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.

VERSIONS

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.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, 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.