Documentation/DocBook/procfs-guide.tmpl

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   5 ]>
   6
   7 <book id="LKProcfsGuide">
   8   <bookinfo>
   9     <title>Linux Kernel Procfs Guide</title>
  10
  11     <authorgroup>
  12       <author>
  13         <firstname>Erik</firstname>
  14         <othername>(J.A.K.)</othername>
  15         <surname>Mouw</surname>
  16         <affiliation>
  17           <orgname>Delft University of Technology</orgname>
  18           <orgdiv>Faculty of Information Technology and Systems</orgdiv>
  19           <address>
  20             <email>J.A.K.Mouw@its.tudelft.nl</email>
  21             <pob>PO BOX 5031</pob>
  22             <postcode>2600 GA</postcode>
  23             <city>Delft</city>
  24             <country>The Netherlands</country>
  25           </address>
  26         </affiliation>
  27       </author>
  28     </authorgroup>
  29
  30     <revhistory>
  31       <revision>
  32         <revnumber>1.0&nbsp;</revnumber>
  33         <date>May 30, 2001</date>
  34         <revremark>Initial revision posted to linux-kernel</revremark>
  35       </revision>
  36       <revision>
  37         <revnumber>1.1&nbsp;</revnumber>
  38         <date>June 3, 2001</date>
  39         <revremark>Revised after comments from linux-kernel</revremark>
  40       </revision>
  41     </revhistory>
  42
  43     <copyright>
  44       <year>2001</year>
  45       <holder>Erik Mouw</holder>
  46     </copyright>
  47
  48
  49     <legalnotice>
  50       <para>
  51         This documentation is free software; you can redistribute it
  52         and/or modify it under the terms of the GNU General Public
  53         License as published by the Free Software Foundation; either
  54         version 2 of the License, or (at your option) any later
  55         version.
  56       </para>
  57
  58       <para>
  59         This documentation is distributed in the hope that it will be
  60         useful, but WITHOUT ANY WARRANTY; without even the implied
  61         warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
  62         PURPOSE.  See the GNU General Public License for more details.
  63       </para>
  64
  65       <para>
  66         You should have received a copy of the GNU General Public
  67         License along with this program; if not, write to the Free
  68         Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  69         MA 02111-1307 USA
  70       </para>
  71
  72       <para>
  73         For more details see the file COPYING in the source
  74         distribution of Linux.
  75       </para>
  76     </legalnotice>
  77   </bookinfo>
  78
  79
  80
  81
  82   <toc>
  83   </toc>
  84
  85
  86
  87
  88   <preface>
  89     <title>Preface</title>
  90
  91     <para>
  92       This guide describes the use of the procfs file system from
  93       within the Linux kernel. The idea to write this guide came up on
  94       the #kernelnewbies IRC channel (see <ulink
  95       url="http://www.kernelnewbies.org/">http://www.kernelnewbies.org/</ulink>),
  96       when Jeff Garzik explained the use of procfs and forwarded me a
  97       message Alexander Viro wrote to the linux-kernel mailing list. I
  98       agreed to write it up nicely, so here it is.
  99     </para>
 100
 101     <para>
 102       I'd like to thank Jeff Garzik
 103       <email>jgarzik@pobox.com</email> and Alexander Viro
 104       <email>viro@parcelfarce.linux.theplanet.co.uk</email> for their input,
 105       Tim Waugh <email>twaugh@redhat.com</email> for his <ulink
 106       url="http://people.redhat.com/twaugh/docbook/selfdocbook/">Selfdocbook</ulink>,
 107       and Marc Joosen <email>marcj@historia.et.tudelft.nl</email> for
 108       proofreading.
 109     </para>
 110
 111     <para>
 112       This documentation was written while working on the LART
 113       computing board (<ulink
 114       url="http://www.lart.tudelft.nl/">http://www.lart.tudelft.nl/</ulink>),
 115       which is sponsored by the Mobile Multi-media Communications
 116       (<ulink
 117       url="http://www.mmc.tudelft.nl/">http://www.mmc.tudelft.nl/</ulink>)
 118       and Ubiquitous Communications (<ulink
 119       url="http://www.ubicom.tudelft.nl/">http://www.ubicom.tudelft.nl/</ulink>)
 120       projects.
