Manpage of GETDENTS

GETDENTS

Section: Linux Programmer's Manual (2)
Updated: 2015-04-12
Index
 

NAME

getdents, getdents64 - get directory entries  

SYNOPSIS

int getdents(unsigned int fd, struct linux_dirent *dirp,             unsigned int count);int getdents64(unsigned int fd, struct linux_dirent64 *dirp,             unsigned int count);

Note: There are no glibc wrappers for these system calls; 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.  

getdents()

The system call getdents() reads several linux_direntstructures from the directory referred to by the open file descriptor fdinto the buffer pointed to by dirp. The argument countspecifies the size of that buffer.

The linux_direntstructure 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_inois an inode number. d_offis the distance from the start of the directory to the start of the next linux_dirent. d_reclenis the size of this entire linux_dirent. d_nameis a null-terminated filename.

d_typeis 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_typefield is implemented since Linux 2.6.4. It occupies a space that was previously a zero-filled padding byte in the linux_direntstructure. Thus, on kernels up to and including 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_inoand d_offfields. In addition, getdents64() supports an explicit d_typefield.

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;    /* 64-bit offset to next structure */
    unsigned short d_reclen; /* Size of this dirent */
    unsigned char  d_type;   /* File type */
    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 errnois 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 these system calls; call them using syscall(2). You will need to define the linux_direntor linux_dirent64structure yourself. However, you probably want to use readdir(3) instead.

These calls supersede 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 ---------------
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 <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("inode#    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)


 

Index

NAME
SYNOPSIS
DESCRIPTION
getdents()
getdents64()
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
EXAMPLE
Program source
SEE ALSO

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