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void frr_init_vtydir(void){
    snprintf(frr_vtydir, sizeof(frr_vtydir), DAEMON_VTY_DIR, "", "");
}

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sudo mv /var/opt/frr/staticd.vty /var/opt/frr/staticd.vty.orig
sudo socat -x -v UNIX-LISTEN:/var/opt/frr/staticd.vty, UNIX-CONNECT:/var/opt/frr/staticd.vty.orig

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$ sudo vtysh
% Can't open configuration file /etc/frr/vtysh.conf due to 'No such file or directory'.

Hello, this is FRRouting (version 8.3-dev-MyOwnFRRVersion-gde416e678).
Copyright 1996-2005 Kunihiro Ishiguro, et al.

This is a git build of frr-8.3-dev-388-gde416e678
Associated branch(es):
        local:master
        github/frrouting/frr.git/master

bullseye# configure
bullseye(config)# ip route 100.0.35.0/24 10.0.2.100
bullseye(config)# exit
bullseye# exit

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> 2022/05/06 14:42:10.163439  length=7 from=0 to=6
 65 6e 61 62 6c 65 00                             enable.
--
< 2022/05/06 14:42:10.164441  length=4 from=0 to=3
 00 00 00 00                                      ....
--
> 2022/05/06 14:42:16.305447  length=11 from=7 to=17
 63 6f 6e 66 69 67 75 72 65 20 00                 configure .
--
< 2022/05/06 14:42:16.305631  length=4 from=4 to=7
 00 00 00 00                                      ....
--
> 2022/05/06 14:42:35.168397  length=34 from=18 to=51
 69 70 20 72 6f 75 74 65 20 31 30 30 2e 30 2e 33  ip route 100.0.3
 35 2e 30 2f 32 34 20 31 30 2e 30 2e 32 2e 31 30  5.0/24 10.0.2.10
 30 00                                            0.
--
< 2022/05/06 14:42:35.169482  length=4 from=8 to=11
 00 00 00 00                                      ....
--
> 2022/05/06 14:42:37.422039  length=5 from=52 to=56
 65 78 69 74 00                                   exit.
--
< 2022/05/06 14:42:37.426787  length=4 from=12 to=15
 00 00 00 00                                      ....
--
> 2022/05/06 14:42:38.509946  length=5 from=57 to=61
 65 78 69 74 00                                   exit.
--
< 2022/05/06 14:42:38.510113  length=4 from=16 to=19
 00 00 00 00                                      ....
--

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static void vty_event(enum vty_event event, struct vty *vty)
{
        switch (event) {
#ifdef VTYSH
        case VTYSH_READ:
                thread_add_read(vty_master, vtysh_read, vty, vty->fd,
                                &vty->t_read);
                break;
        case VTYSH_WRITE:
                thread_add_write(vty_master, vtysh_write, vty, vty->wfd,
                                 &vty->t_write);
                break;

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#define _xref_t_a(addfn, type, m, f, a, v, t)                                  \
        ({                                                                     \
                static const struct xref_threadsched _xref                     \
                                __attribute__((used)) = {                      \
                        .xref = XREF_INIT(XREFT_THREADSCHED, NULL, __func__),  \
                        .funcname = #f,                                        \
                        .dest = #t,                                            \
                        .thread_type = THREAD_ ## type,                        \
                };                                                             \
                XREF_LINK(_xref.xref);                                         \
                _thread_add_ ## addfn(&_xref, m, f, a, v, t);                  \
        })                                                                     \
        /* end */

#define thread_add_read(m,f,a,v,t)       _xref_t_a(read_write, READ,  m,f,a,v,t)
#define thread_add_write(m,f,a,v,t)      _xref_t_a(read_write, WRITE, m,f,a,v,t)
#define thread_add_timer(m,f,a,v,t)      _xref_t_a(timer,      TIMER, m,f,a,v,t)
#define thread_add_timer_msec(m,f,a,v,t) _xref_t_a(timer_msec, TIMER, m,f,a,v,t)
#define thread_add_timer_tv(m,f,a,v,t)   _xref_t_a(timer_tv,   TIMER, m,f,a,v,t)
#define thread_add_event(m,f,a,v,t)      _xref_t_a(event,      EVENT, m,f,a,v,t)

