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
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.
- EBADF
- Invalid directory stream descriptor dirp.
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.
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.
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.
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)