Manpages - getdents.2

Table of Contents

NAME

getdents, getdents64 - get directory entries

SYNOPSIS

  #include <sys/syscall.h> /* Definition of SYS_* constants */
  #include <unistd.h>

  long syscall(SYS_getdents, unsigned int fd",structlinux_dirent*"dirp,
   unsigned int count);

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

  ssize_t getdents64(int fd, void *dirp, 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.

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_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 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_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;    /* 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 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.

CONFORMING TO

SVr4.

NOTES

Library support for *getdents64*() was added in glibc 2.30; Glibc does not provide a wrapper for *getdents*(); call *getdents*() (or *getdents64*() on earlier glibc versions) 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:

  $ ./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 <stdint.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 {
      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;
      long nread;
      char buf[BUF_SIZE];
      struct linux_dirent *d;
      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 (long 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 %10jd  %s\n", d->d_reclen,
                      (intmax_t) d->d_off, d->d_name);
              bpos += d->d_reclen;
          }
      }

      exit(EXIT_SUCCESS);
  }

SEE ALSO

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

COLOPHON

This page is part of release 5.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Author: dt

Created: 2022-02-23 Wed 11:33