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14.2.6 Scanning the Content of a Directory

A higher-level interface to the directory handling functions is the scandir 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.

— Function: int scandir (const char *dir, struct dirent ***namelist, int (*selector) (const struct dirent *), int (*cmp) (const void *, const void *))

The scandir function scans the contents of the directory selected by dir. The result in *namelist is an array of pointers to structure of type struct dirent which describe all selected directory entries and which is allocated using malloc. Instead of always getting all directory entries returned, the user supplied function selector can be used to decide which entries are in the result. Only the entries for which selector returns a non-zero value are selected.

Finally the entries in *namelist are sorted using the user-supplied function cmp. The arguments passed to the cmp function are of type struct dirent **, therefore one cannot directly use the strcmp or strcoll functions; instead see the functions alphasort and versionsort below.

The return value of the function is the number of entries placed in *namelist. If it is -1 an error occurred (either the directory could not be opened for reading or the malloc call failed) and the global variable errno contains more information on the error.

As described above the fourth argument to the scandir 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.

— Function: int alphasort (const void *a, const void *b)

The alphasort function behaves like the strcoll function (see String/Array Comparison). The difference is that the arguments are not string pointers but instead they are of type struct dirent **.

The return value of alphasort is less than, equal to, or greater than zero depending on the order of the two entries a and b.

— Function: int versionsort (const void *a, const void *b)

The versionsort function is like alphasort except that it uses the strverscmp function internally.

If the filesystem supports large files we cannot use the scandir anymore since the dirent structure might not able to contain all the information. The LFS provides the new type struct dirent64. To use this we need a new function.

— Function: int scandir64 (const char *dir, struct dirent64 ***namelist, int (*selector) (const struct dirent64 *), int (*cmp) (const void *, const void *))

The scandir64 function works like the scandir function except that the directory entries it returns are described by elements of type struct dirent64. The function pointed to by selector is again used to select the desired entries, except that selector now must point to a function which takes a struct dirent64 * parameter.

Similarly the cmp function should expect its two arguments to be of type struct dirent64 **.

As cmp is now a function of a different type, the functions alphasort and versionsort cannot be supplied for that argument. Instead we provide the two replacement functions below.

— Function: int alphasort64 (const void *a, const void *b)

The alphasort64 function behaves like the strcoll function (see String/Array Comparison). The difference is that the arguments are not string pointers but instead they are of type struct dirent64 **.

Return value of alphasort64 is less than, equal to, or greater than zero depending on the order of the two entries a and b.

— Function: int versionsort64 (const void *a, const void *b)

The versionsort64 function is like alphasort64, excepted that it uses the strverscmp function internally.

It is important not to mix the use of scandir and the 64-bit comparison functions or vice versa. There are systems on which this works but on others it will fail miserably.