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void _thread_add_read_write(const struct xref_threadsched *xref,
                            struct thread_master *m,
                            void (*func)(struct thread *), void *arg, int fd,
                            struct thread **t_ptr)
{
    if (dir == THREAD_READ)
        frrtrace(9, frr_libfrr, schedule_read, m,
                xref->funcname, xref->xref.file, xref->xref.line,
                t_ptr, fd, 0, arg, 0);
    else
        frrtrace(9, frr_libfrr, schedule_write, m,
                xref->funcname, xref->xref.file, xref->xref.line,
                t_ptr, fd, 0, arg, 0);

    frr_with_mutex(&m->mtx) {
            thread = thread_get(m, dir, func, arg, xref);

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$ find ./yang/ -name '*.yang'
./yang/frr-isisd.yang
./yang/frr-ospfd.yang
./yang/frr-filter.yang
./yang/frr-route-map.yang
./yang/frr-bgp-types.yang
./yang/frr-bgp-peer-group.yang
./yang/frr-bgp-common-structure.yang
./yang/frr-ospf6-route-map.yang
./yang/frr-test-module.yang
./yang/frr-deviations-bgp-datacenter.yang
./yang/frr-bgp-rpki.yang
./yang/frr-ripngd.yang
./yang/frr-route-types.yang
./yang/frr-bgp.yang
./yang/frr-bgp-route-map.yang
./yang/frr-module-translator.yang
./yang/frr-bgp-common-multiprotocol.yang
./yang/frr-bgp-filter.yang
./yang/frr-bgp-bmp.yang
./yang/frr-pim-rp.yang
./yang/frr-pim.yang
./yang/ietf/ietf-routing-types.yang
./yang/ietf/frr-deviations-ietf-rip.yang
./yang/ietf/frr-deviations-ietf-routing.yang
./yang/ietf/frr-deviations-ietf-interfaces.yang
./yang/ietf/ietf-bgp-types.yang
./yang/ietf/ietf-interfaces.yang
./yang/frr-bgp-neighbor.yang
./yang/frr-vrf.yang
./yang/frr-eigrpd.yang
./yang/frr-routing.yang
./yang/frr-nexthop.yang
./yang/confd/confd.frr-ripd.yang
./yang/confd/confd.frr-ripngd.yang
./yang/frr-ospf-route-map.yang
./yang/frr-zebra-route-map.yang
./yang/frr-staticd.yang
./yang/frr-bfdd.yang
./yang/frr-gmp.yang
./yang/frr-ripd.yang
./yang/frr-pathd.yang
./yang/frr-bgp-common.yang
./yang/frr-interface.yang
./yang/frr-zebra.yang
./yang/frr-vrrpd.yang

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struct yang_module *yang_module_load(const char *module_name)
{
        struct yang_module *module;
        const struct lys_module *module_info;

        module_info =
                ly_ctx_load_module(ly_native_ctx, module_name, NULL, NULL);
        if (!module_info) {
                // ...
                exit(1);
        }

        module = XCALLOC(MTYPE_YANG_MODULE, sizeof(*module));
        module->name = module_name;
        module->info = module_info;

        if (RB_INSERT(yang_modules, &yang_modules, module) != NULL) {
                // ...
                exit(1);
        }

        return module;
}

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RB_HEAD(yang_modules, yang_module);
RB_PROTOTYPE(yang_modules, yang_module, entry, yang_module_compare);

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struct yang_module *yang_module_find(const char *module_name)
{
        struct yang_module s;

        s.name = module_name;
        return RB_FIND(yang_modules, &yang_modules, &s);
}

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const struct frr_yang_module_info frr_staticd_info = {
        .name = "frr-staticd",
        .nodes = {
            //...
                {
                    .xpath = "/frr-routing:routing/control-plane-protocols/control-plane-protocol/frr-staticd:staticd/route-list/src-list/path-list/frr-nexthops/nexthop",
                    .cbs = {
                            .apply_finish = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_src_list_path_list_frr_nexthops_nexthop_apply_finish,
                            .create = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_src_list_path_list_frr_nexthops_nexthop_create,
                            .destroy = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_src_list_path_list_frr_nexthops_nexthop_destroy,
                            .pre_validate = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_path_list_frr_nexthops_nexthop_pre_validate,
                            .cli_show = static_src_nexthop_cli_show,
                            .cli_cmp = static_nexthop_cli_cmp,
                    }
                },
            //...
        }