 121     </para>
 122
 123     <para>
 124       Erik
 125     </para>
 126   </preface>
 127
 128
 129
 130
 131   <chapter id="intro">
 132     <title>Introduction</title>
 133
 134     <para>
 135       The <filename class="directory">/proc</filename> file system
 136       (procfs) is a special file system in the linux kernel. It's a
 137       virtual file system: it is not associated with a block device
 138       but exists only in memory. The files in the procfs are there to
 139       allow userland programs access to certain information from the
 140       kernel (like process information in <filename
 141       class="directory">/proc/[0-9]+/</filename>), but also for debug
 142       purposes (like <filename>/proc/ksyms</filename>).
 143     </para>
 144
 145     <para>
 146       This guide describes the use of the procfs file system from
 147       within the Linux kernel. It starts by introducing all relevant
 148       functions to manage the files within the file system. After that
 149       it shows how to communicate with userland, and some tips and
 150       tricks will be pointed out. Finally a complete example will be
 151       shown.
 152     </para>
 153
 154     <para>
 155       Note that the files in <filename
 156       class="directory">/proc/sys</filename> are sysctl files: they
 157       don't belong to procfs and are governed by a completely
 158       different API described in the Kernel API book.
 159     </para>
 160   </chapter>
 161
 162
 163
 164
 165   <chapter id="managing">
 166     <title>Managing procfs entries</title>
 167
 168     <para>
 169       This chapter describes the functions that various kernel
 170       components use to populate the procfs with files, symlinks,
 171       device nodes, and directories.
 172     </para>
 173
 174     <para>
 175       A minor note before we start: if you want to use any of the
 176       procfs functions, be sure to include the correct header file!
 177       This should be one of the first lines in your code:
 178     </para>
 179
 180     <programlisting>
 181 #include &lt;linux/proc_fs.h&gt;
 182     </programlisting>
 183
 184
 185
 186
 187     <sect1 id="regularfile">
 188       <title>Creating a regular file</title>
 189
 190       <funcsynopsis>
 191         <funcprototype>
 192           <funcdef>struct proc_dir_entry* <function>create_proc_entry</function></funcdef>
 193           <paramdef>const char* <parameter>name</parameter></paramdef>
 194           <paramdef>mode_t <parameter>mode</parameter></paramdef>
 195           <paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
 196         </funcprototype>
 197       </funcsynopsis>
 198
 199       <para>
 200         This function creates a regular file with the name
 201         <parameter>name</parameter>, file mode
 202         <parameter>mode</parameter> in the directory
 203         <parameter>parent</parameter>. To create a file in the root of
 204         the procfs, use <constant>NULL</constant> as
 205         <parameter>parent</parameter> parameter. When successful, the
 206         function will return a pointer to the freshly created
 207         <structname>struct proc_dir_entry</structname>; otherwise it
 208         will return <constant>NULL</constant>. <xref
 209         linkend="userland"/> describes how to do something useful with
 210         regular files.
 211       </para>
 212
 213       <para>
 214         Note that it is specifically supported that you can pass a
 215         path that spans multiple directories. For example
 216         <function>create_proc_entry</function>(<parameter>"drivers/via0/info"</parameter>)
 217         will create the <filename class="directory">via0</filename>
 218         directory if necessary, with standard
 219         <constant>0755</constant> permissions.
 220       </para>
 221
 222     <para>
 223       If you only want to be able to read the file, the function
 224       <function>create_proc_read_entry</function> described in <xref
 225       linkend="convenience"/> may be used to create and initialise
 226       the procfs entry in one single call.
 227     </para>
 228     </sect1>
 229
 230
 231
 232
 233     <sect1>
 234       <title>Creating a symlink</title>
 235
 236       <funcsynopsis>
 237         <funcprototype>
 238           <funcdef>struct proc_dir_entry*
 239           <function>proc_symlink</function></funcdef> <paramdef>const
 240           char* <parameter>name</parameter></paramdef>
 241           <paramdef>struct proc_dir_entry*
 242           <parameter>parent</parameter></paramdef> <paramdef>const
 243           char* <parameter>dest</parameter></paramdef>
 244         </funcprototype>
 245       </funcsynopsis>
 246
 247       <para>
 248         This creates a symlink in the procfs directory
 249         <parameter>parent</parameter> that points from
 250         <parameter>name</parameter> to
 251         <parameter>dest</parameter>. This translates in userland to
 252         <literal>ln -s</literal> <parameter>dest</parameter>
 253         <parameter>name</parameter>.
