man2/getdents.2

   1 .\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
   2 .\"
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  12 .\" Since the Linux kernel and libraries are constantly changing, this
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  20 .\" Formatted or processed versions of this manual, if unaccompanied by
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  22 .\"
  23 .\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
  24 .\" Modified 22 July 1995 by Michael Chastain <mec@duracef.shout.net>:
  25 .\"   Derived from 'readdir.2'.
  26 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  27 .TH GETDENTS 2  2010-11-21 "Linux" "Linux Programmer's Manual"
  28 .SH NAME
  29 getdents \- get directory entries
  30 .SH SYNOPSIS
  31 .nf
  32 .BI "int getdents(unsigned int " fd ", struct linux_dirent *" dirp ,
  33 .BI "             unsigned int " count );
  34 .fi
  35 .SH DESCRIPTION
  36 This is not the function you are interested in.
  37 Look at
  38 .BR readdir (3)
  39 for the POSIX conforming C library interface.
  40 This page documents the bare kernel system call interface.
  41 .PP
  42 The system call
  43 .BR getdents ()
  44 reads several
  45 .I linux_dirent
  46 structures from the directory
  47 referred to by the open file descriptor
  48 .I fd
  49 into the buffer pointed to by
  50 .IR dirp .
  51 The argument
  52 .I count
  53 specifies the size of that buffer.
  54 .PP
  55 The
  56 .I linux_dirent
  57 structure is declared as follows:
  58 .PP
  59 .in +4n
  60 .nf
  61 struct linux_dirent {
  62     unsigned long  d_ino;     /* Inode number */
  63     unsigned long  d_off;     /* Offset to next \fIlinux_dirent\fP */
  64     unsigned short d_reclen;  /* Length of this \fIlinux_dirent\fP */
  65     char           d_name[];  /* Filename (null-terminated) */
  66                         /* length is actually (d_reclen \- 2 \-
  67                            offsetof(struct linux_dirent, d_name) */
  68     /*
  69     char           pad;       // Zero padding byte
  70     char           d_type;    // File type (only since Linux 2.6.4;
  71                               // offset is (d_reclen \- 1))
  72     */
  73
  74 }
  75 .fi
  76 .in
  77 .PP
  78 .I d_ino
  79 is an inode number.
  80 .I d_off
  81 is the distance from the start of the directory to the start of the next
  82 .IR linux_dirent .
  83 .I d_reclen
  84 is the size of this entire
  85 .IR linux_dirent .
  86 .I d_name
  87 is a null-terminated filename.
  88
  89 .I d_type
  90 is a byte at the end of the structure that indicates the file type.
  91 It contains one of the following values (defined in
  92 .IR <dirent.h> ):
  93 .TP 12
  94 .B DT_BLK
  95 This is a block device.
  96 .TP
  97 .B DT_CHR
  98 This is a character device.
  99 .TP
 100 .B DT_DIR
 101 This is a directory.
 102 .TP
 103 .B DT_FIFO
 104 This is a named pipe (FIFO).
 105 .TP
 106 .B DT_LNK
 107 This is a symbolic link.
 108 .TP
 109 .B DT_REG
 110 This is a regular file.
 111 .TP
 112 .B DT_SOCK
 113 This is a UNIX domain socket.
 114 .TP
 115 .B DT_UNKNOWN
 116 The file type is unknown.
 117 .PP
 118 The
 119 .I d_type
 120 field is implemented since Linux 2.6.4.
 121 It occupies a space that was previously a zero-filled padding byte in the
 122 .IR linux_dirent
 123 structure.
 124 Thus, on kernels before 2.6.3,
 125 attempting to access this field always provides the value 0
 126 .RB ( DT_UNKNOWN ).
 127 .PP
 128 Currently,
 129 .\" kernel 2.6.27
 130 .\" The same sentence is in readdir.2
 131 only some file systems (among them: Btrfs, ext2, ext3, and ext4)
 132 have full support for returning the file type in
 133 .IR d_type .
 134 All applications must properly handle a return of
 135 .BR DT_UNKNOWN .
 136 .SH "RETURN VALUE"
 137 On success, the number of bytes read is returned.
 138 On end of directory, 0 is returned.
 139 On error, \-1 is returned, and
 140 .I errno
 141 is set appropriately.
 142 .SH ERRORS
 143 .TP
 144 .B EBADF
 145 Invalid file descriptor
 146 .IR fd .
