man2/getdents.2

   1 .\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
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  24 .\"
  25 .\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
  26 .\" Modified 22 July 1995 by Michael Chastain <mec@duracef.shout.net>:
  27 .\"   Derived from 'readdir.2'.
  28 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  29 .TH GETDENTS 2  2012-08-03 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 getdents \- get directory entries
  32 .SH SYNOPSIS
  33 .nf
  34 .BI "int getdents(unsigned int " fd ", struct linux_dirent *" dirp ,
  35 .BI "             unsigned int " count );
  36 .fi
  37
  38 .IR Note :
  39 There is no glibc wrapper for this system call; see NOTES.
  40 .SH DESCRIPTION
  41 This is not the function you are interested in.
  42 Look at
  43 .BR readdir (3)
  44 for the POSIX conforming C library interface.
  45 This page documents the bare kernel system call interface.
  46 .PP
  47 The system call
  48 .BR getdents ()
  49 reads several
  50 .I linux_dirent
  51 structures from the directory
  52 referred to by the open file descriptor
  53 .I fd
  54 into the buffer pointed to by
  55 .IR dirp .
  56 The argument
  57 .I count
  58 specifies the size of that buffer.
  59 .PP
  60 The
  61 .I linux_dirent
  62 structure is declared as follows:
  63 .PP
  64 .in +4n
  65 .nf
  66 struct linux_dirent {
  67     unsigned long  d_ino;     /* Inode number */
  68     unsigned long  d_off;     /* Offset to next \fIlinux_dirent\fP */
  69     unsigned short d_reclen;  /* Length of this \fIlinux_dirent\fP */
  70     char           d_name[];  /* Filename (null-terminated) */
  71                         /* length is actually (d_reclen \- 2 \-
  72                            offsetof(struct linux_dirent, d_name) */
  73     /*
  74     char           pad;       // Zero padding byte
  75     char           d_type;    // File type (only since Linux 2.6.4;
  76                               // offset is (d_reclen \- 1))
  77     */
  78
  79 }
  80 .fi
  81 .in
  82 .PP
  83 .I d_ino
  84 is an inode number.
  85 .I d_off
  86 is the distance from the start of the directory to the start of the next
  87 .IR linux_dirent .
  88 .I d_reclen
  89 is the size of this entire
  90 .IR linux_dirent .
  91 .I d_name
  92 is a null-terminated filename.
  93
  94 .I d_type
  95 is a byte at the end of the structure that indicates the file type.
  96 It contains one of the following values (defined in
  97 .IR <dirent.h> ):
  98 .TP 12
  99 .B DT_BLK
 100 This is a block device.
 101 .TP
 102 .B DT_CHR
 103 This is a character device.
 104 .TP
 105 .B DT_DIR
 106 This is a directory.
 107 .TP
 108 .B DT_FIFO
 109 This is a named pipe (FIFO).
 110 .TP
 111 .B DT_LNK
 112 This is a symbolic link.
 113 .TP
 114 .B DT_REG
 115 This is a regular file.
 116 .TP
 117 .B DT_SOCK
 118 This is a UNIX domain socket.
 119 .TP
 120 .B DT_UNKNOWN
 121 The file type is unknown.
 122 .PP
 123 The
 124 .I d_type
 125 field is implemented since Linux 2.6.4.
 126 It occupies a space that was previously a zero-filled padding byte in the
 127 .IR linux_dirent
 128 structure.
 129 Thus, on kernels before 2.6.3,
 130 attempting to access this field always provides the value 0
 131 .RB ( DT_UNKNOWN ).
 132 .PP
 133 Currently,
 134 .\" kernel 2.6.27
 135 .\" The same sentence is in readdir.2
 136 only some file systems (among them: Btrfs, ext2, ext3, and ext4)
 137 have full support for returning the file type in
 138 .IR d_type .
 139 All applications must properly handle a return of
 140 .BR DT_UNKNOWN .
 141 .SH RETURN VALUE
 142 On success, the number of bytes read is returned.
 143 On end of directory, 0 is returned.
 144 On error, \-1 is returned, and
 145 .I errno
 146 is set appropriately.
 147 .SH ERRORS
 148 .TP
 149 .B EBADF
 150 Invalid file descriptor
 151 .IR fd .
