GETDENTS

Section: Linux Programmer's Manual (2)
Updated: 2009-07-04
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NAME

getdents - get directory entries

SYNOPSIS

int getdents(unsigned int fd, struct linux_dirent *dirp,
             unsigned int count);

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, etx3, 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.

This call supersedes readdir(2).

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, (char *) d->d_name);
            bpos += d->d_reclen;
        }
    }

    exit(EXIT_SUCCESS);
}

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Time: 05:33:03 GMT, December 24, 2015