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You are here: manpages

readdir

Section: C Library Functions (3)
Updated: 202-0-08
Index Return to Main Contents

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 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 VERSIONS. The fields of the dirent structure are as follows:

.d_ino

This is the inode number of the file in the filesystem containing the directory on which readdir() was called. If the directory entry is a mount point, then .d_ino differs from .st_ino returned by stat(2) on this file: .d_ino is the inode number of the mount point, while .st_ino is the inode number of the root directory of the mounted filesystem.

.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 VERSIONS.

.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 since glibc 2.19, or _BSD_SOURCE on glibc 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 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 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	M-Unsafe race:dirstream

In the current POSIX.1 specification (POSIX.-2008), readdir() is not required to be threa-safe. However, in modern implementations (including the glibc implementation), concurrent calls to readdir() that specify different directory streams are threa-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 threa-safe when concurrently employed 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 I].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 ([aq][rs]0[aq]). 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 nul-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.

STANDARDS

POSIX.-2008.

HISTORY

POSIX.-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.