 254       </para>
 255     </sect1>
 256
 257     <sect1>
 258       <title>Creating a directory</title>
 259
 260       <funcsynopsis>
 261         <funcprototype>
 262           <funcdef>struct proc_dir_entry* <function>proc_mkdir</function></funcdef>
 263           <paramdef>const char* <parameter>name</parameter></paramdef>
 264           <paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
 265         </funcprototype>
 266       </funcsynopsis>
 267
 268       <para>
 269         Create a directory <parameter>name</parameter> in the procfs
 270         directory <parameter>parent</parameter>.
 271       </para>
 272     </sect1>
 273
 274
 275
 276
 277     <sect1>
 278       <title>Removing an entry</title>
 279
 280       <funcsynopsis>
 281         <funcprototype>
 282           <funcdef>void <function>remove_proc_entry</function></funcdef>
 283           <paramdef>const char* <parameter>name</parameter></paramdef>
 284           <paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
 285         </funcprototype>
 286       </funcsynopsis>
 287
 288       <para>
 289         Removes the entry <parameter>name</parameter> in the directory
 290         <parameter>parent</parameter> from the procfs. Entries are
 291         removed by their <emphasis>name</emphasis>, not by the
 292         <structname>struct proc_dir_entry</structname> returned by the
 293         various create functions. Note that this function doesn't
 294         recursively remove entries.
 295       </para>
 296
 297       <para>
 298         Be sure to free the <structfield>data</structfield> entry from
 299         the <structname>struct proc_dir_entry</structname> before
 300         <function>remove_proc_entry</function> is called (that is: if
 301         there was some <structfield>data</structfield> allocated, of
 302         course). See <xref linkend="usingdata"/> for more information
 303         on using the <structfield>data</structfield> entry.
 304       </para>
 305     </sect1>
 306   </chapter>
 307
 308
 309
 310
 311   <chapter id="userland">
 312     <title>Communicating with userland</title>
 313
 314     <para>
 315        Instead of reading (or writing) information directly from
 316        kernel memory, procfs works with <emphasis>call back
 317        functions</emphasis> for files: functions that are called when
 318        a specific file is being read or written. Such functions have
 319        to be initialised after the procfs file is created by setting
 320        the <structfield>read_proc</structfield> and/or
 321        <structfield>write_proc</structfield> fields in the
 322        <structname>struct proc_dir_entry*</structname> that the
 323        function <function>create_proc_entry</function> returned:
 324     </para>
 325
 326     <programlisting>
 327 struct proc_dir_entry* entry;
 328
 329 entry->read_proc = read_proc_foo;
 330 entry->write_proc = write_proc_foo;
 331     </programlisting>
 332
 333     <para>
 334       If you only want to use a the
 335       <structfield>read_proc</structfield>, the function
 336       <function>create_proc_read_entry</function> described in <xref
 337       linkend="convenience"/> may be used to create and initialise the
 338       procfs entry in one single call.
 339     </para>
 340
 341
 342
 343     <sect1>
 344       <title>Reading data</title>
 345
 346       <para>
 347         The read function is a call back function that allows userland
 348         processes to read data from the kernel. The read function
 349         should have the following format:
 350       </para>
 351
 352       <funcsynopsis>
 353         <funcprototype>
 354           <funcdef>int <function>read_func</function></funcdef>
 355           <paramdef>char* <parameter>buffer</parameter></paramdef>
 356           <paramdef>char** <parameter>start</parameter></paramdef>
 357           <paramdef>off_t <parameter>off</parameter></paramdef>
 358           <paramdef>int <parameter>count</parameter></paramdef>
 359           <paramdef>int* <parameter>peof</parameter></paramdef>
 360           <paramdef>void* <parameter>data</parameter></paramdef>
 361         </funcprototype>
 362       </funcsynopsis>
 363
 364       <para>
 365         The read function should write its information into the
 366         <parameter>buffer</parameter>, which will be exactly
 367         <literal>PAGE_SIZE</literal> bytes long.
