"Candidate function not viable;" Issues with porting a C library to C++

Question

I'm attempting to use a c library in a c++ implementation, and I get errors often having to do with "cantidate function not viable," and it appears to think that the function in the library will not function with the arguments presented in my code. The thing is, my code is literally an old implementation (in c) that works perfectly fine when compiled in c. Is there a way to make this compile as a part of my c++ environment?

Here are the errors I get:

testbed/des-lsr_routinglogic.cpp:20:27: error: no matching function for call to 'dessert_periodic_add'
periodic_refresh_nh = dessert_periodic_add(refresh_list, NULL, NULL, &refresh_neighbor_t);
                      ^~~~~~~~~~~~~~~~~~~~
/usr/local/include/dessert.h:880:21: note: candidate function not viable: no known conversion from 'dessert_per_result_t ()' to 'dessert_periodiccallback_t *' (aka 'dessert_per_result_t (*)(void *, struct
  timeval *, struct timeval *)') for 1st argument
dessert_periodic_t* dessert_periodic_add(dessert_periodiccallback_t* c, void* data, const struct timeval* scheduled, const struct timeval* interval);
                ^

from this line of code code:

periodic_refresh_nh = dessert_periodic_add(refresh_list, NULL, NULL, &refresh_neighbor_t);

using this excerpt of the library:

dessert_periodic_t* dessert_periodic_add(dessert_periodiccallback_t* c, void* data,     const struct timeval* scheduled, const struct timeval* interval);
dessert_periodic_t* dessert_periodic_add_delayed(dessert_periodiccallback_t* c, void* data, int delay);
int dessert_periodic_del(dessert_periodic_t* p);
void dessert_register_ptr_name(void* ptr, const char* name);
const char* dessert_ptr2name(void* ptr);

I think the compiler output should be self evident to anyone experienced in combining c/c++ but it looks like gibberish to me D:

EDIT

Thank you, it was pointed out to me I left out some important definitions; The definition of refresh_list:

 dessert_per_result_t refresh_list(void *data, struct timeval *scheduled, struct timeval *interval) {
pthread_rwlock_wrlock(&pp_rwlock);
node_neighbors_t *neighbor = dir_neighbors_head;
while (neighbor) {
    if (neighbor->entry_age-- == 0) {
        node_neighbors_t* el_to_delete = neighbor;
        HASH_DEL(dir_neighbors_head, el_to_delete);
        free(el_to_delete);
    } else {
        neighbor->weight = 1;
    }
    neighbor = neighbor->hh.next;
}
pthread_rwlock_unlock(&pp_rwlock);
return 0; }

The definition of dessert_per_result_t:

 typedef enum _dessert_periodic_results { DESSERT_PER_KEEP = 0, DESSERT_PER_UNREGISTER = 1 } dessert_per_result_t;

dessert_periodiccallback_t is defined oddly; I will try to post it.

While the compiler output is fairly easy to understand, it's totally obscure to us what `refresh_list` is, because you chose not to tell us. — Kerrek SB, Jun 10 '13 at 12:27
Please also post the declarations of `dessert_periodiccallback_t`, `dessert_per_result_t` and `refresh_list`. — Sebastian Redl, Jun 10 '13 at 12:35
You are passing a structure and the library expects a pointer. Ah, we all *love*, C, huh? — Bartek Banachewicz, Jun 10 '13 at 12:38
typedef enum _dessert_periodic_results { DESSERT_PER_KEEP = 0, DESSERT_PER_UNREGISTER = 1 } dessert_per_result_t; — KeximusMaximus, Jun 10 '13 at 12:38
@KerrekSB: `refresh_list` has type `dessert_per_result_t ()`. — Eric Postpischil, Jun 10 '13 at 12:41
@EricPostpischil: Errr.. clearly not, given the code you just posted. — Kerrek SB, Jun 10 '13 at 12:47
@KerrekSB: I did not post any code. However, my assertion is consistent with the code posted by Grey Newell. And the type of `result_list` is shown to us in the compiler error message. — Eric Postpischil, Jun 10 '13 at 12:51
@GreyNewell: you have shown the definition of `result_list`. However, the problem is its declaration in a header. — Eric Postpischil, Jun 10 '13 at 12:52

score 2 · Accepted Answer · edited Jun 10 '13 at 12:56

2

C++ is stricter about function declarations than C. In this case, the function refresh_list is declared as dessert_per_result_t refresh_list(), but the first argument to dessert_periodic_add must be a pointer to a function with explicitly specified parameter types.

The best way to fix this would be to add a complete declaration of refresh_list, matching what dessert_periodic_add expects, in place of the abbreviated declaration of it.

edited Jun 10 '13 at 12:56

David Foerster

1,461
1
14
23

answered Jun 10 '13 at 12:48

Eric Postpischil

195,579
13
168
312

Thanks to David Foerster for the correction. I was scrolling back and forth on a small device to copy text and compose an answer. – Eric Postpischil Jun 10 '13 at 13:05
Now I just need to ask the creator of the library what in the hell I should pass as params in a function I don't know when I want to call without global variables... oh, c... – KeximusMaximus Jun 10 '13 at 13:07
@GreyNewell: Global variables are not an issue. The parameter type is. The parameter type is shown: `dessert_periodic_add` expects a pointer to a function taking two parameters, the first a pointer to `void` and the other two pointers to `struct timeval *`, and returning `dessert_per_result_t`. So `refresh_list` should be declared as `dessert_per_result_t refresh_list(void *, struct timeval *, struct timeval *)`. – Eric Postpischil Jun 10 '13 at 15:09

score 0 · Answer 2 · answered Jun 10 '13 at 12:51

0

The compiler message shows that the compiler is of the opinion that refresh_list is a function that takes no arguments and returns a dessert_per_result_t.

The definition you posted disagrees.

This suggests that there is a separate declaration of refresh_list that is inconsistent with the definition.

answered Jun 10 '13 at 12:51

Sebastian Redl

69,373
8
123
157

The declaration may be inconsistent in C++ but not in C. – Eric Postpischil Jun 10 '13 at 12:53
Very possible, yes. In which case he might have to modify the library headers. – Sebastian Redl Jun 10 '13 at 12:54
I would like to avoid modifying headers if possible as my library is used in several places in this project. – KeximusMaximus Jun 10 '13 at 12:58
Well, is refresh_list your own function, or part of the library? – Sebastian Redl Jun 10 '13 at 13:06

score 0 · Answer 3 · edited May 23 '17 at 12:28

The declaration of refresh_list shown in the error message is this:

dessert_per_result_t refresh_list();

This means two different things in C++ and C.

In C++, it means that refresh_list takes zero arguments.

In C, it means that refresh_list takes an unspecified number of arguments of unspecified types.

Hence why it worked in C, but in C++ a pointer to refresh_list is not permitted where a pointer to dessert_per_result_t(void *, struct timeval *, struct timeval *) is expected.

"Candidate function not viable;" Issues with porting a C library to C++

EDIT

3 Answers3