME-formatted messages. The rfc2045 library is used to:

1) Parse the structure of a MIME formatted message

2) Examine the contents of each MIME section

3) Optionally rewrite and reformat the message.

## Creating an rfc2045 structure

    #include <rfc2045.h>

    struct rfc2045 *ptr=rfc2045_alloc();
    void rfc2045_parse(struct rfc2045 *ptr, const char *txt, size_t cnt);

    struct rfc2045 *ptr=rfc2045_fromfd(int fd);
    struct rfc2045 *ptr=rfc2045_fromfp(FILE *fp);

    void rfc2045_free(struct rfc2045 *ptr);

    void rfc2045_error(const char *errmsg)
    {
            perror(errmsg);
            exit(0);
    }

The rfc2045 structure is created from an existing message. The function rfc2045_alloc() allocates the structure, then rfc2045_parse() is called to initialize the structure based on the contents of the message. txt points to the contents of the message, and cnt contains the number of bytes in the message.

Large messages can be parsed by calling rfc2045_parse() multiple number of times, each time passing a portion of the overall message. There is no need to call a separate function after the entire message has been parsed -- the rfc2045 structure is created dynamically, on the fly.

rfc2045_alloc() returns NULL if there was insufficient memory to allocate the structure. The rfc2045_parse() also allocates memory, internally, however no error indication is return in the event of a memory allocation failure. Instead, the function rfc2045_error() is called, with errmsg set to "Out of memory". rfc2045_error() is also called by rfc2045_alloc() also calls rfc2045_error(), prior to returning a NULL pointer.

The rfc2045_error() function is not included in the rfc2045 library, it must be defined by the application to report the error in some appropriate way. All functions below will use rfc2045_error() to report an error condition (currently only insufficient memory is reported), in addition to returning any kind of an error indicator. Some functions do not return an error indicator, so rfc2045_error() is the only reliable way to detect a failure.

The rfc2045_fromfd() function initializes an rfc2045 structure from a file descriptor. It is equivalent to calling rfc2045_alloc(), then reading the contents of the given file descriptor, and calling rfc2045_parse(). The rfc2045_fromfp() function initializes an rfc2045 structure from a FILE.

After the rfc2045 structure is initialized, the functions described below may be used to access and work with the contents of the structure. When the rfc2045 structure is no longer needed, the function rfc2045_free() deallocates and destroys the structure.

## Structure of a MIME message

    struct rfc2045 {

            struct rfc2045 *parent;

            struct rfc2045 *firstpart;
            struct rfc2045 *next;
            int             isdummy;
            int             rfcviolation;
    } ;

The rfc2045 has many fields, only some are publicly documented. A MIME message is represented by a recursive tree of linked rfc2045 structures. Each instance of the rfc2045 structure represents a single MIME section of a MIME message.

The top-level structure representing the entire message is created by the rfc2045_alloc() function. The remaining structures are created dynamically by rfc2045_parse(). Any rfc2045 structure, except ones whose isdummy flag is set, may be used as an argument to any function described in the following chapters.

The rfcviolation field in the top-level rfc2045 is set to indicate any errors encountered while parsing the MIME message. rfcviolation is a bitmask of the following flags:

- `RFC2045_ERR8BITHEADER` - illegal 8-bit characters in MIME headers.
- `RFC2045_ERR8BITCONTENT` - illegal 8-bit contents of a MIME section that has a 7bit transfer encoding.
- `RFC2045_ERR2COMPLEX` - the message has too many MIME sections, this is a potential denial-of-service attack.

In each rfc2045 structure representing a multipart MIME section (or one containing message/rfc822 content), the firstpart pointer points to the first MIME section in the multipart MIME section (or the included "message/rfc822" MIME section). If there are more than one MIME sections in a multipart MIME section firstpart->next gets you the second MIME section, firstpart->next->next gets you the third MIME section, and so on. parent points to the parent MIME section, which is NULL for the top-level MIME section.

