FDOPENDIR
Section: POSIX Programmer's Manual (3P)
Updated: 2017
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PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
fdopendir, opendir
- open directory associated with file descriptor
SYNOPSIS
#include <dirent.h>
DIR *fdopendir(int fd);
DIR *opendir(const char *dirname);
DESCRIPTION
The
fdopendir()
function shall be equivalent to the
opendir()
function except that the directory is specified by a file descriptor
rather than by a name. The file offset associated with the file
descriptor at the time of the call determines which entries are
returned.
Upon successful return from
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
closedir(),
readdir(),
readdir_r(),
rewinddir(),
or
seekdir(),
the behavior is undefined. Upon calling
closedir()
the file descriptor shall be closed.
It is unspecified whether the FD_CLOEXEC flag will be set on the file
descriptor by a successful call to
fdopendir().
The
opendir()
function shall open a directory stream corresponding to the directory
named by the
dirname
argument. The directory stream is positioned at the first entry. If
the type
DIR
is implemented using a file descriptor, applications shall only be
able to open up to a total of
{OPEN_MAX}
files and directories.
If the type
DIR
is implemented using a file descriptor, the descriptor shall be
obtained as if the O_DIRECTORY flag was passed to
open().
RETURN VALUE
Upon successful completion, these functions shall return
a pointer to an object of type
DIR.
Otherwise, these functions shall return a null pointer and set
errno
to indicate the error.
ERRORS
The
fdopendir()
function shall fail if:
- EBADF
-
The
fd
argument is not a valid file descriptor open for reading.
- ENOTDIR
-
The descriptor
fd
is not associated with a directory.
The
opendir()
function shall fail if:
- EACCES
-
Search permission is denied for the component of the path prefix of
dirname
or read permission is denied for
dirname.
- ELOOP
-
A loop exists in symbolic links encountered during resolution of the
dirname
argument.
- ENAMETOOLONG
-
The length of a component of a pathname is longer than
{NAME_MAX}.
- ENOENT
-
A component of
dirname
does not name an existing directory or
dirname
is an empty string.
- ENOTDIR
-
A component of
dirname
names an existing file that is neither a directory nor a symbolic link
to a directory.
The
opendir()
function may fail if:
- ELOOP
-
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the
dirname
argument.
- EMFILE
-
All file descriptors available to the process are currently open.
- ENAMETOOLONG
-
The length of a pathname exceeds
{PATH_MAX},
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
{PATH_MAX}.
- ENFILE
-
Too many files are currently open in the system.
The following sections are informative.
EXAMPLES
Open a Directory Stream
The following program fragment demonstrates how the
opendir()
function is used.
-
#include <dirent.h>
...
DIR *dir;
struct dirent *dp;
...
if ((dir = opendir (".")) == NULL) {
perror ("Cannot open .");
exit (1);
}
while ((dp = readdir (dir)) != NULL) {
...
Find And Open a File
The following program searches through a given directory looking
for files whose name does not begin with a dot and whose size is
larger than 1 MiB.
-
#include <stdio.h>
#include <dirent.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
struct stat statbuf;
DIR *d;
struct dirent *dp;
int dfd, ffd;
if ((d = fdopendir((dfd = open("./tmp", O_RDONLY)))) == NULL) {
fprintf(stderr, "Cannot open ./tmp directoryn");
exit(1);
}
while ((dp = readdir(d)) != NULL) {
if (d->d_name[0] == aq.aq)
continue;
/* there is a possible race condition here as the file
* could be renamed between the readdir and the open */
if ((ffd = openat(dfd, d->d_name, O_RDONLY)) ==-1) {
perror(d->d_name);
continue;
}
if (fstat(ffd, &statbuf) == 0 && statbuf.st_size > (1024*1024)) {
/* found it ... */
printf("%s: %jdKn", d->d_name,
(intmax_t)(statbuf.st_size / 1024));
}
close(ffd);
}
closedir(d); // note this implicitly closes dfd
return 0;
}
APPLICATION USAGE
The
opendir()
function should be used in conjunction with
readdir(),
closedir(),
and
rewinddir()
to examine the contents of the directory (see the EXAMPLES section in
readdir()).
This method is recommended for portability.
RATIONALE
The purpose of the
fdopendir()
function is to enable opening files in directories other than the
current working directory without exposure to race conditions. Any part
of the path of a file could be changed in parallel to a call to
opendir(),
resulting in unspecified behavior.
Based on historical implementations, the rules about file descriptors
apply to directory streams as well. However, this volume of POSIX.1-2017 does not
mandate that the directory stream be implemented using file
descriptors. The description of
closedir()
clarifies that if a file descriptor is used for the directory stream,
it is mandatory that
closedir()
deallocate the file descriptor. When a file descriptor is used to
implement the directory stream, it behaves as if the FD_CLOEXEC
had been set for the file descriptor.
The directory entries for dot
and do-dot
are optional. This volume of POSIX.1-2017 does not provide a way to test
a priori
for their existence because an application that is portable must be
written to look for (and usually ignore) those entries. Writing code
that presumes that they are the first two entries does not always work,
as many implementations permit them to be other than the first two
entries, with a ``normal'' entry preceding them. There is negligible
value in providing a way to determine what the implementation does
because the code to deal with dot and do-dot must be written in any
case and because such a flag would add to the list of those flags
(which has proven in itself to be objectionable) and might be abused.
Since the structure and buffer allocation, if any, for directory
operations are defined by the implementation, this volume of POSIX.1-2017 imposes no
portability requirements for erroneous program constructs, erroneous
data, or the use of unspecified values such as the use or referencing
of a
dirp
value or a
dirent
structure value after a directory stream has been closed or after a
fork()
or one of the
exec
function calls.
FUTURE DIRECTIONS
None.
SEE ALSO
closedir(),
dirfd(),
fstatat(),
open(),
readdir(),
rewinddir(),
symlink()
The Base Definitions volume of POSIX.1-2017,
<dirent.h>,
<sys_types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.-2017, Standard for Information Technology- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/ma-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Open a Directory Stream
-
- Find And Open a File
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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