readdir(3) Library Functions Manual readdir(3)
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 repre-
senting 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 VERSIONS.
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 VER-
SIONS.
d_type This field contains a value indicating the file type, making it
possible to avoid the expense of calling lstat(2) if further ac-
tions 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 re-
turned 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_UN-
KNOWN.
d_name This field contains the null terminated filename; see VERSIONS.
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 er-
rno 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 (includ-
ing the glibc implementation), concurrent calls to readdir() that spec-
ify different directory streams are thread-safe. In cases where multi-
ple threads must read from the same directory stream, using readdir()
with external synchronization is still preferable to the use of the dep-
recated readdir_r(3) function. It is expected that a future version of
POSIX.1 will require that readdir() be thread-safe when concurrently em-
ployed 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 termi-
nating 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 in-
correct; 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 (cor-
rectly) 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.
SEE ALSO
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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