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 14.2.6 Scanning the Content of a Directory
 ------------------------------------------
 
 A higher-level interface to the directory handling functions is the
 'scandir' function.  With its help one can select a subset of the
 entries in a directory, possibly sort them and get a list of names as
 the result.
 
  -- Function: int scandir (const char *DIR, struct dirent ***NAMELIST,
           int (*SELECTOR) (const struct dirent *), int (*CMP) (const
           struct dirent **, const struct dirent **))
      Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | 
      POSIX Safety Concepts.
 
      The 'scandir' function scans the contents of the directory selected
      by DIR.  The result in *NAMELIST is an array of pointers to
      structure of type 'struct dirent' which describe all selected
      directory entries and which is allocated using 'malloc'.  Instead
      of always getting all directory entries returned, the user supplied
      function SELECTOR can be used to decide which entries are in the
      result.  Only the entries for which SELECTOR returns a non-zero
      value are selected.
 
      Finally the entries in *NAMELIST are sorted using the user-supplied
      function CMP.  The arguments passed to the CMP function are of type
      'struct dirent **', therefore one cannot directly use the 'strcmp'
      or 'strcoll' functions; instead see the functions 'alphasort' and
      'versionsort' below.
 
      The return value of the function is the number of entries placed in
      *NAMELIST.  If it is '-1' an error occurred (either the directory
      could not be opened for reading or the malloc call failed) and the
      global variable 'errno' contains more information on the error.
 
    As described above the fourth argument to the 'scandir' function must
 be a pointer to a sorting function.  For the convenience of the
 programmer the GNU C Library contains implementations of functions which
 are very helpful for this purpose.
 
  -- Function: int alphasort (const struct dirent **A, const struct
           dirent **B)
      Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem |
       POSIX Safety Concepts.
 
      The 'alphasort' function behaves like the 'strcoll' function (
      String/Array Comparison).  The difference is that the arguments
      are not string pointers but instead they are of type 'struct dirent
      **'.
 
      The return value of 'alphasort' is less than, equal to, or greater
      than zero depending on the order of the two entries A and B.
 
  -- Function: int versionsort (const struct dirent **A, const struct
           dirent **B)
      Preliminary: | MT-Safe locale | AS-Safe | AC-Safe |  POSIX
      Safety Concepts.
 
      The 'versionsort' function is like 'alphasort' except that it uses
      the 'strverscmp' function internally.
 
    If the filesystem supports large files we cannot use the 'scandir'
 anymore since the 'dirent' structure might not able to contain all the
 information.  The LFS provides the new type 'struct dirent64'.  To use
 this we need a new function.
 
  -- Function: int scandir64 (const char *DIR, struct dirent64
           ***NAMELIST, int (*SELECTOR) (const struct dirent64 *), int
           (*CMP) (const struct dirent64 **, const struct dirent64 **))
      Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | 
      POSIX Safety Concepts.
 
      The 'scandir64' function works like the 'scandir' function except
      that the directory entries it returns are described by elements of
      type 'struct dirent64'.  The function pointed to by SELECTOR is
      again used to select the desired entries, except that SELECTOR now
      must point to a function which takes a 'struct dirent64 *'
      parameter.
 
      Similarly the CMP function should expect its two arguments to be of
      type 'struct dirent64 **'.
 
    As CMP is now a function of a different type, the functions
 'alphasort' and 'versionsort' cannot be supplied for that argument.
 Instead we provide the two replacement functions below.
 
  -- Function: int alphasort64 (const struct dirent64 **A, const struct
           dirent **B)
      Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem |
       POSIX Safety Concepts.
 
      The 'alphasort64' function behaves like the 'strcoll' function
      ( String/Array Comparison).  The difference is that the
      arguments are not string pointers but instead they are of type
      'struct dirent64 **'.
 
      Return value of 'alphasort64' is less than, equal to, or greater
      than zero depending on the order of the two entries A and B.
 
  -- Function: int versionsort64 (const struct dirent64 **A, const struct
           dirent64 **B)
      Preliminary: | MT-Safe locale | AS-Safe | AC-Safe |  POSIX
      Safety Concepts.
 
      The 'versionsort64' function is like 'alphasort64', excepted that
      it uses the 'strverscmp' function internally.
 
    It is important not to mix the use of 'scandir' and the 64-bit
 comparison functions or vice versa.  There are systems on which this
 works but on others it will fail miserably.
 

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