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<h3 class="section">14.3 Working with Directory Trees</h3>

<p><a name="index-directory-hierarchy-1432"></a><a name="index-hierarchy_002c-directory-1433"></a><a name="index-tree_002c-directory-1434"></a>
The functions described so far for handling the files in a directory
have allowed you to either retrieve the information bit by bit, or to
process all the files as a group (see <code>scandir</code>).  Sometimes it is
useful to process whole hierarchies of directories and their contained
files.  The X/Open specification defines two functions to do this.  The
simpler form is derived from an early definition in System&nbsp;V<!-- /@w --> systems
and therefore this function is available on SVID-derived systems.  The
prototypes and required definitions can be found in the <samp><span class="file">ftw.h</span></samp>
header.

   <p>There are four functions in this family: <code>ftw</code>, <code>nftw</code> and
their 64-bit counterparts <code>ftw64</code> and <code>nftw64</code>.  These
functions take as one of their arguments a pointer to a callback
function of the appropriate type.

<!-- ftw.h -->
<!-- GNU -->
<div class="defun">
&mdash; Data Type: <b>__ftw_func_t</b><var><a name="index-g_t_005f_005fftw_005ffunc_005ft-1435"></a></var><br>
<blockquote>
     <pre class="smallexample">          int (*) (const char *, const struct stat *, int)
</pre>
        <p>The type of callback functions given to the <code>ftw</code> function.  The
first parameter points to the file name, the second parameter to an
object of type <code>struct stat</code> which is filled in for the file named
in the first parameter.

     <p class="noindent">The last parameter is a flag giving more information about the current
file.  It can have the following values:

          <dl>
<dt><code>FTW_F</code><a name="index-FTW_005fF-1436"></a><dd>The item is either a normal file or a file which does not fit into one
of the following categories.  This could be special files, sockets etc. 
<br><dt><code>FTW_D</code><a name="index-FTW_005fD-1437"></a><dd>The item is a directory. 
<br><dt><code>FTW_NS</code><a name="index-FTW_005fNS-1438"></a><dd>The <code>stat</code> call failed and so the information pointed to by the
second paramater is invalid. 
<br><dt><code>FTW_DNR</code><a name="index-FTW_005fDNR-1439"></a><dd>The item is a directory which cannot be read. 
<br><dt><code>FTW_SL</code><a name="index-FTW_005fSL-1440"></a><dd>The item is a symbolic link.  Since symbolic links are normally followed
seeing this value in a <code>ftw</code> callback function means the referenced
file does not exist.  The situation for <code>nftw</code> is different.

          <p>This value is only available if the program is compiled with
<code>_BSD_SOURCE</code> or <code>_XOPEN_EXTENDED</code> defined before including
the first header.  The original SVID systems do not have symbolic links. 
</dl>

        <p>If the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> this
type is in fact <code>__ftw64_func_t</code> since this mode changes
<code>struct stat</code> to be <code>struct stat64</code>. 
</p></blockquote></div>

   <p>For the LFS interface and for use in the function <code>ftw64</code>, the
header <samp><span class="file">ftw.h</span></samp> defines another function type.

<!-- ftw.h -->
<!-- GNU -->
<div class="defun">
&mdash; Data Type: <b>__ftw64_func_t</b><var><a name="index-g_t_005f_005fftw64_005ffunc_005ft-1441"></a></var><br>
<blockquote>
     <pre class="smallexample">          int (*) (const char *, const struct stat64 *, int)
</pre>
        <p>This type is used just like <code>__ftw_func_t</code> for the callback
function, but this time is called from <code>ftw64</code>.  The second
parameter to the function is a pointer to a variable of type
<code>struct stat64</code> which is able to represent the larger values. 
</p></blockquote></div>

<!-- ftw.h -->
<!-- GNU -->
<div class="defun">
&mdash; Data Type: <b>__nftw_func_t</b><var><a name="index-g_t_005f_005fnftw_005ffunc_005ft-1442"></a></var><br>
<blockquote>
     <pre class="smallexample">          int (*) (const char *, const struct stat *, int, struct FTW *)
</pre>
        <p><a name="index-FTW_005fDP-1443"></a><a name="index-FTW_005fSLN-1444"></a>The first three arguments are the same as for the <code>__ftw_func_t</code>
type.  However for the third argument some additional values are defined
to allow finer differentiation:
          <dl>
<dt><code>FTW_DP</code><dd>The current item is a directory and all subdirectories have already been
visited and reported.  This flag is returned instead of <code>FTW_D</code> if
the <code>FTW_DEPTH</code> flag is passed to <code>nftw</code> (see below). 
<br><dt><code>FTW_SLN</code><dd>The current item is a stale symbolic link.  The file it points to does
not exist. 
</dl>

        <p>The last parameter of the callback function is a pointer to a structure
with some extra information as described below.

