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           #include <dirent.h>
           struct dirent *readdir(DIR *dirp);
           int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result);
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
               _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
               _SVID_SOURCE || _POSIX_SOURCE


           The readdir() function returns a pointer to a dirent  structure  repre-
           senting  the next directory entry in the directory stream pointed to by
           dirp.  It returns NULL on reaching the end of the directory  stream  or
           if an error occurred.
           On Linux, the dirent structure is defined as follows:
               struct dirent {
                   ino_t          d_ino;       /* inode number */
                   off_t          d_off;       /* not an offset; see NOTES */
                   unsigned short d_reclen;    /* length of this record */
                   unsigned char  d_type;      /* type of file; not supported
                                                  by all filesystem types */
                   char           d_name[256]; /* filename */
           The  only  fields  in the dirent structure that are mandated by POSIX.1
           are: d_name[], of unspecified size, with at  most  NAME_MAX  characters
           preceding  the  terminating null byte ('\0'); and (as an XSI extension)
           d_ino.  The other fields are unstandardized, and  not  present  on  all
           systems; see NOTES below for some further details.
           The  data  returned by readdir() may be overwritten by subsequent calls
           to readdir() for the same directory stream.
           The readdir_r() function is a reentrant version of readdir().  It reads
           the next directory entry from the directory stream dirp, and returns it
           in the caller-allocated buffer pointed to by  entry.   (See  NOTES  for
           information on allocating this buffer.)  A pointer to the returned item
           is placed in *result; if the end of the directory  stream  was  encoun-
           tered, then NULL is instead returned in *result.


           On  success,  readdir() returns a pointer to a dirent structure.  (This
           structure may be statically allocated; do not attempt to  free(3)  it.)
           If  the  end  of  the directory stream is reached, NULL is returned and
           errno is not changed.  If an error occurs, NULL is returned  and  errno
           is set appropriately.


           SVr4, 4.3BSD, POSIX.1-2001.


           Only the fields d_name and d_ino are specified  in  POSIX.1-2001.   The
           remaining  fields  are  available  on many, but not all systems.  Under
           glibc, programs can check  for  the  availability  of  the  fields  not
           defined in POSIX.1 by testing whether the macros _DIRENT_HAVE_D_NAMLEN,
           The value returned in d_off is the same as would be returned by calling
           telldir(3) at the current position in the directory stream.   Be  aware
           that  despite  its type and name, the d_off field is seldom any kind of
           directory offset on modern filesystems.  Applications should treat this
           field as an opaque value, making no assumptions about its contents; see
           also telldir(3).
           Other than Linux, the d_type field is available mainly only on BSD sys-
           tems.   This  field  makes  it possible to avoid the expense of calling
           lstat(2) if further actions depend on the type of  the  file.   If  the
           _BSD_SOURCE  feature test macro is defined, then glibc defines the fol-
           lowing macro constants for the value returned in d_type:
           DT_BLK      This is a block device.
           DT_CHR      This is a character device.
           DT_DIR      This is a directory.
           DT_FIFO     This is a named pipe (FIFO).
           DT_LNK      This is a symbolic link.
           DT_REG      This is a regular file.
           DT_SOCK     This is a UNIX domain socket.
           DT_UNKNOWN  The file type is unknown.
           If the file type could not  be  determined,  the  value  DT_UNKNOWN  is
           returned in d_type.
           Currently,  only  some  filesystems (among them: Btrfs, ext2, ext3, and
           ext4) have full support for returning the file  type  in  d_type.   All
           applications must properly handle a return of DT_UNKNOWN.
           Since  POSIX.1 does not specify the size of the d_name field, and other
           nonstandard fields may precede that field within the dirent  structure,
           portable  applications  that use readdir_r() should allocate the buffer
           whose address is passed in entry as follows:
                                      2013-06-21                        READDIR(3)

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