getdents, getdents64 - get directory entries

#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
long syscall(SYS_getdents, unsigned int fd, struct linux_dirent *dirp,
 unsigned int count);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <dirent.h>
ssize_t getdents64(int fd, void dirp[.count], size_t count);

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.

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.

getdents()

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;     /* Not an offset; see below */
    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 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:

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

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.

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 ---------------
inode#    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 <err.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <unistd.h>
struct linux_dirent {
    unsigned 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;
    char                 d_type;
    char                 buf[BUF_SIZE];
    long                 nread;
    struct linux_dirent  *d;
    fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
    if (fd == -1)
        err(EXIT_FAILURE, "open");
    for (;;) {
        nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
        if (nread == -1)
            err(EXIT_FAILURE, "getdents");
        if (nread == 0)
            break;
        printf("--------------- nread=%ld ---------------\n", nread);
        printf("inode#    file type  d_reclen  d_off   d_name\n");
        for (size_t bpos = 0; bpos < nread;) {
            d = (struct linux_dirent *) (buf + bpos);
            printf("%8lu  ", 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 %10jd  %s\n", d->d_reclen,
                   (intmax_t) d->d_off, d->d_name);
            bpos += d->d_reclen;
        }
    }
    exit(EXIT_SUCCESS);
}

readdir(2), readdir(3), inode(7)