        <p>If the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> this
type is in fact <code>__nftw64_func_t</code> since this mode changes
<code>struct stat</code> to be <code>struct stat64</code>. 
</p></blockquote></div>

   <p>For the LFS interface there is also a variant of this data type
available which has to be used with the <code>nftw64</code> function.

<!-- ftw.h -->
<!-- GNU -->
<div class="defun">
&mdash; Data Type: <b>__nftw64_func_t</b><var><a name="index-g_t_005f_005fnftw64_005ffunc_005ft-1445"></a></var><br>
<blockquote>
     <pre class="smallexample">          int (*) (const char *, const struct stat64 *, int, struct FTW *)
</pre>
        <p>This type is used just like <code>__nftw_func_t</code> for the callback
function, but this time is called from <code>nftw64</code>.  The second
parameter to the function is this time a pointer to a variable of type
<code>struct stat64</code> which is able to represent the larger values. 
</p></blockquote></div>

<!-- ftw.h -->
<!-- XPG4.2 -->
<div class="defun">
&mdash; Data Type: <b>struct FTW</b><var><a name="index-struct-FTW-1446"></a></var><br>
<blockquote><p>The information contained in this structure helps in interpreting the
name parameter and gives some information about the current state of the
traversal of the directory hierarchy.

          <dl>
<dt><code>int base</code><dd>The value is the offset into the string passed in the first parameter to
the callback function of the beginning of the file name.  The rest of
the string is the path of the file.  This information is especially
important if the <code>FTW_CHDIR</code> flag was set in calling <code>nftw</code>
since then the current directory is the one the current item is found
in. 
<br><dt><code>int level</code><dd>Whilst processing, the code tracks how many directories down it has gone
to find the current file.  This nesting level starts at 0 for
files in the initial directory (or is zero for the initial file if a
file was passed). 
</dl>
        </p></blockquote></div>

<!-- ftw.h -->
<!-- SVID -->
<div class="defun">
&mdash; Function: int <b>ftw</b> (<var>const char *filename, __ftw_func_t func, int descriptors</var>)<var><a name="index-ftw-1447"></a></var><br>
<blockquote><p>The <code>ftw</code> function calls the callback function given in the
parameter <var>func</var> for every item which is found in the directory
specified by <var>filename</var> and all directories below.  The function
follows symbolic links if necessary but does not process an item twice. 
If <var>filename</var> is not a directory then it itself is the only object
returned to the callback function.

        <p>The file name passed to the callback function is constructed by taking
the <var>filename</var> parameter and appending the names of all passed
directories and then the local file name.  So the callback function can
use this parameter to access the file.  <code>ftw</code> also calls
<code>stat</code> for the file and passes that information on to the callback
function.  If this <code>stat</code> call was not successful the failure is
indicated by setting the third argument of the callback function to
<code>FTW_NS</code>.  Otherwise it is set according to the description given
in the account of <code>__ftw_func_t</code> above.

        <p>The callback function is expected to return 0 to indicate that no
error occurred and that processing should continue.  If an error
occurred in the callback function or it wants <code>ftw</code> to return
immediately, the callback function can return a value other than
0.  This is the only correct way to stop the function.  The
program must not use <code>setjmp</code> or similar techniques to continue
from another place.  This would leave resources allocated by the
<code>ftw</code> function unfreed.

        <p>The <var>descriptors</var> parameter to <code>ftw</code> specifies how many file
descriptors it is allowed to consume.  The function runs faster the more
descriptors it can use.  For each level in the directory hierarchy at
most one descriptor is used, but for very deep ones any limit on open
file descriptors for the process or the system may be exceeded. 
Moreover, file descriptor limits in a multi-threaded program apply to
all the threads as a group, and therefore it is a good idea to supply a
reasonable limit to the number of open descriptors.

        <p>The return value of the <code>ftw</code> function is 0 if all callback
function calls returned 0 and all actions performed by the
<code>ftw</code> succeeded.  If a function call failed (other than calling
<code>stat</code> on an item) the function returns -1.  If a callback
function returns a value other than 0 this value is returned as
the return value of <code>ftw</code>.

