getdents — get directory entries
int getdents( |
unsigned int fd, |
| struct linux_dirent *dirp, | |
unsigned int count); |
This is not the function you are interested in. Look at readdir(3) for the POSIX conforming C library interface. This page documents the bare kernel system call interface.
The system call getdents()
reads several linux_dirent
structures from the directory referred to by the open file
descriptor fd into
the buffer pointed to by dirp. The argument count specifies the size of
that buffer.
The linux_dirent structure is declared as follows:
struct linux_dirent { unsigned long d_ino;unsigned long d_off;unsigned short d_reclen;char d_name[];
offsetof(struct linux_dirent, d_name) */
char pad; // Zero padding byte
char d_type; // File type (only since Linux 2.6.4;
// offset is (d_reclen \- 1))
*/};
d_ino is an inode
number. d_off is the
distance from the start of the directory to the start of the
next linux_dirent. d_reclen is the size of this
entire linux_dirent.
d_name is a
null-terminated filename.
d_type is a byte
at the end of the structure that indicates the file type. It
contains one of the following values (defined in <dirent.h>
DT_BLKThis is a block device.
DT_CHRThis is a character device.
DT_DIRThis is a directory.
DT_FIFOThis is a named pipe (FIFO).
DT_LNKThis is a symbolic link.
DT_REGThis is a regular file.
DT_SOCKThis is a Unix domain socket.
DT_UNKNOWNThe file type is unknown.
The d_type field
is implemented since Linux 2.6.4. It occupies a space that
was previously a zero-filled padding byte in the linux_dirent structure. Thus, on kernels
before 2.6.3, attempting to access this field always provides
the value 0 (DT_UNKNOWN).
Currently, only some file systems (among them: Btrfs,
ext2, ext3, and ext4) have full support for returning the
file type in d_type. All applications must
properly handle a return of DT_UNKNOWN.
On success, the number of bytes read is returned. On end
of directory, 0 is returned. On error, −1 is returned,
and errno is set
appropriately.
Invalid file descriptor fd.
Argument points outside the calling process's address space.
Result buffer is too small.
No such directory.
File descriptor does not refer to a directory.
Glibc does not provide a wrapper for this system call; call it using syscall(2). You will need to define the linux_dirent structure yourself.
This call supersedes readdir(2).
The program below demonstrates the use of getdents(). The following output shows an
example of what we see when running this program on an ext2
directory:
$ ./a.out /testfs/ --------------- nread=120 --------------- i-node# file type d_reclen d_off d_name 2 directory 16 12 . 2 directory 16 24 .. 11 directory 24 44 lost+found 12 regular 16 56 a 228929 directory 16 68 sub 16353 directory 16 80 sub2 130817 directory 16 4096 sub3
#define _GNU_SOURCE
#include <dirent.h> /* Defines DT_* constants */
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
struct linux_dirent {
long d_ino;
off_t d_off;
unsigned short d_reclen;
char d_name[];
};
#define BUF_SIZE 1024
int
main(int argc, char *argv[])
{
int fd, nread;
char buf[BUF_SIZE];
struct linux_dirent *d;
int bpos;
char d_type;
fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
if (fd == −1)
handle_error("open");
for ( ; ; ) {
nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
if (nread == −1)
handle_error("getdents");
if (nread == 0)
break;
printf("−-------------- nread=%d ---------------\n", nread);
printf("i−node# file type d_reclen d_off d_name\n");
for (bpos = 0; bpos < nread;) {
d = (struct linux_dirent *) (buf + bpos);
printf("%8ld ", d−>d_ino);
d_type = *(buf + bpos + d−>d_reclen - 1);
printf("%−10s ", (d_type == DT_REG) ? "regular" :
(d_type == DT_DIR) ? "directory" :
(d_type == DT_FIFO) ? "FIFO" :
(d_type == DT_SOCK) ? "socket" :
(d_type == DT_LNK) ? "symlink" :
(d_type == DT_BLK) ? "block dev" :
(d_type == DT_CHR) ? "char dev" : "???");
printf("%4d %10lld %s\n", d−>d_reclen,
(long long) d−>d_off, (char *) d->d_name);
bpos += d−>d_reclen;
}
}
exit(EXIT_SUCCESS);
}
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright (C) 1995 Andries Brouwer (aebcwi.nl) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. Written 11 June 1995 by Andries Brouwer <aebcwi.nl> Modified 22 July 1995 by Michael Chastain <mecduracef.shout.net>: Derived from 'readdir.2'. Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esrthyrsus.com> |