FDOPENDIR(3P) POSIX Programmer's Manual FDOPENDIR(3P)
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 resolu-
tion 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 exis-
tence 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 implemen-
tations 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 opera-
tions 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 direc-
tory stream has been closed or after a fork() or one of the exec func-
tion calls.
FUTURE DIRECTIONS
None.
SEE ALSO
closedir(), dirfd(), fstatat(), open(), readdir(), rewinddir(), sym-
link()
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 Electri-
cal 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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FDOPENDIR(3P)
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