Manpage of READDIR


Section: Linux Programmer's Manual (3)
Updated: 2017-09-15


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 direntstructure 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 direntstructure that are mandated by POSIX.1 are d_nameand d_ino. The other fields are unstandardized, and not present on all systems; see NOTES below for some further details.

The fields of the direntstructure are as follows:

This is the inode number of the file.
The value returned in d_offis 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_offfield 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).
This is the size (in bytes) of the returned record. This may not match the size of the structure definition shown above; see NOTES.
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_SOURCEon glibc versions since 2.19, or _BSD_SOURCEon glibc versions 2.19 and earlier), 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 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.
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 direntstructure. (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 errnois not changed. If an error occurs, NULL is returned and errnois set appropriately. To distinguish end of stream and from an error, set errnoto zero before calling readdir() and then check the value of errnoif NULL is returned.  


Invalid directory stream descriptor dirp.


For an explanation of the terms used in this section, see attributes(7).
readdir() Thread safetyMT-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_nameand (as an XSI extension) d_inoare specified in POSIX.1. Other than Linux, the d_typefield 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_TYPEare defined.  

The d_name field

The direntstructure definition shown above is taken from the glibc headers, and shows the d_namefield with a fixed size.

Warning: applications should avoid any dependence on the size of the d_namefield. POSIX defines it as char d_name[],a character array of unspecified size, with at most NAME_MAXcharacters preceding the terminating null byte (aq\0aq).

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_nameis 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_namecan actually exceed this size. In such cases, the d_reclenfield will contain a value that exceeds the size of the glibc direntstructure 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)



The d_name field

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