Tribblix: manual page: readdir.3c

READDIR(3C) Standard C Library Functions READDIR(3C)

NAME

readdir, readdir_r - read directory

SYNOPSIS

#include <sys/types.h>
#include <dirent.h>

struct dirent *
readdir(DIR *dirp);

struct dirent *
readdir_r(DIR *dirp, struct dirent *entry);

Standard Conforming

#define _POSIX_PTHREAD_SEMANTICS

int
readdir_r(DIR *restrict dirp, struct dirent *restrict entry,
struct dirent **restrict result);

DESCRIPTION

The type DIR, which is defined in the header <dirent.h>, represents a
directory stream, which is an ordered sequence of all the directory
entries in a particular directory. Directory entries represent files.
Files can be removed from a directory or added to a directory
asynchronously to the operation of readdir() and readdir_r().

readdir()
The readdir() function returns a pointer to a structure representing
the directory entry at the current position in the directory stream
specified by the argument dirp, and positions the directory stream at
the next entry. It returns a null pointer upon reaching the end of the
directory stream. The structure dirent defined by the <dirent.h>
header describes a directory entry.

The readdir() function will not return directory entries containing
empty names. If entries for . (dot) .. (dot-dot) exist, one entry will
be returned for dot and one entry will be returned for dot-dot;
otherwise they will not be returned.

The pointer returned by readdir() points to data that can be
overwritten by another call to readdir() on the same directory stream.
It will not be overwritten by another call to readdir() on a different
directory stream. The returned pointer will remain valid until the
directory stream is freed with a call to closedir(3C). It is safe to
use readdir() in a threaded application, so long as only one thread
reads from the directory stream at any given time.

If a file is removed from or added to the directory after the most
recent call to opendir(3C) or rewinddir(3C), whether a subsequent call
to readdir() returns an entry for that file is unspecified.

The readdir() function can buffer several directory entries per actual
read operation. It marks for update the st_atime field of the
directory each time the directory is actually read.

After a call to fork(2), either the parent or child (but not both) can
continue processing the directory stream using readdir(),
rewinddir(3C), or seekdir(3C). 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.

readdir_r()
Unless the end of the directory stream has been reached or an error
occurred, the readdir_r() function initializes the dirent structure
referenced by entry to represent the directory entry at the current
position in the directory stream referred to by dirp, and positions the
directory stream at the next entry.

The caller must allocate storage pointed to by entry to be large enough
for a dirent structure with an array of char d_name member containing
at least NAME_MAX (that is, pathconf(directory, _PC_NAME_MAX) plus one
element. (_PC_NAME_MAX is defined in <unistd.h>))

While the readdir_r() function was originally added as a re-entrant
version of readdir(), it is not recommended that readdir_r() be used in
new applications and existing software should instead use readdir().
The readdir_r() function has been deprecated in many systems.
Historically, the data returned from readdir() was not specific to the
directory stream making it unsafe in a multi-threaded environment;
however, that is no longer the case.

The readdir_r() function will not return directory entries containing
empty names. It is unspecified whether entries are returned for .
(dot) or .. (dot-dot).

If a file is removed from or added to the directory after the most
recent call to opendir(3C) or rewinddir(3C), whether a subsequent call
to readdir_r() returns an entry for that file is unspecified.

The readdir_r() function can buffer several directory entries per
actual read operation. It marks for update the st_atime field of the
directory each time the directory is actually read.

The standard-conforming version (see standards(7)) of the readdir_r()
function performs all of the actions described above for readdir_r()
and sets the pointer pointed to by result. If a directory entry is
returned, the pointer will be set to the same value as the entry
argument; otherwise, it will be set to NULL.

RETURN VALUES

Upon successful completion, readdir() and the default readdir_r()
return a pointer to an object of type struct dirent. When an error is
encountered, a null pointer is returned and errno is set to indicate
the error. When the end of the directory is encountered, a null
pointer is returned and errno is not changed.

The standard-conforming readdir_r() returns 0 if the end of the
directory is encountered or a directory entry is stored in the
structure referenced by entry. Otherwise, an error number is returned
to indicate the failure.

EXAMPLES

Example 1 Search the current directory for the entry name.

The following sample program will search the current directory for each
of the arguments supplied on the command line:

#include <sys/types.h>
#include <dirent.h>
#include <errno.h>
#include <stdio.h>
#include <strings.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);
}

int
main(int argc, char *argv[])
{
int i;
for (i = 1; i < argc; i++)
lookup(argv[i]);
return (0);
}

ERRORS

The readdir() and readdir_r() functions will fail if:

EOVERFLOW One of the values in the structure to be returned
cannot be represented correctly.

The readdir() and readdir_r() 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.

USAGE

The readdir() and readdir_r() functions should be used in conjunction
with opendir(3C), closedir(3C), and rewinddir(3C) to examine the
contents of the directory. Since readdir() and the default readdir_r()
return a null pointer both at the end of the directory and on error, an
application wanting to check for error situations should set errno to 0
before calling either of these functions. If errno is set to non-zero
on return, an error occurred.

The standard-conforming readdir_r() returns the error number if an
error occurred. It returns 0 on success (including reaching the end of
the directory stream).

The readdir() and readdir_r() functions have transitional interfaces
for 64-bit file offsets. See lf64(7).

INTERFACE STABILITY

Committed

MT-LEVEL
The readdir() function is Unsafe; however, it is Safe if different
directory streams are used concurrently. The readdir_r() function is
Safe.

NOTES

When compiling multithreaded programs, see the MULTITHREADED
APPLICATIONS section of Intro(3).

Solaris 2.4 and earlier releases provided a readdir_r() interface as
specified in POSIX.1c Draft 6. The final POSIX.1c standard changed the
interface as described above. Support for the Draft 6 interface is
provided for compatibility only. New applications and libraries should
use the standard-conforming interface, though preferably readdir().

For POSIX.1c-conforming applications, the _POSIX_PTHREAD_SEMANTICS and
_REENTRANT flags are automatically turned on by defining the
_POSIX_C_SOURCE flag with a value >= 199506L.

illumos February 25, 2021 illumos