 368       </para>
 369
 370       <para>
 371         The parameter
 372         <parameter>peof</parameter> should be used to signal that the
 373         end of the file has been reached by writing
 374         <literal>1</literal> to the memory location
 375         <parameter>peof</parameter> points to.
 376       </para>
 377
 378       <para>
 379         The <parameter>data</parameter>
 380         parameter can be used to create a single call back function for
 381         several files, see <xref linkend="usingdata"/>.
 382       </para>
 383
 384       <para>
 385         The rest of the parameters and the return value are described
 386         by a comment in <filename>fs/proc/generic.c</filename> as follows:
 387       </para>
 388
 389       <blockquote>
 390         <para>
 391         You have three ways to return data:
 392         </para>
 393         <orderedlist>
 394           <listitem>
 395             <para>
 396               Leave <literal>*start = NULL</literal>.  (This is the default.)
 397               Put the data of the requested offset at that
 398               offset within the buffer.  Return the number (<literal>n</literal>)
 399               of bytes there are from the beginning of the
 400               buffer up to the last byte of data.  If the
 401               number of supplied bytes (<literal>= n - offset</literal>) is
 402               greater than zero and you didn't signal eof
 403               and the reader is prepared to take more data
 404               you will be called again with the requested
 405               offset advanced by the number of bytes
 406               absorbed.  This interface is useful for files
 407               no larger than the buffer.
 408             </para>
 409           </listitem>
 410           <listitem>
 411             <para>
 412               Set <literal>*start</literal> to an unsigned long value less than
 413               the buffer address but greater than zero.
 414               Put the data of the requested offset at the
 415               beginning of the buffer.  Return the number of
 416               bytes of data placed there.  If this number is
 417               greater than zero and you didn't signal eof
 418               and the reader is prepared to take more data
 419               you will be called again with the requested
 420               offset advanced by <literal>*start</literal>.  This interface is
 421               useful when you have a large file consisting
 422               of a series of blocks which you want to count
 423               and return as wholes.
 424               (Hack by Paul.Russell@rustcorp.com.au)
 425             </para>
 426           </listitem>
 427           <listitem>
 428             <para>
 429               Set <literal>*start</literal> to an address within the buffer.
 430               Put the data of the requested offset at <literal>*start</literal>.
 431               Return the number of bytes of data placed there.
 432               If this number is greater than zero and you
 433               didn't signal eof and the reader is prepared to
 434               take more data you will be called again with the
 435               requested offset advanced by the number of bytes
 436               absorbed.
 437             </para>
 438           </listitem>
 439         </orderedlist>
 440       </blockquote>
 441
 442       <para>
 443         <xref linkend="example"/> shows how to use a read call back
 444         function.
 445       </para>
 446     </sect1>
 447
 448
 449
 450
 451     <sect1>
 452       <title>Writing data</title>
 453
 454       <para>
 455         The write call back function allows a userland process to write
 456         data to the kernel, so it has some kind of control over the
 457         kernel. The write function should have the following format:
 458       </para>
 459
 460       <funcsynopsis>
 461         <funcprototype>
 462           <funcdef>int <function>write_func</function></funcdef>
 463           <paramdef>struct file* <parameter>file</parameter></paramdef>
 464           <paramdef>const char* <parameter>buffer</parameter></paramdef>
 465           <paramdef>unsigned long <parameter>count</parameter></paramdef>
 466           <paramdef>void* <parameter>data</parameter></paramdef>
 467         </funcprototype>
 468       </funcsynopsis>
 469
 470       <para>
 471         The write function should read <parameter>count</parameter>
 472         bytes at maximum from the <parameter>buffer</parameter>. Note
 473         that the <parameter>buffer</parameter> doesn't live in the
 474         kernel's memory space, so it should first be copied to kernel
 475         space with <function>copy_from_user</function>. The
 476         <parameter>file</parameter> parameter is usually
 477         ignored. <xref linkend="usingdata"/> shows how to use the
 478         <parameter>data</parameter> parameter.
 479       </para>
 480
 481       <para>
 482         Again, <xref linkend="example"/> shows how to use this call back
 483         function.