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static int nb_transaction_process(enum nb_event event,
                                  struct nb_transaction *transaction,
                                  char *errmsg, size_t errmsg_len)
{
    struct nb_config_cb *cb;
    // ...
    RB_FOREACH (cb, nb_config_cbs, &transaction->changes) {
        struct nb_config_change *change = (struct nb_config_change *)cb;
        int ret;
    ret = nb_callback_configuration(transaction->context, event,
                                    change, errmsg, errmsg_len);
    // ...
    }
    return NB_OK;
}

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// in lib/northbound.c
int nb_candidate_commit_prepare(struct nb_context *context,
                                struct nb_config *candidate,
                                const char *comment,
                                struct nb_transaction **transaction,
                                /* ... */) {
    // ...
    nb_transaction_new()
    // ...
    nb_transaction_process(NB_EV_PREPARE, ...)
}

int nb_candidate_commit(struct nb_context *context, struct nb_config *candidate,
                    bool save_transaction, const char *comment,
                    /* attrs */) {
    // ...
    nb_candidate_commit_prepare()
    // ...
    nb_candidate_commit_apply()
}

void nb_candidate_commit_apply(struct nb_transaction *transaction, 
                               /* attrs */) {
    nb_transaction_process(NB_EV_APPLY, ...)
    nb_transaction_apply_finish()
    // ...
}

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// in lib/northbound_cli.c
static int nb_cli_classic_commit(struct vty *vty) {
    // ...
    nb_candidate_commit()
    // ...
}

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void nb_cli_enqueue_change(struct vty *vty, const char *xpath,
                           enum nb_operation operation, const char *value)
{
        struct vty_cfg_change *change;
        // ...
        change = &vty->cfg_changes[vty->num_cfg_changes++];
        strlcpy(change->xpath, xpath, sizeof(change->xpath));
        change->operation = operation;
        change->value = value;
}

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nb_init(master, di->yang_modules, di->n_yang_modules, true);

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static struct frr_daemon_info staticd_di;

static const struct frr_yang_module_info *const staticd_yang_modules[] = {
        &frr_filter_info,
        &frr_interface_info,
        &frr_vrf_info,
        &frr_routing_info,
        &frr_staticd_info,
};

FRR_DAEMON_INFO(staticd, STATIC,
                .yang_modules = staticd_yang_modules,
                .n_yang_modules = array_size(staticd_yang_modules),
);

int main(int argc, char **argv, char **envp) {
    frr_preinit(&staticd_di, argc, argv);
    frr_init();
}

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#define FRR_DAEMON_INFO(execname, constname, ...)                              \
        static struct frr_daemon_info execname##_di = {.name = #execname,      \
                                                       .logname = #constname,  \
                                                       .module = THIS_MODULE,  \
                                                       __VA_ARGS__};           \
        FRR_COREMOD_SETUP(.name = #execname,                                   \
                          .description = #execname " daemon",                  \
                          .version = FRR_VERSION, );                           \
        MACRO_REQUIRE_SEMICOLON() /* end */

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const struct frr_yang_module_info frr_staticd_info = {
        .name = "frr-staticd",
        .nodes = {
            //...
                {
                    .xpath = "/frr-routing:routing/control-plane-protocols/control-plane-protocol/frr-staticd:staticd/route-list/src-list/path-list/frr-nexthops/nexthop",
                    .cbs = {
                            .apply_finish = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_src_list_path_list_frr_nexthops_nexthop_apply_finish,
                            .create = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_src_list_path_list_frr_nexthops_nexthop_create,
                            .destroy = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_src_list_path_list_frr_nexthops_nexthop_destroy,
                            .pre_validate = routing_control_plane_protocols_control_plane_protocol_staticd_route_list_path_list_frr_nexthops_nexthop_pre_validate,
                            .cli_show = static_src_nexthop_cli_show,
                            .cli_cmp = static_nexthop_cli_cmp,
                    }
                },
            //...
        }

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void (*const zserv_handlers[])(ZAPI_HANDLER_ARGS) = {
    // ...
        [ZEBRA_ROUTE_ADD] = zread_route_add,
        [ZEBRA_ROUTE_DELETE] = zread_route_del,
        [ZEBRA_REDISTRIBUTE_ADD] = zebra_redistribute_add,
        [ZEBRA_REDISTRIBUTE_DELETE] = zebra_redistribute_delete,
    // ...

