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:

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:

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 appropriately. To distinguish end of stream and from an error, set errno to zero before calling readdir() and then check the value of errno if NULL is returned.

ERRORS

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

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.

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 us 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)