• GenericEscapingEngine

Rules of the game:

Note: Wiki not updated, actual design header: http:/people/ct/escaping.h

Set Up

define an array of str_esc

Where each entry is in the form of:

 {
  { "unescaped", "escaped", flags, special_parser },
  ...
  { 0,0,0,0 }
 }

• unescaped

  • might be set to NOCHARS to indicate that escaped can't be trivially converted to unescaped

  • The string size of at least one byte is assumed so "\0" stands for a literal nullbyte
  • This strings might not contain "\0" on another than the first position, "" is illegal

  escaped

  • is the escaped representation of unescaped (or at least the start of it)
  • escaped strings never ever contain "\0"

  special_parser

  • is either NULL or points to a parser_func which parses the remainder of an escape sequence

  flags can be:

  • ESCAPE_GENERIC : use a generic escape instead of "escaped" when doing escaping (Example: accept "\ " but generate "\x20" for spaces)

• Note that encodings which contain zero bytes on another than the first position need a special_parser.

generate the lookup tables

 str_esclookup * str_esclookup_alloc (alloc_limits limits,
                                      str_esc * conv,
                                      isprint_cnt_func,
                                      escape_func)

• conv

  • is the table from 1).

  isprint_cnt_func

  • is a pointer to a function which returns the number of next bytes which form a character which do not need escaping

  escape_func

  • is a pointer to a function which can generate a generic escape sequence for the next character('s)
  Returns: an lookup structure which is used to lookup strings, see below

  Note: the lookup structure contains arrays/trees of integer indices rather than pointers, so it is relocateable and static. That makes it possible to generate it when compiling the program instead dynamically generating it.

  The user has to lim_free() the lookup table after use.

converting unescaped to escaped strings:

 t_uchar * str_save_escaped (alloc_limits limits,
                             const t_uchar * str,
                             int size,
                             const str_conv_lookup * lookup)

• limits

  • allocation domain, see hackerlab doc

  str

  • is the unescaped string

  size

  • is either the size if str (in case it might contain zero bytes) or -1 where the size is determined automatically

  lookup

  • is a pointer to the datastructure described before

  the string is freshly allocated and need to be freed by the user

  Semantic:

  for every char in str
    is_print_cnt is invoked, found characters are copied verbatim
    if the return was zero
      it lookups the longest matching string from unescaped
      if found and GENERIC flag is not set
        copy the escaped string
      else
        escape_func is invoked to generate a generic escape

converting escaped to unescaped strings:

 t_uchar * str_save_unescaped (alloc_limits limits,
                               const t_uchar * str,
                               size_t* size,
                               const str_esc_lookup * lookup)

• limits

  • allocation domain, see hackerlab doc

  str

  • pointer to an escaped C-String

  size

  • pointer where the function should store the final size of the unescaped string

  lookup

  • lookup datastructure, see above.

  Semantic:

  for ever character in str
    is_print_cnt is invoked, found characters are copied verbatim
    if return was zero
      lookup longest matching escaped string
      if found
        if unescaped != NOCHARS
          copy the unescaped string
        if parser_func != 0
          invoke parserfunc
      else
        free memory, abort, return zero ( this is the only case of an error, illegal escape sequence)

• Note: the string is freshly allocated and need to be freed by the user

DISCLAIMER

• This is only a very terse and not fully correct explanation, human imagination is required to get it right!