 152 .TP
 153 .B EFAULT
 154 Argument points outside the calling process's address space.
 155 .TP
 156 .B EINVAL
 157 Result buffer is too small.
 158 .TP
 159 .B ENOENT
 160 No such directory.
 161 .TP
 162 .B ENOTDIR
 163 File descriptor does not refer to a directory.
 164 .SH CONFORMING TO
 165 SVr4.
 166 .\" SVr4 documents additional ENOLINK, EIO error conditions.
 167 .SH NOTES
 168 Glibc does not provide a wrapper for this system call; call it using
 169 .BR syscall (2).
 170 You will need to define the
 171 .I linux_dirent
 172 structure yourself.
 173 However, you probably want to use
 174 .BR readdir (3)
 175 instead.
 176
 177 This call supersedes
 178 .BR readdir (2).
 179
 180 The original Linux
 181 .BR getdents ()
 182 system call did not handle large file systems and large file offsets.
 183 Consequently, Linux 2.4 added
 184 .BR getdents64 (),
 185 with wider types for the
 186 .I d_ino
 187 and
 188 .I d_off
 189 fields employed in the
 190 .IR linux_dirent
 191 structure.
 192 .SH EXAMPLE
 193 .\" FIXME: This program uses the older getdents(0 system call
 194 .\" and the structure with smaller field widths.
 195 The program below demonstrates the use of
 196 .BR getdents ().
 197 The following output shows an example of what we see when running this
 198 program on an ext2 directory:
 199
 200 .in +4n
 201 .nf
 202 .RB "$" " ./a.out /testfs/"
 203 --------------- nread=120 ---------------
 204 i-node#  file type  d_reclen  d_off   d_name
 205        2  directory    16         12  .
 206        2  directory    16         24  ..
 207       11  directory    24         44  lost+found
 208       12  regular      16         56  a
 209   228929  directory    16         68  sub
 210    16353  directory    16         80  sub2
 211   130817  directory    16       4096  sub3
 212 .fi
 213 .in
 214 .SS Program source
 215 \&
 216 .nf
 217 #define _GNU_SOURCE
 218 #include <dirent.h>     /* Defines DT_* constants */
 219 #include <fcntl.h>
 220 #include <stdio.h>
 221 #include <unistd.h>
 222 #include <stdlib.h>
 223 #include <sys/stat.h>
 224 #include <sys/syscall.h>
 225
 226 #define handle_error(msg) \\
 227         do { perror(msg); exit(EXIT_FAILURE); } while (0)
 228
 229 struct linux_dirent {
 230     long           d_ino;
 231     off_t          d_off;
 232     unsigned short d_reclen;
 233     char           d_name[];
 234 };
 235
 236 #define BUF_SIZE 1024
 237
 238 int
 239 main(int argc, char *argv[])
 240 {
 241     int fd, nread;
 242     char buf[BUF_SIZE];
 243     struct linux_dirent *d;
 244     int bpos;
 245     char d_type;
 246
 247     fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
 248     if (fd == \-1)
 249         handle_error("open");
 250
 251     for ( ; ; ) {
 252         nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
 253         if (nread == \-1)
 254             handle_error("getdents");
 255
 256         if (nread == 0)
 257             break;
 258
 259         printf("\-\-\-\-\-\-\-\-\-\-\-\-\-\-\- nread=%d \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\\n", nread);
 260         printf("i\-node#  file type  d_reclen  d_off   d_name\\n");
 261         for (bpos = 0; bpos < nread;) {
 262             d = (struct linux_dirent *) (buf + bpos);
 263             printf("%8ld  ", d\->d_ino);
 264             d_type = *(buf + bpos + d\->d_reclen \- 1);
 265             printf("%\-10s ", (d_type == DT_REG) ?  "regular" :
 266                              (d_type == DT_DIR) ?  "directory" :
 267                              (d_type == DT_FIFO) ? "FIFO" :
 268                              (d_type == DT_SOCK) ? "socket" :
 269                              (d_type == DT_LNK) ?  "symlink" :
 270                              (d_type == DT_BLK) ?  "block dev" :
 271                              (d_type == DT_CHR) ?  "char dev" : "???");
 272             printf("%4d %10lld  %s\\n", d\->d_reclen,
 273                     (long long) d\->d_off, d\->d_name);
 274             bpos += d\->d_reclen;
 275         }
 276     }
 277
 278     exit(EXIT_SUCCESS);
 279 }
 280 .fi
 281 .SH SEE ALSO
 282 .BR readdir (2),
 283 .BR readdir (3)