 484       </para>
 485     </sect1>
 486
 487
 488
 489
 490     <sect1 id="usingdata">
 491       <title>A single call back for many files</title>
 492
 493       <para>
 494          When a large number of almost identical files is used, it's
 495          quite inconvenient to use a separate call back function for
 496          each file. A better approach is to have a single call back
 497          function that distinguishes between the files by using the
 498          <structfield>data</structfield> field in <structname>struct
 499          proc_dir_entry</structname>. First of all, the
 500          <structfield>data</structfield> field has to be initialised:
 501       </para>
 502
 503       <programlisting>
 504 struct proc_dir_entry* entry;
 505 struct my_file_data *file_data;
 506
 507 file_data = kmalloc(sizeof(struct my_file_data), GFP_KERNEL);
 508 entry->data = file_data;
 509       </programlisting>
 510
 511       <para>
 512           The <structfield>data</structfield> field is a <type>void
 513           *</type>, so it can be initialised with anything.
 514       </para>
 515
 516       <para>
 517         Now that the <structfield>data</structfield> field is set, the
 518         <function>read_proc</function> and
 519         <function>write_proc</function> can use it to distinguish
 520         between files because they get it passed into their
 521         <parameter>data</parameter> parameter:
 522       </para>
 523
 524       <programlisting>
 525 int foo_read_func(char *page, char **start, off_t off,
 526                   int count, int *eof, void *data)
 527 {
 528         int len;
 529
 530         if(data == file_data) {
 531                 /* special case for this file */
 532         } else {
 533                 /* normal processing */
 534         }
 535
 536         return len;
 537 }
 538       </programlisting>
 539
 540       <para>
 541         Be sure to free the <structfield>data</structfield> data field
 542         when removing the procfs entry.
 543       </para>
 544     </sect1>
 545   </chapter>
 546
 547
 548
 549
 550   <chapter id="tips">
 551     <title>Tips and tricks</title>
 552
 553
 554
 555
 556     <sect1 id="convenience">
 557       <title>Convenience functions</title>
 558
 559       <funcsynopsis>
 560         <funcprototype>
 561           <funcdef>struct proc_dir_entry* <function>create_proc_read_entry</function></funcdef>
 562           <paramdef>const char* <parameter>name</parameter></paramdef>
 563           <paramdef>mode_t <parameter>mode</parameter></paramdef>
 564           <paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
 565           <paramdef>read_proc_t* <parameter>read_proc</parameter></paramdef>
 566           <paramdef>void* <parameter>data</parameter></paramdef>
 567         </funcprototype>
 568       </funcsynopsis>
 569
 570       <para>
 571         This function creates a regular file in exactly the same way
 572         as <function>create_proc_entry</function> from <xref
 573         linkend="regularfile"/> does, but also allows to set the read
 574         function <parameter>read_proc</parameter> in one call. This
 575         function can set the <parameter>data</parameter> as well, like
 576         explained in <xref linkend="usingdata"/>.
 577       </para>
 578     </sect1>
 579
 580
 581
 582     <sect1>
 583       <title>Modules</title>
 584
 585       <para>
 586         If procfs is being used from within a module, be sure to set
 587         the <structfield>owner</structfield> field in the
 588         <structname>struct proc_dir_entry</structname> to
 589         <constant>THIS_MODULE</constant>.
 590       </para>
 591
 592       <programlisting>
 593 struct proc_dir_entry* entry;
 594
 595 entry->owner = THIS_MODULE;
 596       </programlisting>
 597     </sect1>
 598
 599
 600
 601
 602     <sect1>
 603       <title>Mode and ownership</title>
 604
 605       <para>
 606         Sometimes it is useful to change the mode and/or ownership of
 607         a procfs entry. Here is an example that shows how to achieve
 608         that:
 609       </para>
 610
 611       <programlisting>
 612 struct proc_dir_entry* entry;
 613
 614 entry->mode =  S_IWUSR |S_IRUSR | S_IRGRP | S_IROTH;
 615 entry->uid = 0;
 616 entry->gid = 100;
 617       </programlisting>
 618
 619     </sect1>
 620   </chapter>
 621
 622
 623
 624
 625   <chapter id="example">
 626     <title>Example</title>
 627
 628     <!-- be careful with the example code: it shouldn't be wider than
 629     approx. 60 columns, or otherwise it won't fit properly on a page
 630     -->
 631
 632 &procfsexample;
 633
 634   </chapter>
 635 </book>