Not all MIME sections are created equal. In a multipart MIME section, there is an initial, unused, "filler" section before the first MIME delimiter (see RFC 2045 for more information). This filler section typically contains a terse message saying that this is a MIME-formatted message, or something similar of that kind. This is not considered to be a "real" MIME section, and all MIME-aware software must ignore those. These filler sections are designated by setting the "isdummy" flag to non-zero. All rfc2045 structures that have "isdummy" set must be completely ignored, and skipped over, when traversing the rfc2045 tree.

## Basic MIME information

    const char *content_type, *content_transfer_encoding,
               *content_character_set;

    void rfc2045_mimeinfo(const struct rfc2045 *ptr,
            &content_type, &content_transfer_encoding,
            &content_character_set);

    off_t start_pos, end_pos, start_body, nlines, nbodylines;

    void rfc2045_mimepos(const struct rfc2045 *ptr,
            &start_pos, &end_pos, &start_body, &nlines,
            &nbodylines);

The rfc2045_mimeinfo() function returns the content type, encoding method, and the character set of a given MIME section. Where a MIME section does not specify any property, rfc2045_mimeinfo() automatically supplies a default value. The character set is only meaningful for MIME sections containing a text content type, however it is still defaulted for other sections. It is not permissible to supply a NULL pointer for any argument to rfc2045_mimeinfo().

The rfc2045_mimepos() function is used to locate the position of the given MIME section in the original message. It is not permissible to supply a NULL pointer for any argument to rfc2045_mimepos(). All arguments must be used.

start_pos and end_pos point to the starting and the ending offset, from the beginning of the message, of this MIME section. nlines is initialized to the number of lines of text in this MIME section. The starting offset points to the start of MIME headers in this section. start_body is initialized to point to the starting offset of the actual contents of this MIME section, and nbodylines is set to the number of lines of actual content in this MIME section.

    const char *id=rfc2045_content_id(
                           const struct rfc2045 *ptr);

    const char *desc=rfc2045_content_description(
                           const struct rfc2045 *ptr);

    const char *lang=rfc2045_content_language(
                           const struct rfc2045 *ptr);

    const char *md5=rfc2045_content_md5(
                           const struct rfc2045 *ptr);

These functions return the contents of the corresponding MIME headers. If these headers do not exist, these functions return an empty string, "", NOT a null pointer.

    char *id=rfc2045_related_start(const struct rfc2045 *ptr);

This function returns the start parameter of the Content-Type: header, which is used by multipart/related content. This function returns a dynamically-allocated buffer, which must be free(3)-ed after use (a null pointer is returned if there was insufficient memory for the buffer, and rfc2045_error() is called).

    const char *disposition, *name, *filename;

    void rfc2045_dispositioninfo(const struct rfc2045 *ptr,
            &disposition, &name, &filename);

rfc2045_dispositioninfo() returns the disposition specifications of a MIME section. For MIME sections that do not specify the type of disposition (inline or attachment), the name or the filename of the attachment, the corresponding pointer is initialized to NULL.

    char *url=rfc2045_content_base(struct rfc2045 *ptr);

    char *url=rfc2045_append_url(const char *base, const char *url);

These functions are used to work with multipart/related MIME messages. The rfc2045_content_base() returns the contents of either the Content-Base: or the Content-Location: header. If both are present, they are logically combined. The rfc2045_append_url() function combines two URLs, base and url, and returns the absolute URL that results from the combination.

Both functions return a pointer to a dynamically-allocated buffer that must be free(3)-ed after it is no longer needed. Both functions return NULL if there was no sufficient memory to allocate the buffer. rfc2045_content_base() returns an empty string in the event that there are no Content-Base: or Content-Location: headers. Either argument to rfc2045_append_url() may be a NULL, or an empty string.

