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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 directory\n");
exit(1);
}
while ((dp = readdir(d)) != NULL) {
if (dp->d_name[0] == '.')
continue;
/* there is a possible race condition here as the file
* could be renamed between the readdir and the open */
if ((ffd = openat(dfd, dp->d_name, O_RDONLY)) == -1) {
perror(dp->d_name);
continue;
}
if (fstat(ffd, &statbuf) == 0 && statbuf.st_size > (1024*1024)) {
/* found it ... */
printf("%s: %jdK\n", dp->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.1‐2008 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 dot-dot are optional. This volume of POSIX.1‐2008 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 dot-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.1‐2008 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.1‐2008, <dirent.h>, <sys_types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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/man-pages/reporting_bugs.html .