 147 .TP
 148 .B EFAULT
 149 Argument points outside the calling process's address space.
 150 .TP
 151 .B EINVAL
 152 Result buffer is too small.
 153 .TP
 154 .B ENOENT
 155 No such directory.
 156 .TP
 157 .B ENOTDIR
 158 File descriptor does not refer to a directory.
 159 .SH "CONFORMING TO"
 160 SVr4.
 161 .\" SVr4 documents additional ENOLINK, EIO error conditions.
 162 .SH NOTES
 163 Glibc does not provide a wrapper for this system call; call it using
 164 .BR syscall (2).
 165 You will need to define the
 166 .I linux_dirent
 167 structure yourself.
 168
 169 This call supersedes
 170 .BR readdir (2).
 171
 172 The original Linux
 173 .BR getdents ()
 174 system call did not handle large file systems and large file offsets.
 175 Consequently, Linux 2.4 added
 176 .BR getdents64 (),
 177 with wider types for the
 178 .I d_ino
 179 and
 180 .I d_off
 181 fields employed in the
 182 .IR linux_dirent
 183 structure.
 184 .SH EXAMPLE
 185 .\" FIXME: This program uses the older getdents(0 system call
 186 .\" and the structure with smaller field widths.
 187 The program below demonstrates the use of
 188 .BR getdents ().
 189 The following output shows an example of what we see when running this
 190 program on an ext2 directory:
 191
 192 .in +4n
 193 .nf
 194 .RB "$" " ./a.out /testfs/"
 195 --------------- nread=120 ---------------
 196 i-node#  file type  d_reclen  d_off   d_name
 197        2  directory    16         12  .
 198        2  directory    16         24  ..
 199       11  directory    24         44  lost+found
 200       12  regular      16         56  a
 201   228929  directory    16         68  sub
 202    16353  directory    16         80  sub2
 203   130817  directory    16       4096  sub3
 204 .fi
 205 .in
 206 .SS Program source
 207 \&
 208 .nf
 209 #define _GNU_SOURCE
 210 #include <dirent.h>     /* Defines DT_* constants */
 211 #include <fcntl.h>
 212 #include <stdio.h>
 213 #include <unistd.h>
 214 #include <stdlib.h>
 215 #include <sys/stat.h>
 216 #include <sys/syscall.h>
 217
 218 #define handle_error(msg) \\
 219         do { perror(msg); exit(EXIT_FAILURE); } while (0)
 220
 221 struct linux_dirent {
 222     long           d_ino;
 223     off_t          d_off;
 224     unsigned short d_reclen;
 225     char           d_name[];
 226 };
 227
 228 #define BUF_SIZE 1024
 229
 230 int
 231 main(int argc, char *argv[])
 232 {
 233     int fd, nread;
 234     char buf[BUF_SIZE];
 235     struct linux_dirent *d;
 236     int bpos;
 237     char d_type;
 238
 239     fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
 240     if (fd == \-1)
 241         handle_error("open");
 242
 243     for ( ; ; ) {
 244         nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
 245         if (nread == \-1)
 246             handle_error("getdents");
 247
 248         if (nread == 0)
 249             break;
 250
 251         printf("\--------------- nread=%d ---------------\\n", nread);
 252         printf("i\-node#  file type  d_reclen  d_off   d_name\\n");
 253         for (bpos = 0; bpos < nread;) {
 254             d = (struct linux_dirent *) (buf + bpos);
 255             printf("%8ld  ", d\->d_ino);
 256             d_type = *(buf + bpos + d\->d_reclen - 1);
 257             printf("%\-10s ", (d_type == DT_REG) ?  "regular" :
 258                              (d_type == DT_DIR) ?  "directory" :
 259                              (d_type == DT_FIFO) ? "FIFO" :
 260                              (d_type == DT_SOCK) ? "socket" :
 261                              (d_type == DT_LNK) ?  "symlink" :
 262                              (d_type == DT_BLK) ?  "block dev" :
 263                              (d_type == DT_CHR) ?  "char dev" : "???");
 264             printf("%4d %10lld  %s\\n", d\->d_reclen,
 265                     (long long) d\->d_off, (char *) d->d_name);
 266             bpos += d\->d_reclen;
 267         }
 268     }
 269
 270     exit(EXIT_SUCCESS);
 271 }
 272 .fi
 273 .SH "SEE ALSO"
 274 .BR readdir (2),
 275 .BR readdir (3)