READDIR(3P) POSIX Programmer's Manual READDIR(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
readdir, readdir_r -- read a directory
SYNOPSIS
#include <dirent.h>
struct dirent *readdir(DIR *dirp);
int readdir_r(DIR *restrict dirp, struct dirent *restrict entry,
struct dirent **restrict result);
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 asyn-
chronously to the operation of readdir().
The readdir() function shall return a pointer to a structure represent-
ing 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; other-
wise, 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.
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 func-
tions, 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 refer-
enced 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 direc-
tory 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 ele-
ments.
Upon successful return, the pointer returned at *result shall have the
same value as the argument entry. Upon reaching the end of the direc-
tory 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.
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.
ERRORS
These functions shall fail if:
EOVERFLOW
One of the values in the structure to be returned cannot be rep-
resented 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.
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);
}
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.
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 perfor-
mance-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; other-
wise, 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 val-
ues in a user-supplied buffer instead of possibly using a static data
area that may be overwritten by each call. Either the {NAME_MAX} com-
pile-time constant or the corresponding pathconf() option can be used
to determine the maximum sizes of returned pathnames.
FUTURE DIRECTIONS
None.
SEE ALSO
closedir(), dirfd(), exec, fdopendir(), fstatat(), 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 READDIR(3P)
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