readdir(3) — Linux manual page


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

NAME         top

       readdir - read a directory

SYNOPSIS         top

       #include <dirent.h>

       struct dirent *readdir(DIR *dirp);

DESCRIPTION         top

       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

           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).

              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 fur‐
              ther 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

       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         top

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

ERRORS         top

       EBADF  Invalid directory stream descriptor dirp.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │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.

CONFORMING TO         top

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

NOTES         top

       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
       _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.

SEE ALSO         top

       getdents(2), read(2), closedir(3), dirfd(3), ftw(3), offsetof(3),
       opendir(3), readdir_r(3), rewinddir(3), scandir(3), seekdir(3),

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

                                 2019-03-06                       READDIR(3)

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