man2/getdents.2

   1 .\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
   2 .\" and Copyright 2008, 2015 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" %%%LICENSE_START(VERBATIM)
   5 .\" Permission is granted to make and distribute verbatim copies of this
   6 .\" manual provided the copyright notice and this permission notice are
   7 .\" preserved on all copies.
   8 .\"
   9 .\" Permission is granted to copy and distribute modified versions of this
  10 .\" manual under the conditions for verbatim copying, provided that the
  11 .\" entire resulting derived work is distributed under the terms of a
  12 .\" permission notice identical to this one.
  13 .\"
  14 .\" Since the Linux kernel and libraries are constantly changing, this
  15 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  16 .\" responsibility for errors or omissions, or for damages resulting from
  17 .\" the use of the information contained herein.  The author(s) may not
  18 .\" have taken the same level of care in the production of this manual,
  19 .\" which is licensed free of charge, as they might when working
  20 .\" professionally.
  21 .\"
  22 .\" Formatted or processed versions of this manual, if unaccompanied by
  23 .\" the source, must acknowledge the copyright and authors of this work.
  24 .\" %%%LICENSE_END
  25 .\"
  26 .\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
  27 .\" Modified 22 July 1995 by Michael Chastain <mec@duracef.shout.net>:
  28 .\"   Derived from 'readdir.2'.
  29 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  30 .\"
  31 .TH GETDENTS 2  2017-05-03 "Linux" "Linux Programmer's Manual"
  32 .SH NAME
  33 getdents, getdents64 \- get directory entries
  34 .SH SYNOPSIS
  35 .nf
  36 .BI "int getdents(unsigned int " fd ", struct linux_dirent *" dirp ,
  37 .BI "             unsigned int " count );
  38 .BI "int getdents64(unsigned int " fd ", struct linux_dirent64 *" dirp ,
  39 .BI "             unsigned int " count );
  40 .fi
  41 .PP
  42 .IR Note :
  43 There are no glibc wrappers for these system calls; see NOTES.
  44 .SH DESCRIPTION
  45 These are not the interfaces you are interested in.
  46 Look at
  47 .BR readdir (3)
  48 for the POSIX-conforming C library interface.
  49 This page documents the bare kernel system call interfaces.
  50 .SS getdents()
  51 The system call
  52 .BR getdents ()
  53 reads several
  54 .I linux_dirent
  55 structures from the directory
  56 referred to by the open file descriptor
  57 .I fd
  58 into the buffer pointed to by
  59 .IR dirp .
  60 The argument
  61 .I count
  62 specifies the size of that buffer.
  63 .PP
  64 The
  65 .I linux_dirent
  66 structure is declared as follows:
  67 .PP
  68 .in +4n
  69 .nf
  70 struct linux_dirent {
  71     unsigned long  d_ino;     /* Inode number */
  72     unsigned long  d_off;     /* Offset to next \fIlinux_dirent\fP */
  73     unsigned short d_reclen;  /* Length of this \fIlinux_dirent\fP */
  74     char           d_name[];  /* Filename (null-terminated) */
  75                       /* length is actually (d_reclen \- 2 \-
  76                          offsetof(struct linux_dirent, d_name)) */
  77     /*
  78     char           pad;       // Zero padding byte
  79     char           d_type;    // File type (only since Linux
  80                               // 2.6.4); offset is (d_reclen \- 1)
  81     */
  82 }
  83 .fi
  84 .in
  85 .PP
  86 .I d_ino
  87 is an inode number.
  88 .I d_off
  89 is the distance from the start of the directory to the start of the next
  90 .IR linux_dirent .
  91 .I d_reclen
  92 is the size of this entire
  93 .IR linux_dirent .
  94 .I d_name
  95 is a null-terminated filename.
  96 .PP
  97 .I d_type
  98 is a byte at the end of the structure that indicates the file type.
  99 It contains one of the following values (defined in
 100 .IR <dirent.h> ):
 101 .TP 12
 102 .B DT_BLK
 103 This is a block device.
 104 .TP
 105 .B DT_CHR
 106 This is a character device.
 107 .TP
 108 .B DT_DIR
 109 This is a directory.
 110 .TP
 111 .B DT_FIFO
 112 This is a named pipe (FIFO).
 113 .TP
 114 .B DT_LNK
 115 This is a symbolic link.
 116 .TP
 117 .B DT_REG
 118 This is a regular file.
 119 .TP
 120 .B DT_SOCK
 121 This is a UNIX domain socket.
 122 .TP
 123 .B DT_UNKNOWN
 124 The file type is unknown.
 125 .PP
 126 The
 127 .I d_type
 128 field is implemented since Linux 2.6.4.
 129 It occupies a space that was previously a zero-filled padding byte in the
 130 .IR linux_dirent
 131 structure.
 132 Thus, on kernels up to and including 2.6.3,
 133 attempting to access this field always provides the value 0
 134 .RB ( DT_UNKNOWN ).
 135 .PP
 136 Currently,
 137 .\" kernel 2.6.27
 138 .\" The same sentence is in readdir.2
 139 only some filesystems (among them: Btrfs, ext2, ext3, and ext4)
 140 have full support for returning the file type in
 141 .IR d_type .
 142 All applications must properly handle a return of
 143 .BR DT_UNKNOWN .
 144 .SS getdents64()
 145 The original Linux
 146 .BR getdents ()
 147 system call did not handle large filesystems and large file offsets.
