Well, you've got a number of answers, most of which I agree with.

I'll add my opinion to the mix, though.

IMHO, the existing definition of the C language already has enough in
it to permit "directory" operations without needing additional
features.

Without violating the semantics of the current C standard, a
hypothetical implementation /could/ provide mechanisms within the
existing stdio definition to permit any sort of directory operations
that you like.

Want to enumerate the files within a directory? This hypothetical
implementation could offer a stream consisting of newline delimited
filenames when a specific filename is read. As in

  #include <stdio.h>

  {
    FILE *directory;

    if ((directory = fopen("/some/implementation/defined/path/to/a/
directory/*","r")) != NULL)
    {
       int byte;

       printf("contents of directory:");
       while ((byte=fgetc(directory)) != EOF) putc(byte); /* print
file names */
       fclose(directory);
    }
   }

Want to delete a directory?
   remove("/some/implementation/defined/path/to/a/directory");

Want to rename a directory
  rename("/some/implementation/defined/path/to/a/directory","/some/
other/implementation/defined/path");

Want to change current working directories?
  {
    FILE *cwd;

    if ((cwd = fopen("SYSTEM:current working directory","w") != NULL)
    {  fprintf(cwd,"/some/new/path"); fclose(cwd); }

   }

And so on.

All you need is an implementation-defined filename string and the
existing standard file functions, and a hypothetical implementation of
a standard C compiler and runtime could offer directory operations /
without violation or extension/ of the existing C standard