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READDIR(3) Linux Programmer's Manual READDIR(3)
NAME
readdir - read a directory
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 on glibc versions since 2.19, or _BSD_SOURCE on glibc versions 2.19 and earlier), glibc de‐
fines 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 imple‐
mentation), concurrent calls to readdir() that specify different directory streams are thread-safe. In cases where multiple threads must read from the same di‐
rectory stream, using readdir() with external synchronization is still preferable to the use of the deprecated readdir_r(3) function. It is expected that a fu‐
ture version of POSIX.1 will require that readdir() be thread-safe when concurrently employed on different directory streams.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, 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.
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.
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)
2021-03-22 READDIR(3)