[m-users.] C pointers, strings, seg fault

[Mark Brown]

Hi Tomas,

On Tue, Nov 18, 2014 at 3:50 AM, Tomas By <tomas at basun.net> wrote:
> So the problem seems to be the call to "intstring_c"
>
> What am I supposed to pass as the string output parameter to a Mercury
> procedure?

For C it is MR_String*, just as you have done.

>
> Since I do not know the length of the string before the call, allocating
> it in advance might get awkward.
>
> Plus I still don't understand the seg fault, as I do not use that value?

int_str_m is annotated with will_not_call_mercury, but it does call
Mercury. That could be the problem.

Cheers,
Mark.


>
>
>
> On Mon, November 17, 2014 16:48, Tomas By wrote:
>> Hello,
>>
>> Would appreciate if somebody could explain why this code causes a
>> segmentation fault.
>>
>> (If it looks messed up, it is because I have simplified a `real' program
>> in several stages, and then experimented a bit with the procedure
>> signatures. Am on very thin ice here.)
>>
>> /Tomas
>>
>>
>>
>> ---------- begin code ----------
>> % -*- fundamental -*-
>> :- module test.
>> :- interface.
>> :- import_module io.
>> :- pred main(io::di,io::uo) is det.
>> :- implementation.
>> :- import_module list, string.
>>
>> :- pragma foreign_decl("C","void int_str_c(MR_Integer,MR_String*);").
>> :- pragma foreign_decl("C","void intstring_c(MR_Integer,MR_String*);").
>>
>> main(!IO) :-
>>   Ns = [1,2,3,4,5,6,7,8,9,10],
>>   map(int_str_m,Ns,Ss),
>>   write_list(Ss,"\n",write_string,!IO), io.nl(!IO).
>>
>> :- pragma foreign_decl("C","#define _POSIX_C_SOURCE 200809L").
>> :- pragma foreign_decl("C","#include <stdlib.h>").
>> :- pragma foreign_decl("C","#include <string.h>").
>> :- pragma foreign_decl("C","#include <stdint.h>").
>> :- pragma foreign_decl("C","#include <stdio.h>").
>>
>> :- pred int_str_m(int::in,string::out) is det.
>> :- pragma foreign_proc("C",int_str_m(Loc::in,Str::out),
>>               [will_not_call_mercury,promise_pure,thread_safe,tabled_for_io],"
>> int_str_c(Loc,&Str);
>> ").
>>
>> :- pragma foreign_decl("C","void int_str_c(MR_Integer n,MR_String* p) {
>>   MR_String s;
>>   char temp[1024];
>>   intstring_c(n,&s);
>>   sprintf(temp,\"%i\",(int)n);
>>   MR_make_aligned_string_copy(*p,temp);
>> }
>> ").
>>
>> :- pragma foreign_export("C",intstring_m(in,out),"intstring_c").
>> :- pred intstring_m(int,string).
>> :- mode intstring_m(in,out) is det.
>> intstring_m(Int,Str) :-
>>   Str = string.from_int(Int).
>>
>> :- end_module test.
>>
>>
>
>
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