illumos: 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