читать каталог (read a directory)
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)
readdir, readdir_r — read a directory
Синопсис (Synopsis)
#include <dirent.h>
struct dirent *readdir(DIR *dirp);
int readdir_r(DIR *restrict dirp, struct dirent *restrict entry,
struct dirent **restrict result);
Описание (Description)
The type DIR, which is defined in the <dirent.h> header,
represents a directory stream, which is an ordered sequence of
all the directory entries in a particular directory. Directory
entries represent files; files may be removed from a directory or
added to a directory asynchronously to the operation of
readdir().
The readdir() function shall return a pointer to a structure
representing the directory entry at the current position in the
directory stream specified by the argument dirp, and position the
directory stream at the next entry. It shall return a null
pointer upon reaching the end of the directory stream. The
structure dirent defined in the <dirent.h> header describes a
directory entry. The value of the structure's d_ino member shall
be set to the file serial number of the file named by the d_name
member. If the d_name member names a symbolic link, the value of
the d_ino member shall be set to the file serial number of the
symbolic link itself.
The readdir() function shall not return directory entries
containing empty names. If entries for dot or dot-dot exist, one
entry shall be returned for dot and one entry shall be returned
for dot-dot; otherwise, they shall not be returned.
The application shall not modify the structure to which the
return value of readdir() points, nor any storage areas pointed
to by pointers within the structure. The returned pointer, and
pointers within the structure, might be invalidated or the
structure or the storage areas might be overwritten by a
subsequent call to readdir() on the same directory stream. They
shall not be affected by a call to readdir() on a different
directory stream. The returned pointer, and pointers within the
structure, might also be invalidated if the calling thread is
terminated.
If a file is removed from or added to the directory after the
most recent call to opendir() or rewinddir(), whether a
subsequent call to readdir() returns an entry for that file is
unspecified.
The readdir() function may buffer several directory entries per
actual read operation; readdir() shall mark for update the last
data access timestamp of the directory each time the directory is
actually read.
After a call to fork(), either the parent or child (but not both)
may continue processing the directory stream using readdir(),
rewinddir(), or seekdir(). If both the parent and child
processes use these functions, the result is undefined.
The readdir() function need not be thread-safe.
Applications wishing to check for error situations should set
errno to 0 before calling readdir(). If errno is set to non-zero
on return, an error occurred.
The readdir_r() function shall initialize the dirent structure
referenced by entry to represent the directory entry at the
current position in the directory stream referred to by dirp,
store a pointer to this structure at the location referenced by
result, and position the directory stream at the next entry.
The storage pointed to by entry shall be large enough for a
dirent with an array of char d_name members containing at least
{NAME_MAX}+1 elements.
Upon successful return, the pointer returned at *result shall
have the same value as the argument entry. Upon reaching the end
of the directory stream, this pointer shall have the value NULL.
The readdir_r() function shall not return directory entries
containing empty names.
If a file is removed from or added to the directory after the
most recent call to opendir() or rewinddir(), whether a
subsequent call to readdir_r() returns an entry for that file is
unspecified.
The readdir_r() function may buffer several directory entries per
actual read operation; readdir_r() shall mark for update the last
data access timestamp of the directory each time the directory is
actually read.
Возвращаемое значение (Return value)
Upon successful completion, readdir() shall return a pointer to
an object of type struct dirent. When an error is encountered, a
null pointer shall be returned and errno shall be set to indicate
the error. When the end of the directory is encountered, a null
pointer shall be returned and errno is not changed.
If successful, the readdir_r() function shall return zero;
otherwise, an error number shall be returned to indicate the
error.
Ошибки (Error)
These functions shall fail if:
EOVERFLOW
One of the values in the structure to be returned cannot
be represented correctly.
These functions may fail if:
EBADF The dirp argument does not refer to an open directory
stream.
ENOENT The current position of the directory stream is invalid.
The following sections are informative.
Примеры (Examples)
The following sample program searches the current directory for
each of the arguments supplied on the command line.
#include <dirent.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
static void lookup(const char *arg)
{
DIR *dirp;
struct dirent *dp;
if ((dirp = opendir(".")) == NULL) {
perror("couldn't open '.'");
return;
}
do {
errno = 0;
if ((dp = readdir(dirp)) != NULL) {
if (strcmp(dp->d_name, arg) != 0)
continue;
(void) printf("found %s\n", arg);
(void) closedir(dirp);
return;
}
} while (dp != NULL);
if (errno != 0)
perror("error reading directory");
else
(void) printf("failed to find %s\n", arg);
(void) closedir(dirp);
return;
}
int main(int argc, char *argv[])
{
int i;
for (i = 1; i < argc; i++)
lookup(argv[i]);
return (0);
}
Использование в приложениях (Application usage)
The readdir() function should be used in conjunction with
opendir(), closedir(), and rewinddir() to examine the contents of
the directory.
The readdir_r() function is thread-safe and shall return values
in a user-supplied buffer instead of possibly using a static data
area that may be overwritten by each call.
Обоснование (Rationale)
The returned value of readdir() merely represents a directory
entry. No equivalence should be inferred.
Historical implementations of readdir() obtain multiple directory
entries on a single read operation, which permits subsequent
readdir() operations to operate from the buffered information.
Any wording that required each successful readdir() operation to
mark the directory last data access timestamp for update would
disallow such historical performance-oriented implementations.
When returning a directory entry for the root of a mounted file
system, some historical implementations of readdir() returned the
file serial number of the underlying mount point, rather than of
the root of the mounted file system. This behavior is considered
to be a bug, since the underlying file serial number has no
significance to applications.
Since readdir() returns NULL when it detects an error and when
the end of the directory is encountered, an application that
needs to tell the difference must set errno to zero before the
call and check it if NULL is returned. Since the function must
not change errno in the second case and must set it to a non-zero
value in the first case, a zero errno after a call returning NULL
indicates end-of-directory; otherwise, an error.
Routines to deal with this problem more directly were proposed:
int derror (dirp)
DIR *dirp;
void clearderr (dirp)
DIR *dirp;
The first would indicate whether an error had occurred, and the
second would clear the error indication. The simpler method
involving errno was adopted instead by requiring that readdir()
not change errno when end-of-directory is encountered.
An error or signal indicating that a directory has changed while
open was considered but rejected.
The thread-safe version of the directory reading function returns
values in a user-supplied buffer instead of possibly using a
static data area that may be overwritten by each call. Either the
{NAME_MAX} compile-time constant or the corresponding pathconf()
option can be used to determine the maximum sizes of returned
pathnames.
Будущие направления (Future directions)
None.
Смотри также (See also)
closedir(3p), dirfd(3p), exec(1p), fdopendir(3p), fstatat(3p),
rewinddir(3p), symlink(3p)
The Base Definitions volume of POSIX.1‐2017, dirent.h(0p),
sys_types.h(0p)