 148 Consequently, Linux 2.4 added
 149 .BR getdents64 (),
 150 with wider types for the
 151 .I d_ino
 152 and
 153 .I d_off
 154 fields.
 155 In addition,
 156 .BR getdents64 ()
 157 supports an explicit
 158 .I d_type
 159 field.
 160 .PP
 161 The
 162 .BR getdents64 ()
 163 system call is like
 164 .BR getdents (),
 165 except that its second argument is a pointer to a buffer containing
 166 structures of the following type:
 167 .PP
 168 .nf
 169 .in +4n
 170 struct linux_dirent64 {
 171     ino64_t        d_ino;    /* 64-bit inode number */
 172     off64_t        d_off;    /* 64-bit offset to next structure */
 173     unsigned short d_reclen; /* Size of this dirent */
 174     unsigned char  d_type;   /* File type */
 175     char           d_name[]; /* Filename (null-terminated) */
 176 };
 177 .in
 178 .fi
 179 .SH RETURN VALUE
 180 On success, the number of bytes read is returned.
 181 On end of directory, 0 is returned.
 182 On error, \-1 is returned, and
 183 .I errno
 184 is set appropriately.
 185 .SH ERRORS
 186 .TP
 187 .B EBADF
 188 Invalid file descriptor
 189 .IR fd .
 190 .TP
 191 .B EFAULT
 192 Argument points outside the calling process's address space.
 193 .TP
 194 .B EINVAL
 195 Result buffer is too small.
 196 .TP
 197 .B ENOENT
 198 No such directory.
 199 .TP
 200 .B ENOTDIR
 201 File descriptor does not refer to a directory.
 202 .SH CONFORMING TO
 203 SVr4.
 204 .\" SVr4 documents additional ENOLINK, EIO error conditions.
 205 .SH NOTES
 206 Glibc does not provide a wrapper for these system calls; call them using
 207 .BR syscall (2).
 208 You will need to define the
 209 .I linux_dirent
 210 or
 211 .I linux_dirent64
 212 structure yourself.
 213 However, you probably want to use
 214 .BR readdir (3)
 215 instead.
 216 .PP
 217 These calls supersede
 218 .BR readdir (2).
 219 .SH EXAMPLE
 220 .\" FIXME The example program needs to be revised, since it uses the older
 221 .\" getdents() system call and the structure with smaller field widths.
 222 The program below demonstrates the use of
 223 .BR getdents ().
 224 The following output shows an example of what we see when running this
 225 program on an ext2 directory:
 226 .PP
 227 .in +4n
 228 .nf
 229 .RB "$" " ./a.out /testfs/"
 230 --------------- nread=120 ---------------
 231 inode#    file type  d_reclen  d_off   d_name
 232        2  directory    16         12  .
 233        2  directory    16         24  ..
 234       11  directory    24         44  lost+found
 235       12  regular      16         56  a
 236   228929  directory    16         68  sub
 237    16353  directory    16         80  sub2
 238   130817  directory    16       4096  sub3
 239 .fi
 240 .in
 241 .SS Program source
 242 \&
 243 .EX
 244 #define _GNU_SOURCE
 245 #include <dirent.h>     /* Defines DT_* constants */
 246 #include <fcntl.h>
 247 #include <stdio.h>
 248 #include <unistd.h>
 249 #include <stdlib.h>
 250 #include <sys/stat.h>
 251 #include <sys/syscall.h>
 252
 253 #define handle_error(msg) \\
 254         do { perror(msg); exit(EXIT_FAILURE); } while (0)
 255
 256 struct linux_dirent {
 257     long           d_ino;
 258     off_t          d_off;
 259     unsigned short d_reclen;
 260     char           d_name[];
 261 };
 262
 263 #define BUF_SIZE 1024
 264
 265 int
 266 main(int argc, char *argv[])
 267 {
 268     int fd, nread;
 269     char buf[BUF_SIZE];
 270     struct linux_dirent *d;
 271     int bpos;
 272     char d_type;
 273
 274     fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
 275     if (fd == \-1)
 276         handle_error("open");
 277
 278     for ( ; ; ) {
 279         nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
 280         if (nread == \-1)
 281             handle_error("getdents");
 282
 283         if (nread == 0)
 284             break;
 285
 286         printf("\-\-\-\-\-\-\-\-\-\-\-\-\-\-\- nread=%d \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\\n", nread);
 287         printf("inode#    file type  d_reclen  d_off   d_name\\n");
 288         for (bpos = 0; bpos < nread;) {
 289             d = (struct linux_dirent *) (buf + bpos);
 290             printf("%8ld  ", d\->d_ino);
 291             d_type = *(buf + bpos + d\->d_reclen \- 1);
 292             printf("%\-10s ", (d_type == DT_REG) ?  "regular" :
 293                              (d_type == DT_DIR) ?  "directory" :
 294                              (d_type == DT_FIFO) ? "FIFO" :
 295                              (d_type == DT_SOCK) ? "socket" :
 296                              (d_type == DT_LNK) ?  "symlink" :
 297                              (d_type == DT_BLK) ?  "block dev" :
 298                              (d_type == DT_CHR) ?  "char dev" : "???");
 299             printf("%4d %10lld  %s\\n", d\->d_reclen,
 300                     (long long) d\->d_off, d\->d_name);
 301             bpos += d\->d_reclen;
 302         }
 303     }
 304
 305     exit(EXIT_SUCCESS);
 306 }
 307 .EE
 308 .SH SEE ALSO
 309 .BR readdir (2),
 310 .BR readdir (3),
 311 .BR inode (7)