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// in zebra/zapi_msg.c
static void zread_route_add(ZAPI_HANDLER_ARGS)
{
    // ...
    struct route_entry *re;
    // reを組み立てる
    ret = rib_add_multipath_nhe(afi, api.safi, &api.prefix, src_p,
                                re, &nhe, false);
    // ...
}

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/* Routing table instance.  */
struct zebra_vrf {
        struct vrf *vrf;

        /* Description.  */
        char *desc;

        /* FIB identifier.  */
        uint8_t fib_id;
// ...
        uint32_t table_id;

        /* Routing table.  */
        struct route_table *table[AFI_MAX][SAFI_MAX];

        /* Recursive Nexthop table */
        struct route_table *rnh_table[AFI_MAX];
        struct route_table *rnh_table_multicast[AFI_MAX];

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struct route_node;
struct route_table;
// ...
struct route_table {
        struct route_node *top;
        struct rn_hash_node_head hash;

        /*
         * Delegate that performs certain functions for this table.
         */
        route_table_delegate_t *delegate;
        void (*cleanup)(struct route_table *, struct route_node *);

        unsigned long count;
        void *info;
};

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#define ROUTE_NODE_FIELDS                                                      \
        /* Actual prefix of this radix. */                                     \
        struct prefix p;                                                       \
                                                                               \
        /* Tree link. */                                                       \
        struct route_table *table_rdonly(table);                               \
        struct route_node *table_rdonly(parent);                               \
        struct route_node *table_rdonly(link[2]);                              \
                                                                               \
        /* Lock of this radix */                                               \
        unsigned int table_rdonly(lock);                                       \
                                                                               \
        struct rn_hash_node_item nodehash;                                     \
        /* Each node of route. */                                              \
        void *info;                                                            \


/* Each routing entry. */
struct route_node {
        ROUTE_NODE_FIELDS

#define l_left   link[0]
#define l_right  link[1]
};

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static inline rib_dest_t *rib_dest_from_rnode(struct route_node *rn)
{
    return (rib_dest_t *)rn->info;
}

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// l.198 in zebra/rib.h
typedef struct rib_dest_t_ {
    struct route_node *rnode;
    struct re_list_head routes;
    struct route_entry *selected_fib;
    struct rnh_list_head nht;
} rib_dest_t;
DECLARE_LIST(rnh_list, struct rnh, rnh_list_item);
DECLARE_LIST(re_list, struct route_entry, next);

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int rib_add_multipath_nhe(afi_t afi, safi_t safi, struct prefix *p,
                          struct prefix_ipv6 *src_p, struct route_entry *re,
                          struct nhg_hash_entry *re_nhe, bool startup)
{
    // ...
    rn = srcdest_rnode_get(table, p, src_p);
    // 同じルートノード(prefix, src_prefix)に紐づくroute_entryの数を確認したり
    RNODE_FOREACH_RE (rn, same) {
        if (rib_compare_routes(re, same)) {
            // ROUTE_ENTRY_REMOVEDなのをスキップ
            // 全く同じエントリであるか確認している
            if (rib_compare_routes(re, same)) {
                if (first_same == NULL) first_same = same;
            }
        }
    }

    // kernel/connectedを特別扱いするコード
    SET_FLAG(re->status, ROUTE_ENTRY_CHANGED);
    rib_addnode(rn, re, 1); // 中でrib_link()を呼び出す
}

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static void rib_link(struct route_node *rn, struct route_entry *re, int process)
{
    
    dest = rib_dest_from_rnode(rn);
    if (!dest) {
        // route_nodeに向き先: route_entryのリストがなかったら作成する
        dest = zebra_rib_create_dest(rn);
    }
    // 今回の route_entryを登録する
    re_list_add_head(&dest->routes, re);

    afi = (rn->p.family == AF_INET)
          ? AFI_IP
          : (rn->p.family == AF_INET6) ? AFI_IP6 : AFI_MAX;
    if (is_zebra_import_table_enabled(afi, re->vrf_id, re->table)) {
    } else if (process)
        rib_queue_add(rn);

