configfile, config_read,  config_delete,  config_renewed,  config_length,
     config_issub, config_isatom, config_isstring - generic configuration file

     #include <configfile.h>

     config_t *config_read(const char *file, int flags, config_t *cfg)
     void config_delete(config_t *cfg)
     int config_renewed(config_t *cfg)
     size_t config_length(config_t *cfg)
     int config_issub(config_t *cfg)
     int config_isatom(config_t *cfg)
     int config_isstring(config_t *cfg)

     The configfile routines operate on  a  generic  configuration  file  that
     follows the syntax described in configfile(5).

     The interface presented by the functions above uses  the  following  type
     and definitions from <configfile.h>:

       typedef const struct config {
           config_t    *next;   /* Next configuration file thing. */
           config_t    *list;   /* For a { sublist }. */
           const char  *file;   /* File and line where this is found. */
           unsigned    line;
           int         flags;   /* Special flags. */
           char        word[];  /* Payload. */
       } config_t;

       #define CFG_CLONG   0x0001  /* strtol(word, &end, 0) is valid. */
       #define CFG_OLONG   0x0002  /* strtol(word, &end, 010). */
       #define CFG_DLONG   0x0004  /* strtol(word, &end, 10). */
       #define CFG_XLONG   0x0008  /* strtol(word, &end, 0x10). */
       #define CFG_CULONG  0x0010  /* strtoul(word, &end, 0). */
       #define CFG_OULONG  0x0020  /* strtoul(word, &end, 010). */
       #define CFG_DULONG  0x0040  /* strtoul(word, &end, 10). */
       #define CFG_XULONG  0x0080  /* strtoul(word, &end, 0x10). */
       #define CFG_STRING  0x0100  /* The word is enclosed in quotes. */
       #define CFG_SUBLIST 0x0200  /* This is a sublist, so no word. */
       #define CFG_ESCAPED 0x0400  /* Escapes are still marked with \. */

     In memory a configuration file is represented as a list of config_t cells
     linked  together  with  the  next  field  ending  with a null pointer.  A
     sublist between braces is attached to a cell at the  list  field.   Words
     and  strings  are  put  in the word field, a null terminated string.  The
     flags field records the type and features of a cell.  The CFG_*LONG flags
     are  set  if a word is a number according to one of the strtol or strtoul
     calls.  Purely a number, no quotes or trailing garbage.   The  CFG_STRING
     flag  is  set  if  the  object  was  enclosed  in  double quotes.  Lastly
     CFG_SUBLIST tells if the cell is only a pointer to a sublist in braces.

     Characters in a word or string may have been formed  with  the  \  escape
     character.   They  have  been  parsed  and  expanded,  but the \ is still
     present if CFG_ESCAPED is set.  The word array may be changed, as long as
     it doesn't grow longer, so one may remove the \s like this:

          if (cfg->flags & CFG_ESCAPED) {
              char *p, *q;
              p= q= cfg->word;
              for (;;) {
                  if ((*q = *p) == '\\') *q = *++p;
                  if (*q == 0) break;

     The low level syntax of a config file is checked when it is read.  If  an
     error is encountered a message is printed and the program exits with exit
     code 1.  What the data means is not checked, that should be done  by  the
     program using the data.  Only the atom include at the beginning of a list
     is special.  It should be followed by a string.  The string  is  seen  as
     the  name  of  a file, that is opened, read, and inserted in place of the
     include.  Unless the name of the file starts  with  a  /,  it  is  sought
     relative  to  the  directory  the  current file is found in.  Nonexistent
     files are treated as being empty.

     The file and line fields in each cell tell where the cell was read.

     A configuration file is read with config_read.  The first argument is the
     file  to  read.   The second is either 0 or CFG_ESCAPED to tell whether \
     escapes should be fully expanded without leaving  a  trace,  or  if  they
     should  still  be  marked  with  a  \  so that the caller knows where the
     excapes are.  The third argument, cfg, should be a null  pointer  on  the
     first  call.   If  you want to reread a config file that may have changed
     then cfg should be what you previously read.

     With config_delete one can free up the memory that has been acquired with
     malloc(3) to hold the contents of the configuration file.

     To determine if the contents  of  configuration  file  has  changed  when
     reread one uses config_renewed after config_read.  It returns a "changed"
     flag that is set when the configuration file changed and then clears that
     flag.   It returns true on the very first call.  For the function to work
     you need to feed the old data  back  into  config_read,  not  delete  and
     The length of a series of config structures is told by config_length.  It
     follows the next fields, so a sublist between braces counts as one extra.

     The config_issub, config_isatom and config_isstring  functions  are  just
     pretty  macros  to test if a cell references a sublist, is a word/string,
     or is just a string.  CFG_SUBLIST and CFG_STRING tell the same story.


     */etc/*.conf    Several files in several etc directories.


     The syntax of a config file puts some constraints on  what  you  find  in
     memory.   The  top  level list consists entirely of sublist cells.  These
     point to lists that start with at least an atom, followed  by  a  mix  of
     atoms  and sublist cells.  These sublists in turn point to a list of only
     sublist cells (recurse now.)

     The struct config shown above is not exactly proper C to aid readability,
     read <configfile.h> itself to see why.

     Kees J. Bot (