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
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 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 pointer returned by readdir() points to data which may be
overwritten by another call to readdir() on the same directory stream.
This data is not overwritten by another 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 st_atime field 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.
If the entry names a symbolic link, the value of the d_ino member is
unspecified.
The readdir() function need not be reentrant. A function that is not
required to be reentrant is not required to be thread-safe.
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; the readdir_r() function shall mark for update
the st_atime field of the directory each time the directory is actually
read.
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.
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
The readdir() function shall fail if:
EOVERFLOW
One of the values in the structure to be returned cannot be
represented correctly.
The readdir() function 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 readdir_r() function may fail if:
EBADF The dirp argument does not refer to an open directory stream.
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(arvg[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
st_atime field for update would disallow such historical performance-
oriented implementations.
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.
FUTURE DIRECTIONS
None.
SEE ALSO
closedir() , lstat() , opendir() , rewinddir() , symlink() , the Base
Definitions volume of IEEE Std 1003.1-2001, <dirent.h>, <sys/types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .