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| <h4 class="subsection">14.2.6 Scanning the Content of a Directory</h4> |
| |
| <p>A higher-level interface to the directory handling functions is the |
| <code>scandir</code> function. With its help one can select a subset of the |
| entries in a directory, possibly sort them and get a list of names as |
| the result. |
| |
| <!-- dirent.h --> |
| <!-- BSD/SVID --> |
| <div class="defun"> |
| — Function: int <b>scandir</b> (<var>const char *dir, struct dirent ***namelist, int </var>(<var>*selector</var>) (<var>const struct dirent *</var>)<var>, int </var>(<var>*cmp</var>) (<var>const void *, const void *</var>))<var><a name="index-scandir-1426"></a></var><br> |
| <blockquote> |
| <p>The <code>scandir</code> function scans the contents of the directory selected |
| by <var>dir</var>. The result in *<var>namelist</var> is an array of pointers to |
| structure of type <code>struct dirent</code> which describe all selected |
| directory entries and which is allocated using <code>malloc</code>. Instead |
| of always getting all directory entries returned, the user supplied |
| function <var>selector</var> can be used to decide which entries are in the |
| result. Only the entries for which <var>selector</var> returns a non-zero |
| value are selected. |
| |
| <p>Finally the entries in *<var>namelist</var> are sorted using the |
| user-supplied function <var>cmp</var>. The arguments passed to the <var>cmp</var> |
| function are of type <code>struct dirent **</code>, therefore one cannot |
| directly use the <code>strcmp</code> or <code>strcoll</code> functions; instead see |
| the functions <code>alphasort</code> and <code>versionsort</code> below. |
| |
| <p>The return value of the function is the number of entries placed in |
| *<var>namelist</var>. If it is <code>-1</code> an error occurred (either the |
| directory could not be opened for reading or the malloc call failed) and |
| the global variable <code>errno</code> contains more information on the error. |
| </p></blockquote></div> |
| |
| <p>As described above the fourth argument to the <code>scandir</code> function |
| must be a pointer to a sorting function. For the convenience of the |
| programmer the GNU C library contains implementations of functions which |
| are very helpful for this purpose. |
| |
| <!-- dirent.h --> |
| <!-- BSD/SVID --> |
| <div class="defun"> |
| — Function: int <b>alphasort</b> (<var>const void *a, const void *b</var>)<var><a name="index-alphasort-1427"></a></var><br> |
| <blockquote><p>The <code>alphasort</code> function behaves like the <code>strcoll</code> function |
| (see <a href="String_002fArray-Comparison.html#String_002fArray-Comparison">String/Array Comparison</a>). The difference is that the arguments |
| are not string pointers but instead they are of type |
| <code>struct dirent **</code>. |
| |
| <p>The return value of <code>alphasort</code> is less than, equal to, or greater |
| than zero depending on the order of the two entries <var>a</var> and <var>b</var>. |
| </p></blockquote></div> |
| |
| <!-- dirent.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>versionsort</b> (<var>const void *a, const void *b</var>)<var><a name="index-versionsort-1428"></a></var><br> |
| <blockquote><p>The <code>versionsort</code> function is like <code>alphasort</code> except that it |
| uses the <code>strverscmp</code> function internally. |
| </p></blockquote></div> |
| |
| <p>If the filesystem supports large files we cannot use the <code>scandir</code> |
| anymore since the <code>dirent</code> structure might not able to contain all |
| the information. The LFS provides the new type <code>struct dirent64</code><!-- /@w -->. To use this we need a new function. |
| |
| <!-- dirent.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>scandir64</b> (<var>const char *dir, struct dirent64 ***namelist, int </var>(<var>*selector</var>) (<var>const struct dirent64 *</var>)<var>, int </var>(<var>*cmp</var>) (<var>const void *, const void *</var>))<var><a name="index-scandir64-1429"></a></var><br> |
| <blockquote><p>The <code>scandir64</code> function works like the <code>scandir</code> function |
| except that the directory entries it returns are described by elements |
| of type <code>struct dirent64</code><!-- /@w -->. The function pointed to by |
| <var>selector</var> is again used to select the desired entries, except that |
| <var>selector</var> now must point to a function which takes a |
| <code>struct dirent64 *</code><!-- /@w --> parameter. |
| |
| <p>Similarly the <var>cmp</var> function should expect its two arguments to be |
| of type <code>struct dirent64 **</code>. |
| </p></blockquote></div> |
| |
| <p>As <var>cmp</var> is now a function of a different type, the functions |
| <code>alphasort</code> and <code>versionsort</code> cannot be supplied for that |
| argument. Instead we provide the two replacement functions below. |
| |
| <!-- dirent.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>alphasort64</b> (<var>const void *a, const void *b</var>)<var><a name="index-alphasort64-1430"></a></var><br> |
| <blockquote><p>The <code>alphasort64</code> function behaves like the <code>strcoll</code> function |
| (see <a href="String_002fArray-Comparison.html#String_002fArray-Comparison">String/Array Comparison</a>). The difference is that the arguments |
| are not string pointers but instead they are of type |
| <code>struct dirent64 **</code>. |
| |
| <p>Return value of <code>alphasort64</code> is less than, equal to, or greater |
| than zero depending on the order of the two entries <var>a</var> and <var>b</var>. |
| </p></blockquote></div> |
| |
| <!-- dirent.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>versionsort64</b> (<var>const void *a, const void *b</var>)<var><a name="index-versionsort64-1431"></a></var><br> |
| <blockquote><p>The <code>versionsort64</code> function is like <code>alphasort64</code>, excepted that it |
| uses the <code>strverscmp</code> function internally. |
| </p></blockquote></div> |
| |
| <p>It is important not to mix the use of <code>scandir</code> and the 64-bit |
| comparison functions or vice versa. There are systems on which this |
| works but on others it will fail miserably. |
| |
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