path: root/man2/getdents.2

.\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
.\"
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.\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 22 July 1995 by Michael Chastain <mec@duracef.shout.net>:
.\"   Derived from 'readdir.2'.
.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\"
.TH GETDENTS 2  2012-08-03 "Linux" "Linux Programmer's Manual"
.SH NAME
getdents \- get directory entries
.SH SYNOPSIS
.nf
.BI "int getdents(unsigned int " fd ", struct linux_dirent *" dirp ,
.BI "             unsigned int " count );
.fi

.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
This is not the function you are interested in.
Look at
.BR readdir (3)
for the POSIX conforming C library interface.
This page documents the bare kernel system call interface.
.PP
The system call
.BR getdents ()
reads several
.I linux_dirent
structures from the directory
referred to by the open file descriptor
.I fd
into the buffer pointed to by
.IR dirp .
The argument
.I count
specifies the size of that buffer.
.PP
The
.I linux_dirent
structure is declared as follows:
.PP
.in +4n
.nf
struct linux_dirent {
    unsigned long  d_ino;     /* Inode number */
    unsigned long  d_off;     /* Offset to next \fIlinux_dirent\fP */
    unsigned short d_reclen;  /* Length of this \fIlinux_dirent\fP */
    char           d_name[];  /* Filename (null-terminated) */
                      /* length is actually (d_reclen \- 2 \-
                         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)
    */

}
.fi
.in
.PP
.I d_ino
is an inode number.
.I d_off
is the distance from the start of the directory to the start of the next
.IR linux_dirent .
.I d_reclen
is the size of this entire
.IR linux_dirent .
.I d_name
is a null-terminated filename.

.I 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
.IR <dirent.h> ):
.TP 12
.B DT_BLK
This is a block device.
.TP
.B DT_CHR
This is a character device.
.TP
.B DT_DIR
This is a directory.
.TP
.B DT_FIFO
This is a named pipe (FIFO).
.TP
.B DT_LNK
This is a symbolic link.
.TP
.B DT_REG
This is a regular file.
.TP
.B DT_SOCK
This is a UNIX domain socket.
.TP
.B DT_UNKNOWN
The file type is unknown.
.PP
The
.I d_type
field is implemented since Linux 2.6.4.
It occupies a space that was previously a zero-filled padding byte in the
.IR linux_dirent
structure.
Thus, on kernels before 2.6.3,
attempting to access this field always provides the value 0
.RB ( DT_UNKNOWN ).
.PP
Currently,
.\" kernel 2.6.27
.\" The same sentence is in readdir.2
only some filesystems (among them: Btrfs, ext2, ext3, and ext4)
have full support for returning the file type in
.IR d_type .
All applications must properly handle a return of
.BR DT_UNKNOWN .
.SH RETURN VALUE
On success, the number of bytes read is returned.
On end of directory, 0 is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
Invalid file descriptor
.IR fd .
.TP
.B EFAULT
Argument points outside the calling process's address space.
.TP
.B EINVAL
Result buffer is too small.
.TP
.B ENOENT
No such directory.
.TP
.B ENOTDIR
File descriptor does not refer to a directory.
.SH CONFORMING TO
SVr4.
.\" SVr4 documents additional ENOLINK, EIO error conditions.
.SH NOTES
Glibc does not provide a wrapper for this system call; call it using
.BR syscall (2).
You will need to define the
.I linux_dirent
structure yourself.
However, you probably want to use
.BR readdir (3)
instead.

This call supersedes
.BR readdir (2).

The original Linux
.BR getdents ()
system call did not handle large filesystems and large file offsets.
Consequently, Linux 2.4 added
.BR getdents64 (),
with wider types for the
.I d_ino
and
.I d_off
fields employed in the
.IR linux_dirent
structure.
.SH EXAMPLE
.\" FIXME: This program uses the older getdents() system call
.\" and the structure with smaller field widths.
The program below demonstrates the use of
.BR getdents ().
The following output shows an example of what we see when running this
program on an ext2 directory:

.in +4n
.nf
.RB "$" " ./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
.fi
.in
.SS Program source
\&
.nf
#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, d\->d_name);
            bpos += d\->d_reclen;
        }
    }

    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR readdir (2),
.BR readdir (3)