[m-users.] Compiler warning in my readdir.m

[Julian Fondren]

On 2019-10-13 23:08, Volker Wysk wrote:
> Hi
> 
> There might be an issue in my fixed version of readdir.m. When building
> the module, I get a compiler warning:
> 
> Making Mercury/os/readdir.o
> readdir.m: In function ‘readdir_module0’:
> readdir.m:70:18: warning: assignment discards ‘const’ qualifier from
> pointer target type [-Wdiscarded-qualifiers]
> readdir.m: In function ‘readdir_module1’:
> readdir.m:70:18: warning: assignment discards ‘const’ qualifier from
> pointer target type [-Wdiscarded-qualifiers]
> 
> This is in the C code at this place (the line in the middle):
> 
>         if (errno == 0) {
>            Entry = \"\";
>            Result = 0;
> 
> I've attached the complete file.
> 
> Changing the 'Entry =' line to 'Entry = (char*) \"\"' avoids the
> warning message. I guess it's not an error, but I'm not completely
> sure. I'm not experienced with the foreign language interface (yet).
> Maybe it's the type of "Entry" should be "char * const", rahter than
> "char *".
> 

A string literal like "" is going to be in read-only memory, so the
const stuff helps slightly to prevent accidental writing to it.
What you're doing though is pretty weird: returning a string just
so that Mercury can test for that string.

Probably you arrived at this design because per the pred
declaration you're obliged to return a string even when there's no
useful string to return. What you can do though, is just not set
Entry at all (untested):

:- type low_readdir_result
     --->    ok
     ;       eod
     ;       error.
:- pragma foreign_export_enum("C",
     low_readdir_result/0, [prefix("LRR_"), uppercase]).

readdir(Dir, Result, !IO) :-
     readdir0(Dir, Entry, Res, !IO),
     (
         Res = ok,
         Result = ok(yes(Entry))
     ;
         Res = eod,
         % !!
         % what is Entry in this case?
         % ... let's just promise not to look at it.
         % !!
         Result = ok(no)
     ;
         Res = error,
         % !!
         errno(Err, !IO),
         Result = error(Err),
     ).

:- pred readdir0(dir::in, string::out, low_readdir_result::out, io::di, 
io::uo) is det.
:- pragma foreign_proc("C",
     readdir0(Dir::in, Entry::out, Result::out, _IO0::di, _IO::uo),
     [will_not_call_mercury, promise_pure, thread_safe, tabled_for_io],
"
     struct dirent *ent = NULL;
     errno = 0;
     ent = readdir(Dir);
     if (ent != NULL) {
         MR_make_aligned_string_copy(Entry, ent->d_name);
         Result = LRR_OK;
     } else if (errno == 0) {
        Result = LRR_EOD;
     else {
        Result = LRR_ERROR;
     }
").

I've wondered if predicates like this should just be impure, so
that callers at least have to explicitly opt in, but it's only
come up in small libraries, in unexported code.