sys/net/netif: add helpers for IPv6 addresses

- add `netif_get_ipv6()` to query IPv6 (as a slim convenience wrapper
  for `netif_get_opt()`)
- add `netifs_get_ipv6()` to query IPv6 addresses of all netifs
- add `netif_print_ipv6()` to print the IPv6 address(es) of a single
  netif
- add `netifs_print_ipv6()` to print the IPv6 address(es) of all netifs

sys/include/net/netif.h

@@ -35,9 +35,11 @@
 
 #include <stdint.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "list.h"
 #include "net/netopt.h"
+#include "net/ipv6.h"
 
 #ifdef MODULE_NETSTATS_NEIGHBOR
 #include "cib.h"
@@ -200,6 +202,65 @@ int netif_set_opt(netif_t *netif, netopt_t opt, uint16_t context,
  */
 int netif_register(netif_t *netif);
 
+/**
+ * @brief   Get IPv6 address(es) of the given interface
+ * @param[in]   netif       Interface to get the IPv6 address(es) from
+ * @param[out]  dest        Array of IPv6 addresses to write to
+ * @param[in]   numof       Size of @p dest in array elements (not in bytes!)
+ * @retval  -1              Failed
+ * @return      Number of addresses written to @p dest
+ */
+static inline ssize_t netif_get_ipv6(netif_t *netif, ipv6_addr_t *dest,
+                                     size_t numof)
+{
+    int res = netif_get_opt(netif, NETOPT_IPV6_ADDR, 0, dest, sizeof(*dest) * numof);
+    if (res < 0) {
+        /* standard says at ssize_t's value range is [-1, SSIZE_MAX], so do
+         * not rely on smaller numbers that -1 being passed through correctly */
+        return -1;
+    }
+
+    return res / sizeof(*dest);
+}
+
+/**
+ * @brief   Get IPv6 address(es) of **all** interfaces
+ * @param[out]  dest        Array of IPv6 addresses to write to
+ * @param[in]   numof       Size of @p dest in array elements (not in bytes!)
+ * @retval  -1              Failed
+ * @return      Number of addresses written to @p dest
+ */
+ssize_t netifs_get_ipv6(ipv6_addr_t *dest, size_t numof);
+
+/**
+ * @brief   Print the IPv6 address(es) of the given interface
+ * @param[in]   netif       Interface to print the IPv6 address(es) of
+ * @param[in]   separator   Separator to print between the IPv6 addresses
+ *
+ * Usage:
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.c}
+ *     // print IPv6 addrs of netif as JSON
+ *     printf("{\"IPv6 addresses\": [\"");
+ *     netif_print_ipv6(netif, "\", \"");
+ *     puts("\"]}");
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ */
+void netif_print_ipv6(netif_t *netif, const char *separator);
+
+/**
+ * @brief   Print the IPv6 address(es) of **all** interface
+ * @param[in]   separator   Separator to print between the IPv6 addresses
+ *
+ * Usage:
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.c}
+ *     // print all IPv6 addrs as JSON
+ *     printf("{\"IPv6 addresses\": [\"");
+ *     netifs_print_ipv6("\", \"");
+ *     puts("\"]}");
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ */
+void netifs_print_ipv6(const char *separator);
+
 #ifdef __cplusplus
 }
 #endif

sys/net/netif/netif.c

@@ -14,10 +14,12 @@
  */
 
 #include <assert.h>
+#include <errno.h>
+#include <stdio.h>
 #include <string.h>
 
-#include "errno.h"
 #include "irq.h"
+#include "kernel_defines.h"
 #include "net/netif.h"
 #include "utlist.h"
 
@@ -91,4 +93,52 @@ __attribute__((weak)) netif_t *netif_get_by_id(int16_t id)
     return NULL;
 }
 
+ssize_t netifs_get_ipv6(ipv6_addr_t *dest, size_t numof)
+{
+    ssize_t result = 0;
+    netif_t *netif = NULL;
+    while (((netif = netif_iter(netif)) != NULL) && (numof > 0)) {
+        ssize_t addrs_numof = netif_get_ipv6(netif, dest, numof);
+        if (addrs_numof <= 0) {
+            continue;
+        }
+        result += addrs_numof;
+        dest += addrs_numof;
+        numof -= addrs_numof;
+    }
+
+    return result;
+}
+
+#ifndef NETIF_PRINT_IPV6_NUMOF
+#define NETIF_PRINT_IPV6_NUMOF 4
+#endif
+
+void netif_print_ipv6(netif_t *netif, const char *separator)
+{
+    ipv6_addr_t addrs[NETIF_PRINT_IPV6_NUMOF];
+    ssize_t num = netif_get_ipv6(netif, addrs, ARRAY_SIZE(addrs));
+    if (num > 0) {
+        ipv6_addrs_print(addrs, num, separator);
+    }
+}
+
+void netifs_print_ipv6(const char *separator)
+{
+    netif_t *netif = 0;
+    bool first = true;
+    while ((netif = netif_iter(netif)) != NULL) {
+        ipv6_addr_t addrs[NETIF_PRINT_IPV6_NUMOF];
+        ssize_t num = netif_get_ipv6(netif, addrs, ARRAY_SIZE(addrs));
+        if (num > 0) {
+            if (first) {
+                first = false;
+            }
+            else {
+                printf("%s", separator);
+            }
+            ipv6_addrs_print(addrs, num, separator);
+        }
+    }
+}
 /** @} */