Manpages - readdir.3

readdir - read a directory

  #include <dirent.h>

  struct dirent *readdir(DIR *dirp);

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 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.

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.

EBADF

Invalid directory stream descriptor dirp.

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

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.

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

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.

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

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