DIRECTORY(3) BSD Library Functions Manual DIRECTORY(3)NAME
fdopendir, opendir, readdir, readdir_r, telldir, seekdir, rewinddir,
closedir, dirfd — directory operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <dirent.h>
DIR *
opendir(const char *filename);
DIR *
fdopendir(int fd);
struct dirent *
readdir(DIR *dirp);
int
readdir_r(DIR * restrict dirp, struct dirent * restrict entry,
struct dirent ** restrict result);
long
telldir(DIR *dirp);
void
seekdir(DIR *dirp, long loc);
void
rewinddir(DIR *dirp);
int
closedir(DIR *dirp);
int
dirfd(DIR *dirp);
DESCRIPTION
The type DIR represents a directory stream; an ordered sequence of all
directory entries in a particular directory. The purpose of the DIR
structure is similar to that of the FILE structure maintained by the
stdio(3) library functions.
FUNCTIONS
The following standard directory operations are defined.
opendir(filename)
The opendir() function opens the directory named by filename and as‐
sociates a directory stream with it. The directory stream is posi‐
tioned at the first entry. Upon successful completion, a pointer to
DIR type is returned. Otherwise, opendir() returns NULL.
fdopendir(fd)
The fdopendir() function associates a directory stream with the
directory file descriptor fd. The file offset associated with fd at
the time of the call determines which entries are returned.
Upon failure, fdopendir() returns NULL. Otherwise the file descrip‐
tor is under the control of the system, and if any attempt is made
to close the file descriptor, or to modify the state of the associ‐
ated description, other than by means of closedir(), readdir(),
readdir_r(), rewinddir(), the behavior is undefined. The file
descriptor can be closed by calling closedir().
readdir(dirp)
The readdir() function returns a pointer to the directory entry at
the current position in the directory stream specified by dirp, and
positions the directory stream at the next entry. It returns NULL
upon reaching the end of the directory or detecting an invalid
seekdir() operation. The returned structure is described in
dirent(3).
The returned pointer to the dirent structure points to data which
may be overwritten by another call to readdir() on the same direc‐
tory stream. This data is not however overwritten by another call
to readdir() on a different directory stream.
readdir_r(dirp, entry, result)
The readdir_r() function provides the same functionality as
readdir(), but the caller must provide a directory entry buffer to
store the results in. If the read succeeds, result is pointed at
the entry; upon reaching the end of the directory result is set to
NULL. The readdir_r() function returns 0 on success or an error
number to indicate failure.
Like readdir(), the readdir_r() function may buffer several direc‐
tory entries per actual read operation. Both functions mark for
update the st_atime field (see stat(2)) of the directory each time
the directory is actually read.
telldir(dirp)
The telldir() function returns the current location associated with
the directory stream specified by dirp.
If the most recent operation on the particular directory stream was
a seekdir(), the directory position returned from telldir() is the
same as loc supplied as an argument to the seekdir() call.
seekdir(dirp, loc)
The seekdir() function sets the position of the next readdir() oper‐
ation on the directory stream specified by dirp. The value of loc
should come from a previous call to telldir() using the same direc‐
tory stream.
The new position reverts to the one associated with the directory
stream when the telldir() operation was performed. Values returned
by telldir() are good only for the lifetime of the DIR pointer,
dirp, from which they are derived. If the directory is closed and
then reopened, the telldir() value cannot be re-used.
rewinddir(dirp)
The rewinddir() function resets the position of the named directory
stream to the beginning of the directory. It also causes the direc‐
tory stream to refer to the current state of the corresponding
directory, as if a call to opendir() would have been made.
If dirp does not refer to a valid directory stream, the behavior is
undefined.
closedir(dirp)
The closedir() function closes the directory stream and frees the
structure associated with the dirp pointer, returning 0 on success
and -1 on failure.
dirfd(dirp)
The dirfd() function returns the integer file descriptor associated
with the directory stream specified by dirp. Upon failure, dirfd()
returns -1. The returned file descriptor should be closed by
closedir() instead of close(2).
The rationale of dirfd() is to provide a mechanism by which a file
descriptor can be obtained for the use of the fchdir(2) function.
EXAMPLES
Sample code which searches a directory for entry “name” is:
len = strlen(name);
dirp = opendir(".");
if (dirp != NULL) {
while ((dp = readdir(dirp)) != NULL)
if (dp->d_namlen == len &&
!strcmp(dp->d_name, name)) {
(void)closedir(dirp);
return (FOUND);
}
(void)closedir(dirp);
}
return (NOT_FOUND);
COMPATIBILITY
The described directory operations have traditionally been problematic in
terms of portability. A good example is the semantics around ‘.’ (dot)
and ‘..’ (dot-dot). Based on historical implementations, the rules about
file descriptors apply to directory streams as well. The IEEE Std
1003.1-2008 (“POSIX.1”) standard does not however any more mandate that
directory streams are necessarily implemented by using file descriptors.
The following additional remarks can be noted from the IEEE Std
1003.1-2008 (“POSIX.1”) standard.
· If the type DIR is implemented using a file descriptor, like in
NetBSD, applications should be able to open only OPEN_MAX files and
directories. Otherwise the limit is left as unspecified.
· When a file descriptor is used to implement the directory stream,
the closedir() function behaves as if the FD_CLOEXEC had been set
for the file descriptor. In another words, it is mandatory that
closedir() deallocates the file descriptor.
· If directory streams are not implemented by using file descriptors,
functions such as dirfd() may fail with ENOTSUP.
· If a file is removed from or added to the directory after the most
recent call to opendir() or rewinddir(), it is unspecified whether
a subsequent call to readdir() returns an entry for that file.
· When using the function seekdir(), note that if the value of loc
was not obtained from an earlier call to telldir(), or if a call to
rewinddir() occurred between the calls to telldir() and seekdir(),
any subsequent call to readdir() is unspecified, possibly resulting
undefined behavior.
· After a call to fork(2), either the parent or child (but not both)
can continue processing the directory stream using readdir(),
rewinddir(), or seekdir(). However, if both the parent and child
processes use these functions, the result is undefined.
ERRORS
All described functions may set errno to indicate the error.
SEE ALSOclose(2), lseek(2), open(2), read(2), dirent(3)STANDARDS
The opendir(), readdir(), rewinddir() and closedir() functions conform to
ISO/IEC 9945-1:1990 (“POSIX.1”). The other functions conform to IEEE Std
1003.1-2008 (“POSIX.1”).
HISTORY
The opendir(), readdir(), telldir(), seekdir(), rewinddir(), closedir(),
and dirfd() functions appeared in 4.2BSD. The fdopendir() function
appeared in NetBSD 6.0.
BSD October 15, 2011 BSD