## Decoding a MIME section

    void rfc2045_cdecode_start(struct rfc2045 *ptr,
            int (*callback_func)(const char *, size_t, void *),
            void *callback_arg);

    int rfc2045_cdecode(struct rfc2045 *ptr, const char *stuff,
            size_t nstuff);

    int rfc2045_cdecode_end(struct rfc2045 *ptr);

These functions are used to return the raw contents of the given MIME section, transparently decoding quoted-printable or base64-encoded content. Because the rfc2045 library does not require the message to be read from a file (it can be stored in a memory buffer), the application is responsible for reading the contents of the message and calling rfc2045_cdecode().

The rfc2045_cdecode_start() function is used to begin the process of decoding the given MIME section. After calling rfc2045_cdecode_start(), the application must the repeatedly call rfc2045_cdecode() with the contents of the MIME message between the offsets given by the start_body and end_pos return values from rfc2045_mimepos() function. The rfc2045_cdecode() function can be called repeatedly, if necessary, for successive portions of the MIME section. After the last MIME section, the rfc2045_cdecode_end() function is called to finish decoding the MIME section.

rfc2045_cdecode() and rfc2045_cdecode_end() repeatedly call the callback_func() function, with the decoded contents of the MIME section. The first argument to callback_func() is a pointer to a portion of the decoded content, the second argument is the number of bytes in this portion. The third argument is callback_arg.

callback_func() is required to return zero, to continue decoding. If callback_func() returns non-zero, the decoding immediately stops and rfc2045_cdecode() or rfc2045_cdecode_end() will terminate with callback_func's return code.

## Rewriting MIME messages

The rfc2045 library contains functions that can be used to rewrite a MIME message in order to convert 8-bit data to 7-bit encoding method, or to convert 7-bit encoded data to full 8-bit data, if possible.

    struct rfc2045 *ptr=rfc2045_alloc_ac();
    int necessary=rfc2045_ac_check(struct rfc2045 *ptr, int mode);

    int error=rfc2045_rewrite(struct rfc2045 *ptr,
                    int fdin,
                    int fdout,
                    const char *appname);

    int rfc2045_rewrite_func(struct rfc2045 *p, int fdin,
            int (*funcout)(const char *, int, void *), void *funcout_arg,
            const char *appname);

When rewriting will be used, the rfc2045_alloc_ac() function must be used to create the initial rfc2045 structure. This function allocates some additional structures that are used in rewriting. The rfc2045_parse() function is used to parse the message, as usual. The rfc2045_free() function will also be used normally to destroy the rfc2045 structure, when all is said and done.

The rfc2045_ac_check() function must be called to determine whether rewriting is necessary. mode must be set to one of the following values:

- `RFC2045_RW_7BIT` - we want to generate 7-bit content. If the original message contains any 8-bit content it will be converted to 7-bit content using quoted-printable encoding.
- `RFC2045_RW_8BIT` - we want to generate 8-bit content. If the original message contains any 7-bit quoted-printable content it should be rewritten as 8-bit content.

The rfc2045_ac_check() function returns non-zero if there's any content in the MIME message that should be converted, OR if there are any missing MIME headers. rfc2045_ac_check() returns zero if there's no need to rewrite the message. However it might still be worthwhile to rewrite the message anyway. There are some instances where it is desirable to provide defaults for some missing MIME headers, but they are too trivial to require the message to be rewritten. One such case would be a missing Content-Transfer-Encoding: header for a multipart section.

Either the rfc2045_rewrite() or the rfc2045_rewrite_func() function is used to rewrite the message. The only difference is that rfc2045_rewrite() writes the new message to a given file descriptor, fdout, while rfc2045_rewrite_func() repeatedly calls the funcout function. Both function read the original message from fdin. funcout receives to a portion of the MIME message, the number of bytes in the specified portion, and funcout_arg. When either function rewrites a MIME section, an informational header gets appended, noting that the message was converted by appname.

## SEE ALSO

rfc822(3), reformime(1), reformail(1).