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READDIR(3) Linux Programmer's Manual READDIR(3)

NAME

     readdir - read a directory

SYNOPSIS

     #include <dirent.h>

     struct dirent *readdir(DIR *dirp);

DESCRIPTION

     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.

     In the glibc implementation, the dirent structure is  defined  as  fol-
     lows:

         struct dirent {
             ino_t          d_ino;       /* Inode number */
             off_t          d_off;       /* Not an offset; see below */
             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]; /* Null-terminated filename */ };

     The  only  fields  in the dirent structure that are mandated by POSIX.1
     are d_name and d_ino.  The other fields  are  unstandardized,  and  not
     present on all systems; see NOTES below for some further details.

     The fields of the dirent structure are as follows:

     d_ino  This is the inode number of the file.

     d_off  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  filesys-
            tems.   Applications should treat this field as an opaque value,
            making no assumptions about its contents; see also telldir(3).

     d_reclen
            This is the size (in bytes) of the returned  record.   This  may
            not  match the size of the structure definition shown above; see
            NOTES.

     d_type This field contains a value indicating the file type, making  it
            possible  to  avoid  the  expense of calling lstat(2) if further
            actions depend on the type of the file.

            When a suitable feature test macro is  defined  (_DEFAULT_SOURCE
            on  glibc  versions since 2.19, or _BSD_SOURCE on glibc versions
            2.19 and earlier), glibc defines the following  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 could not be determined.

            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.

     d_name This field contains the null terminated filename.  See NOTES.

     The data returned by readdir() may be overwritten by  subsequent  calls
     to readdir() for the same directory stream.

RETURN VALUE

     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.  To distinguish end of stream and from an  error,
     set  errno to zero before calling readdir() and then check the value of
     errno if NULL is returned.

ERRORS

     EBADF  Invalid directory stream descriptor dirp.

ATTRIBUTES

     For  an  explanation  of  the  terms  used   in   this   section,   see
     attributes(7).

     +----------+---------------+--------------------------+
     |Interface | Attribute     | Value                    |
     +----------+---------------+--------------------------+
     |readdir() | Thread safety | MT-Unsafe race:dirstream |
     +----------+---------------+--------------------------+

     In  the  current POSIX.1 specification (POSIX.1-2008), readdir() is not
     required  to  be  thread-safe.   However,  in  modern   implementations
     (including  the  glibc  implementation),  concurrent calls to readdir()
     that specify different directory streams  are  thread-safe.   In  cases
     where  multiple threads must read from the same directory stream, using
     readdir() with external synchronization is still preferable to the  use
     of  the deprecated readdir_r(3) function.  It is expected that a future
     version of POSIX.1 will require that readdir() be thread-safe when con-
     currently employed on different directory streams.

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES

     A directory stream is opened using opendir(3).

     The  order in which filenames are read by successive calls to readdir()
     depends on the filesystem implementation; it is unlikely that the names
     will be sorted in any fashion.

     Only the fields d_name and (as an XSI extension) d_ino are specified in
     POSIX.1.  Other than Linux, the d_type field is available  mainly  only
     on  BSD  systems.   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,  _DIRENT_HAVE_D_RECLEN,  _DIRENT_HAVE_D_OFF,  or
     _DIRENT_HAVE_D_TYPE are defined.

 The d_name field
     The  dirent  structure  definition  shown above is taken from the glibc
     headers, and shows the d_name field with a fixed size.

     Warning: applications should avoid any dependence on the  size  of  the
     d_name  field.  POSIX defines it as char d_name[], a character array of
     unspecified size, with at most NAME_MAX characters preceding the termi-
     nating null byte ('\0').

     POSIX.1  explicitly  notes  that  this  field  should not be used as an
     lvalue.  The standard also notes that  the  use  of  sizeof(d_name)  is
     incorrect; use strlen(d_name) instead.  (On some systems, this field is
     defined as char d_name[1]!)   By  implication,  the  use  sizeof(struct
     dirent)  to capture the size of the record including the size of d_name
     is also incorrect.

     Note that while the call

         fpathconf(fd, _PC_NAME_MAX)

     returns the value 255 for most filesystems, on some filesystems  (e.g.,
     CIFS,  Windows SMB servers), the null-terminated filename that is (cor-
     rectly) returned in d_name can actually  exceed  this  size.   In  such
     cases, the d_reclen field will contain a value that exceeds the size of
     the glibc dirent structure shown above.