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static void rib_process(struct route_node *rn)
{
    // VRFを特定したり優先度を特定する
    best = rib_choose_best(new_fib, re)
    if (new_fib && best != new_fib) {
        new_fib = best
    }
    // 
    if (new_fib && old_fib)
        rib_process_update_fib(zvrf, rn, old_fib, new_fib);
    else if (new_fib)
        rib_process_add_fib(zvrf, rn, new_fib);
    else if (old_fib)
        rib_process_del_fib(zvrf, rn, old_fib);
}

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static void dplane_thread_loop(struct thread *event)
{
    // 最初のproviderのためにwork_listを準備している
    TAILQ_INIT(&work_list);
    TAILQ_INIT(&error_list);
    error_counter = 0;

    DPLANE_LOCK(); // DPLANEの操作受付を一旦止める
    prov = TAILQ_FIRST(&zdplane_info.dg_providers_q);

    // 最大で limit 数の操作の塊(ctx)を上で取得したproviderに割り当ててwork_listに登録する
    for (counter = 0; counter < limit; counter++) {
        ctx = TAILQ_FIRST(&zdplane_info.dg_update_ctx_q);
        if (ctx) {
            TAILQ_REMOVE(&zdplane_info.dg_update_ctx_q, ctx,
                         zd_q_entries);

            ctx->zd_provider = prov->dp_id;

            TAILQ_INSERT_TAIL(&work_list, ctx, zd_q_entries);
        } else {
            break;
        }
    }
    DPLANE_UNLOCK(); // DPLANEへの操作要求を受け付け開始
    while (prov) {
        TAILQ_FOREACH_SAFE(ctx, &work_list, zd_q_entries, tctx) {
            if (dplane_ctx_get_status(ctx) ==
                ZEBRA_DPLANE_REQUEST_SUCCESS) {
                ctx->zd_provider = prov->dp_id;
            } else {
                // エラーなら扱うべきじゃない
                TAILQ_REMOVE(&work_list, ctx, zd_q_entries);
                TAILQ_INSERT_TAIL(&error_list,
                                  ctx, zd_q_entries);
                error_counter++;
            }
        }

        dplane_provider_lock(prov);
        // providermにwork_listを登録する
        if (TAILQ_FIRST(&work_list))
            TAILQ_CONCAT(&(prov->dp_ctx_in_q), &work_list,
                         zd_q_entries);
        // 色々やって
        (*prov->dp_fp)(prov); // (*provider->dp_fp)はkernel_dplane_process_func

        // 後処理 ...
        // 次のprovのためにwork_listを準備
        prov = TAILQ_NEXT(prov, dp_prov_link); // provに次のproviderを入れる
    }
    // 処理後のcallbackを登録している (失敗, 成功)
    (zdplane_info.dg_results_cb)(&error_list);
    TAILQ_INIT(&error_list);
    (zdplane_info.dg_results_cb)(&work_list);
    TAILQ_INIT(&work_list);

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nl = kernel_netlink_nlsock_lookup(dp_info->sock);
n->nlmsg_seq = dp_info->seq;
n->nlmsg_pid = nl->snl.nl_pid;
if (netlink_send_msg(nl, n, n->nlmsg_len) == -1)
    return -1;

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vty_read() {
 	// 入力処理
 	switch {
 	case '\n':
 		// まずvty->wfdに書き込みバッファをフラッシュしてから受け取ったコマンドを実行する
 		buffer_flush_available(vty->obuf, vty->wfd);
 		vty_execute(vty);
 	}
 }

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void static_vty_init(void)
{
        install_node(&debug_node);

        install_element(CONFIG_NODE, &ip_mroute_dist_cmd);

        install_element(CONFIG_NODE, &ip_route_blackhole_cmd);
        install_element(VRF_NODE, &ip_route_blackhole_vrf_cmd);
        install_element(CONFIG_NODE, &ip_route_address_interface_cmd);
        install_element(VRF_NODE, &ip_route_address_interface_vrf_cmd);
        install_element(CONFIG_NODE, &ip_route_cmd);
        install_element(VRF_NODE, &ip_route_vrf_cmd);

        install_element(CONFIG_NODE, &ipv6_route_blackhole_cmd);
        install_element(VRF_NODE, &ipv6_route_blackhole_vrf_cmd);
// ....
}