readdir_r(3) read a directory

Other Alias



#include <dirent.h>

struct dirent *readdir(DIR *dirp);

int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):




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.

On Linux, the dirent structure is defined as follows:

struct dirent {
    ino_t          d_ino;       /* inode number */
    off_t          d_off;       /* not an offset; see NOTES */
    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]; /* filename */

The only fields in the dirent structure that are mandated by POSIX.1 are: d_name[], of unspecified size, with at most NAME_MAX characters preceding the terminating null byte ('\0'); and (as an XSI extension) d_ino. The other fields are unstandardized, and not present on all systems; see NOTES below for some further details.

The data returned by readdir() may be overwritten by subsequent calls to readdir() for the same directory stream.

The readdir_r() function is a reentrant version of readdir(). It reads the next directory entry from the directory stream dirp, and returns it in the caller-allocated buffer pointed to by entry. (See NOTES for information on allocating this buffer.) A pointer to the returned item is placed in *result; if the end of the directory stream was encountered, then NULL is instead returned in *result.


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.

The readdir_r() function returns 0 on success. On error, it returns a positive error number (listed under ERRORS). If the end of the directory stream is reached, readdir_r() returns 0, and returns NULL in *result.


Invalid directory stream descriptor dirp.


For an explanation of the terms used in this section, see attributes(7).
readdir() Thread safetyMT-Unsafe race:dirstream
readdir_r() Thread safetyMT-Safe


SVr4, 4.3BSD, POSIX.1-2001.


Only the fields d_name and d_ino are specified in POSIX.1-2001. 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 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).

Other than Linux, the d_type field is available mainly only on BSD systems. This field makes it possible to avoid the expense of calling lstat(2) if further actions depend on the type of the file. If the _BSD_SOURCE feature test macro is defined, then glibc defines the following macro constants for the value returned in d_type:

This is a block device.
This is a character device.
This is a directory.
This is a named pipe (FIFO).
This is a symbolic link.
This is a regular file.
This is a UNIX domain socket.
The file type is unknown.

If the file type could not be determined, the value DT_UNKNOWN is returned in d_type.

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.

Since POSIX.1 does not specify the size of the d_name field, and other nonstandard fields may precede that field within the dirent structure, portable applications that use readdir_r() should allocate the buffer whose address is passed in entry as follows:

name_max = pathconf(dirpath, _PC_NAME_MAX);
if (name_max == -1)         /* Limit not defined, or error */
    name_max = 255;         /* Take a guess */
len = offsetof(struct dirent, d_name) + name_max + 1;
entryp = malloc(len);
(POSIX.1 requires that d_name is the last field in a struct dirent.)


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