|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
|
|
|
READDIR(3P) POSIX Programmer's Manual READDIR(3P)
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.
readdir, readdir_r — read a directory
#include <dirent.h>
struct dirent *readdir(DIR *dirp);
int readdir_r(DIR *restrict dirp, struct dirent *restrict entry,
struct dirent **restrict result);
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.
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.
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.
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);
}
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.
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.
None.
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)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 READDIR(3P)
Pages that refer to this page: dirent.h(0p), dirfd(3p), exec(3p), fdopendir(3p), glob(3p), nftw(3p), rewinddir(3p), seekdir(3p), telldir(3p)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|