This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       If  dir  contains  filenames  that do not form character strings, or which contain characters outside the
       domain of the collating sequence of the current locale, the alphasort() function need not provide a total
       ordering. This condition is not possible if all filenames within the directory consist only of characters
       from the portable filename character set.

       The scandir() function may allocate dynamic storage  during  its  operation.  If  scandir()  is  forcibly
       terminated,  such  as  by  longjmp()  or siglongjmp() being executed by the function pointed to by sel or
       compar, or by an interrupt routine, scandir() does not have a chance to free that storage, so it  remains
       permanently  allocated.  A  safe  way  to  handle  interrupts  is to store the fact that an interrupt has
       occurred, then wait until scandir() returns to act on the interrupt.

       For functions that allocate memory as if by malloc(), the application should release such memory when  it
       is  no  longer  required  by  a  call  to  free().  For scandir(), this is namelist (including all of the
       individual strings in namelist).

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                     ALPHASORT(3POSIX)

       The  alphasort()  function  can be used as the comparison function for the scandir() function to sort the
       directory entries, d1 and d2, into alphabetical order. Sorting happens as if  by  calling  the  strcoll()
       function  on  the  d_name element of the dirent structures passed as the two parameters. If the strcoll()
       function fails, the return value of alphasort() is unspecified.

       The alphasort() function shall not change the setting of errno if successful. Since no  return  value  is
       reserved  to  indicate an error, an application wishing to check for error situations should set errno to
       0, then call alphasort(), then check errno.

       The scandir() function shall scan the directory dir, calling the  function  referenced  by  sel  on  each
       directory  entry.  Entries  for  which the function referenced by sel returns non-zero shall be stored in
       strings allocated as if by a call to malloc(), and sorted as if by a call to qsort() with the  comparison
       function  compar,  except that compar need not provide total ordering. The strings are collected in array
       namelist which shall be allocated as if by a call to malloc().  If sel is a  null  pointer,  all  entries
       shall be selected.  If the comparison function compar does not provide total ordering, the order in which
       the directory entries are stored is unspecified.

       The scandir() function shall fail if:

       EACCES Search  permission  is  denied  for  the component of the path prefix of dir or read permission is
              denied for dir.

       ELOOP  A loop exists in symbolic links encountered during resolution of the dir argument.

       ENAMETOOLONG
              The length of a component of a pathname is longer than {NAME_MAX}.

       ENOENT A component of dir does not name an existing directory or dir is an empty string.

       ENOMEM Insufficient storage space is available.

       ENOTDIR
              A component of dir names an existing file that is neither a directory nor a  symbolic  link  to  a
              directory.

       EOVERFLOW
              One of the values to be returned or passed to a callback function cannot be represented correctly.

       The scandir() function may fail if:

       ELOOP  More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the dir argument.

       EMFILE All file descriptors available to the process are currently open.

       ENAMETOOLONG
              The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an
              intermediate result with a length that exceeds {PATH_MAX}.

       ENFILE Too many files are currently open in the system.

       Thefollowingsectionsareinformative.

       An example to print the files in the current directory:

           #include <dirent.h>
           #include <stdio.h>
           #include <stdlib.h>
           ...
           struct dirent **namelist;
           int i,n;

               n = scandir(".", &namelist, 0, alphasort);
               if (n < 0)
                   perror("scandir");
               else {
                   for (i = 0; i < n; i++) {
                       printf("%s\n", namelist[i]->d_name);
                       free(namelist[i]);
                       }
                   }
               free(namelist);
           ...

       None.

       alphasort, scandir — scan a directory

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       None.

       Upon  successful  completion, the alphasort() function shall return an integer greater than, equal to, or
       less than 0, according to whether the name of the directory entry pointed to by d1 is  lexically  greater
       than,  equal  to, or less than the directory pointed to by d2 when both are interpreted as appropriate to
       the current locale. There is no return value reserved to indicate an error.

       Upon successful completion, the scandir() function shall return the number of entries in the array and  a
       pointer to the array through the parameter namelist.  Otherwise, the scandir() function shall return -1.

qsort(), strcoll()

       The Base Definitions volume of POSIX.1‐2017, <dirent.h>

       #include <dirent.h>

       int alphasort(const struct dirent **d1, const struct dirent **d2);
       int scandir(const char *dir, struct dirent ***namelist,
           int (*sel)(const struct dirent *),
           int (*compar)(const struct dirent **, const struct dirent **));