freebsd-skq/lib/libc/gen/directory.3

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.\"     @(#)directory.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd May 22, 2017
.Dt DIRECTORY 3
.Os
.Sh NAME
.Nm opendir ,
.Nm fdopendir ,
.Nm readdir ,
.Nm readdir_r ,
.Nm telldir ,
.Nm seekdir ,
.Nm rewinddir ,
.Nm closedir ,
.Nm fdclosedir ,
.Nm dirfd
.Nd directory operations
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In dirent.h
.Ft DIR *
.Fn opendir "const char *filename"
.Ft DIR *
.Fn fdopendir "int fd"
.Ft struct dirent *
.Fn readdir "DIR *dirp"
.Ft int
.Fn readdir_r "DIR *dirp" "struct dirent *entry" "struct dirent **result"
.Ft long
.Fn telldir "DIR *dirp"
.Ft void
.Fn seekdir "DIR *dirp" "long loc"
.Ft void
.Fn rewinddir "DIR *dirp"
.Ft int
.Fn closedir "DIR *dirp"
.Ft int
.Fn fdclosedir "DIR *dirp"
.Ft int
.Fn dirfd "DIR *dirp"
.Sh DESCRIPTION
.Bf -symbolic
The
.Fn readdir_r
interface is deprecated
because it cannot be used correctly unless
.Brq Va NAME_MAX
is a fixed value.
.Ef
.Pp
The
.Fn opendir
function
opens the directory named by
.Fa filename ,
associates a
.Em directory stream
with it
and
returns a pointer to be used to identify the
.Em directory stream
in subsequent operations.
The pointer
.Dv NULL
is returned if
.Fa filename
cannot be accessed, or if it cannot
.Xr malloc 3
enough memory to hold the whole thing.
.Pp
The
.Fn fdopendir
function is equivalent to the
.Fn opendir
function except that the directory is specified by a file descriptor
.Fa fd
rather than by a name.
The file offset associated with the file descriptor at the time of the call
determines which entries are returned.
.Pp
Upon successful return from
.Fn fdopendir ,
the file descriptor 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 associated description other than by means
of
.Fn closedir ,
.Fn readdir ,
.Fn readdir_r ,
or
.Fn rewinddir ,
the behavior is undefined.
Upon calling
.Fn closedir
the file descriptor is closed.
The
.Dv FD_CLOEXEC
flag is set on the file descriptor by a successful call to
.Fn fdopendir .
.Pp
The
.Fn readdir
function
returns a pointer to the next directory entry.
The directory entry remains valid until the next call to
.Fn readdir
or
.Fn closedir
on the same
.Em directory stream .
The function returns
.Dv NULL
upon reaching the end of the directory or on error.
In the event of an error,
.Va errno
may be set to any of the values documented for the
.Xr getdirentries 2
system call.
.Pp
The
.Fn readdir_r
function
provides the same functionality as
.Fn readdir ,
but the caller must provide a directory
.Fa entry
buffer to store the results in.
The buffer must be large enough for a
.Vt struct dirent
with a
.Va d_name
array with
.Brq Va NAME_MAX
+ 1 elements.
If the read succeeds,
.Fa result
is pointed at the
.Fa entry ;
upon reaching the end of the directory
.Fa result
is set to
.Dv NULL .
The
.Fn readdir_r
function
returns 0 on success or an error number to indicate failure.
.Pp
The
.Fn telldir
function
returns a token representing the current location associated with the named
.Em directory stream .
Values returned by
.Fn telldir
are good only for the lifetime of the
.Dv DIR
pointer,
.Fa dirp ,
from which they are derived.
If the directory is closed and then
reopened, prior values returned by
.Fn telldir
will no longer be valid.
Values returned by
.Fn telldir
are also invalidated by a call to
.Fn rewinddir .
.Pp
The
.Fn seekdir
function
sets the position of the next
.Fn readdir
operation on the
.Em directory stream .
The new position reverts to the one associated with the
.Em directory stream
when the
.Fn telldir
operation was performed.
.Pp
The
.Fn rewinddir
function
resets the position of the named
.Em directory stream
to the beginning of the directory.
.Pp
The
.Fn closedir
function
closes the named
.Em directory stream
and frees the structure associated with the
.Fa dirp
pointer,
returning 0 on success.
On failure, \-1 is returned and the global variable
.Va errno
is set to indicate the error.
.Pp
The
.Fn fdclosedir
function is equivalent to the
.Fn closedir
function except that this function returns directory file descriptor instead of
closing it.
.Pp
The
.Fn dirfd
function
returns the integer file descriptor associated with the named
.Em directory stream ,
see
.Xr open 2 .
.Pp
Sample code which searches a directory for entry ``name'' is:
.Bd -literal -offset indent
dirp = opendir(".");
if (dirp == NULL)
	return (ERROR);
len = strlen(name);
while ((dp = readdir(dirp)) != NULL) {
	if (dp->d_namlen == len && strcmp(dp->d_name, name) == 0) {
		(void)closedir(dirp);
		return (FOUND);
	}
}
(void)closedir(dirp);
return (NOT_FOUND);
.Ed
.Sh SEE ALSO
.Xr close 2 ,
.Xr lseek 2 ,
.Xr open 2 ,
.Xr read 2 ,
.Xr dir 5
.Sh HISTORY
The
.Fn opendir ,
.Fn readdir ,
.Fn telldir ,
.Fn seekdir ,
.Fn rewinddir ,
.Fn closedir ,
and
.Fn dirfd
functions appeared in
.Bx 4.2 .
The
.Fn fdopendir
function appeared in
.Fx 8.0 .
.Fn fdclosedir
function appeared in
.Fx 10.0 .
.Sh BUGS
The behaviour of
.Fn telldir
and
.Fn seekdir
is likely to be wrong if there are parallel unlinks happening
and the directory is larger than one page.
There is code to ensure that a
.Fn seekdir
to the location given by a
.Fn telldir
immediately before the last
.Fn readdir
will always set the correct location to return the same value as that last
.Fn readdir
performed.
This is enough for some applications which want to
"push back the last entry read", e.g., Samba.
Seeks back to any other location,
other than the beginning of the directory,
may result in unexpected behaviour if deletes are present.
It is hoped that this situation will be resolved with changes to
.Fn getdirentries
and the VFS.