getdents(2)

—

Options · 14

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.
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.
getdents64()	glibc 2.30.

NAME

getdents, getdents64 - get directory entries

SYNOPSIS

bash

#include <sys/syscall.h>\n      /* Definition of \nSYS_*\n constants */
\n#include <unistd.h>

bash

long syscall(SYS_getdents, unsigned int \nfd\n, struct linux_dirent *\ndirp\n,\n
\n             unsigned int \ncount\n);

bash

#define _GNU_SOURCE\n           /* See feature_test_macros(7) */
\n#include <dirent.h>

bash

ssize_t getdents64(int \nfd\n, void \ndirp\n[.\ncount\n], size_t \ncount\n);

Note: glibc provides no wrapper for getdents(), necessitating the use of syscall(2).

Note: There is no definition of struct linux_dirent in glibc; see NOTES.

DESCRIPTION

These are not the interfaces you are interested in. Look at readdir(3) for the POSIX-conforming C library interface. This page documents the bare kernel system call interfaces.

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:

bash

struct linux_dirent {
\n
    unsigned long  d_ino;     /* Inode number */
\n
    unsigned long  d_off;     /* Not an offset; see below */
\n
    unsigned short d_reclen;  /* Length of this \nlinux_dirent\n */
\n
    char           d_name[];  /* Filename (null-terminated) */
\n
                      /* length is actually (d_reclen - 2 -
\n
                         offsetof(struct linux_dirent, d_name)) */
\n
    /*
\n
    char           pad;       // Zero padding byte
\n
    char           d_type;    // File type (only since Linux
\n
                              // 2.6.4); offset is (d_reclen - 1)
\n
    */
}

d_ino is an inode number. d_off is a filesystem-specific value with no specific meaning to user space, though on older filesystems it used to be the distance from the start of the directory to the start of the next linux_dirent; see readdir(3).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 up to and including Linux 2.6.3, attempting to access this field always provides the value 0 (DT_UNKNOWN).

Currently, only some filesystems (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.

getdents64()

The original Linux getdents() system call did not handle large filesystems and large file offsets. Consequently, Linux 2.4 added getdents64(), with wider types for the d_ino and d_off fields. In addition, getdents64() supports an explicit d_type field.

The getdents64() system call is like getdents(), except that its second argument is a pointer to a buffer containing structures of the following type:

bash

struct linux_dirent64 {
\n
    ino64_t        d_ino;    /* 64-bit inode number */
\n
    off64_t        d_off;    /* Not an offset; see getdents() */
\n
    unsigned short d_reclen; /* Size of this dirent */
\n
    unsigned char  d_type;   /* File type */
\n
    char           d_name[]; /* Filename (null-terminated) */
};

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 to indicate the error.

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.

STANDARDS

None.

HISTORY

SVr4.

getdents64(): glibc 2.30.

NOTES

glibc does not provide a wrapper for getdents(); call getdents() using syscall(2). In that case you will need to define the linux_dirent or linux_dirent64 structure yourself.

Probably, you want to use readdir(3) instead of these system calls.

These calls supersede readdir(2).

EXAMPLES

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:

bash

$\n ./a.out /testfs/\n
--------------- nread=120 ---------------
inode#    file type  d_reclen  d_off   d_name
\n
       2  directory    16         12  .
\n
       2  directory    16         24  ..
\n
      11  directory    24         44  lost+found
\n
      12  regular      16         56  a
\n
  228929  directory    16         68  sub
\n
   16353  directory    16         80  sub2
\n
  130817  directory    16       4096  sub3

Program source

bash

#define _GNU_SOURCE
#include <dirent.h>     /* Defines DT_* constants */
#include <err.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
struct linux_dirent {
\n
    unsigned long  d_ino;
\n
    off_t          d_off;
\n
    unsigned short d_reclen;
\n
    char           d_name[];
};
#define BUF_SIZE 1024
int
main(int argc, char *argv[])
{
\n
    int                  fd;
\n
    char                 d_type;
\n
    char                 buf[BUF_SIZE];
\n
    long                 nread;
\n
    struct linux_dirent  *d;
\n
    fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
\n
    if (fd == -1)
\n
        err(EXIT_FAILURE, "open");
\n
    for (;;) {
\n
        nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
\n
        if (nread == -1)
\n
            err(EXIT_FAILURE, "getdents");
\n
        if (nread == 0)
\n
            break;
\n
        printf("--------------- nread=%ld ---------------\n", nread);
\n
        printf("inode#    file type  d_reclen  d_off   d_name\n");
\n
        for (size_t bpos = 0; bpos < nread;) {
\n
            d = (struct linux_dirent *) (buf + bpos);
\n
            printf("%8lu  ", d->d_ino);
\n
            d_type = *(buf + bpos + d->d_reclen - 1);
\n
            printf("%-10s ", (d_type == DT_REG) ?  "regular" :
\n
                             (d_type == DT_DIR) ?  "directory" :
\n
                             (d_type == DT_FIFO) ? "FIFO" :
\n
                             (d_type == DT_SOCK) ? "socket" :
\n
                             (d_type == DT_LNK) ?  "symlink" :
\n
                             (d_type == DT_BLK) ?  "block dev" :
\n
                             (d_type == DT_CHR) ?  "char dev" : "???");
\n
            printf("%4d %10jd  %s\n", d->d_reclen,
\n
                   (intmax_t) d->d_off, d->d_name);
\n
            bpos += d->d_reclen;
\n
        }
\n
    }
\n
    exit(EXIT_SUCCESS);
}

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

getdents()

getdents64()

RETURN VALUE

ERRORS

STANDARDS

HISTORY

NOTES

EXAMPLES

Program source

SEE ALSO