Hi all, I am trying to write a concurrent stack. Code is below:
%{
#include <stdatomic.h>
#include <stdlib.h>
struct stack_t {
void *value;
struct stack_t *next;
};
void __cats_new(struct stack_t *st) {
st->value = NULL;
st->next = NULL;
}
void __cats_push(struct stack_t *st, void *val) {
for (;;) {
struct stack_t old_st = *st;
struct stack_t new_st = {val, &old_st};
if (atomic_compare_exchange_strong(st, &old_st, new_st))
return;
}
}
// ignore ABA problem
void *__cats_pop(struct stack_t *st) {
for (;;) {
if (st->next == NULL)
return NULL;
struct stack_t *old_st = st;
struct stack_t xs1 = *(st->next);
void *x = st->value;
if (atomic_compare_exchange_strong(st, old_st, xs1))
return x;
}
}
atstype_boxed pop_ats(atstype_ref st) { return __cats_pop(st); }
atsvoid_t0ype new_ats(atstype_ref st) { __cats_new(st); }
atsvoid_t0ype push_ats(atstype_ref st, atstype_var val) {
__cats_push(st, val);
}
%}
typedef stack_t(a: t@ype+) = $extype "struct stack_t"
extern
fun new {a:t@ype+}(&stack_t(a)? >> _) : void =
"ext#new_ats"
extern
fun push {a:t@ype+}(&stack_t(a) >> _, a) : void =
"ext#push_ats"
extern
fun pop {a:t@ype+}(&stack_t(a) >> _) : Option(a) =
"ext#pop_ats"
fn print_str(x : string) : void =
println!(x)
implement main0 (argc, argv) =
let
var st: stack_t(string)
val () = new(st)
val () = push(st, "res")
val () = push(st, "res2")
val- Some (x) = pop(st)
val () = print_str(x)
in end
(This compiles with gcc; I can run it with patscc simple.dats -latomic ;
./a.out)
Unfortunately, it immediately segfaults. I know that the offending line
is the
val () = print_str(x)
because removing it makes the program run fine.
I tried writing an equivalent program in C, viz.
#include <stdio.h>
int main(int argc, char *argv[]) {
struct stack_t *st;
__cats_push(st, "res");
__cats_push(st, "res2");
char *res;
res = __cats_pop(st);
printf("%s\n", res);
res = __cats_pop(st);
printf("%s\n", res);
}
...which works as expected. So I believe I am misunderstanding how ATS
handles FFI and how structures exist in memory.
Any insight is appreciated!
Thanks,
Vanessa
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