открытый каталог, связанный с файловым дескриптором (open directory associated with file descriptor)
Пролог (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.
Имя (Name)
fdopendir, opendir — open directory associated with file
descriptor
Синопсис (Synopsis)
#include <dirent.h>
DIR *fdopendir(int fd);
DIR *opendir(const char *dirname);
Описание (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().
Возвращаемое значение (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.
Ошибки (Error)
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.
Примеры (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;
}
Использование в приложениях (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(3p)). This
method is recommended for portability.
Обоснование (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 dot-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 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.1‐2017 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.
Будущие направления (Future directions)
None.
Смотри также (See also)
closedir(3p), dirfd(3p), fstatat(3p), open(3p), readdir(3p),
rewinddir(3p), symlink(3p)
The Base Definitions volume of POSIX.1‐2017, dirent.h(0p),
sys_types.h(0p)
