Ubuntu Manpage: readdir - read a directory

NAME

       readdir - read a directory

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <dirent.h>

       struct dirent *readdir(DIR *dirp);

DESCRIPTION

       The  readdir()  function  returns  a  pointer  to a dirent structure representing 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 follows:

           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
              filesystems.  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 since glibc 2.19, or
              _BSD_SOURCE on glibc 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  to  indicate  the  error.   To
       distinguish  end  of  stream 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 concurrently employed on different directory streams.

STANDARDS

       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 terminating 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 (correctly) 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.