        <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> on a
32-bit system this function is in fact <code>ftw64</code>, i.e., the LFS
interface transparently replaces the old interface. 
</p></blockquote></div>

<!-- ftw.h -->
<!-- Unix98 -->
<div class="defun">
&mdash; Function: int <b>ftw64</b> (<var>const char *filename, __ftw64_func_t func, int descriptors</var>)<var><a name="index-ftw64-1448"></a></var><br>
<blockquote><p>This function is similar to <code>ftw</code> but it can work on filesystems
with large files.  File information is reported using a variable of type
<code>struct stat64</code> which is passed by reference to the callback
function.

        <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> on a
32-bit system this function is available under the name <code>ftw</code> and
transparently replaces the old implementation. 
</p></blockquote></div>

<!-- ftw.h -->
<!-- XPG4.2 -->
<div class="defun">
&mdash; Function: int <b>nftw</b> (<var>const char *filename, __nftw_func_t func, int descriptors, int flag</var>)<var><a name="index-nftw-1449"></a></var><br>
<blockquote><p>The <code>nftw</code> function works like the <code>ftw</code> functions.  They call
the callback function <var>func</var> for all items found in the directory
<var>filename</var> and below.  At most <var>descriptors</var> file descriptors
are consumed during the <code>nftw</code> call.

        <p>One difference is that the callback function is of a different type.  It
is of type <code>struct&nbsp;FTW&nbsp;*</code><!-- /@w --> and provides the callback function
with the extra information described above.

        <p>A second difference is that <code>nftw</code> takes a fourth argument, which
is 0 or a bitwise-OR combination of any of the following values.

          <dl>
<dt><code>FTW_PHYS</code><a name="index-FTW_005fPHYS-1450"></a><dd>While traversing the directory symbolic links are not followed.  Instead
symbolic links are reported using the <code>FTW_SL</code> value for the type
parameter to the callback function.  If the file referenced by a
symbolic link does not exist <code>FTW_SLN</code> is returned instead. 
<br><dt><code>FTW_MOUNT</code><a name="index-FTW_005fMOUNT-1451"></a><dd>The callback function is only called for items which are on the same
mounted filesystem as the directory given by the <var>filename</var>
parameter to <code>nftw</code>. 
<br><dt><code>FTW_CHDIR</code><a name="index-FTW_005fCHDIR-1452"></a><dd>If this flag is given the current working directory is changed to the
directory of the reported object before the callback function is called. 
When <code>ntfw</code> finally returns the current directory is restored to
its original value. 
<br><dt><code>FTW_DEPTH</code><a name="index-FTW_005fDEPTH-1453"></a><dd>If this option is specified then all subdirectories and files within
them are processed before processing the top directory itself
(depth-first processing).  This also means the type flag given to the
callback function is <code>FTW_DP</code> and not <code>FTW_D</code>. 
<br><dt><code>FTW_ACTIONRETVAL</code><a name="index-FTW_005fACTIONRETVAL-1454"></a><dd>If this option is specified then return values from callbacks
are handled differently.  If the callback returns <code>FTW_CONTINUE</code>,
walking continues normally.  <code>FTW_STOP</code> means walking stops
and <code>FTW_STOP</code> is returned to the caller.  If <code>FTW_SKIP_SUBTREE</code>
is returned by the callback with <code>FTW_D</code> argument, the subtree
is skipped and walking continues with next sibling of the directory. 
If <code>FTW_SKIP_SIBLINGS</code> is returned by the callback, all siblings
of the current entry are skipped and walking continues in its parent. 
No other return values should be returned from the callbacks if
this option is set.  This option is a GNU extension. 
</dl>

        <p>The return value is computed in the same way as for <code>ftw</code>. 
<code>nftw</code> returns 0 if no failures occurred and all callback
functions returned 0.  In case of internal errors, such as memory
problems, the return value is -1 and <var>errno</var> is set
accordingly.  If the return value of a callback invocation was non-zero
then that value is returned.

        <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> on a
32-bit system this function is in fact <code>nftw64</code>, i.e., the LFS
interface transparently replaces the old interface. 
</p></blockquote></div>

<!-- ftw.h -->
<!-- Unix98 -->
<div class="defun">
&mdash; Function: int <b>nftw64</b> (<var>const char *filename, __nftw64_func_t func, int descriptors, int flag</var>)<var><a name="index-nftw64-1455"></a></var><br>
<blockquote><p>This function is similar to <code>nftw</code> but it can work on filesystems
with large files.  File information is reported using a variable of type
<code>struct stat64</code> which is passed by reference to the callback
function.

        <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> on a
32-bit system this function is available under the name <code>nftw</code> and
transparently replaces the old implementation. 
</p></blockquote></div>

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