Manual Reference Pages  - GETDENTS (2)

NAME

getdents - get directory entries

CONTENTS

Synopsis
Description
Errors
Notes
Example
     Program source
Colophon

SYNOPSIS


int getdents(unsigned int fd, struct linux_dirent *dirp, 

             unsigned int count); 

Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

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;     /* Inode number */
    unsigned long  d_off;     /* Offset to next linux_dirent */
    unsigned short d_reclen;  /* Length of this linux_dirent */
    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))
    */

}

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_BLK This is a block device.
DT_CHR This is a character device.
DT_DIR This is a directory.
DT_FIFO This is a named pipe (FIFO).
DT_LNK This is a symbolic link.
DT_REG This is a regular file.
DT_SOCK This is a UNIX domain socket.
DT_UNKNOWN The 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.

RETURN VALUE

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.

ERRORS

EBADF Invalid file descriptor fd.
EFAULT Argument points outside the calling process’s address space.
EINVAL Result buffer is too small.
ENOENT No such directory.
ENOTDIR
  File descriptor does not refer to a directory.

CONFORMING TO

SVr4.

NOTES

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. However, you probably want to use readdir(3) instead.

This call supersedes readdir(2).

The original Linux getdents() system call did not handle large file systems and large file offsets. Consequently, Linux 2.4 added getdents64(), with wider types for the d_ino and d_off fields employed in the linux_dirent structure.

EXAMPLE

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

    Program source

#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); }

SEE ALSO

readdir(2), readdir(3)

COLOPHON

This page is part of release 3.44 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/.


Linux GETDENTS (2) 2012-08-03
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