basic/errno-list: add helper to print errno names

src/basic/errno-list.c

@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include "errno-list.h"
@@ -33,7 +35,6 @@ const char* errno_to_name(int id) {
 #  include "errno-to-name.inc"
 
 const char* errno_to_name(int id) {
-
         if (id < 0)
                 id = -id;
 
@@ -43,3 +44,11 @@ const char* errno_to_name(int id) {
         return errno_names[id];
 }
 #endif
+
+const char* errno_name_full(int id, char buf[static ERRNO_NAME_BUF_LEN]) {
+        const char *a = errno_to_name(id);
+        if (a)
+                return a;
+        snprintf(buf, ERRNO_NAME_BUF_LEN, "%d", abs(id));
+        return buf;
+}

src/basic/errno-list.h

@@ -9,3 +9,10 @@ int errno_from_name(const char *name) _pure_;
 static inline bool errno_is_valid(int n) {
         return n > 0 && n <= ERRNO_MAX;
 }
+
+#define ERRNO_NAME_BUF_LEN DECIMAL_STR_MAX(int)
+/* Like errno_name, but always returns a string. */
+const char* errno_name_full(int id, char buf[static ERRNO_NAME_BUF_LEN]);
+
+/* A helper to print the errno "name" or number if name is not defined. */
+#define ERRNO_NAME_FULL(errnum) errno_name_full(errnum, (char[ERRNO_NAME_BUF_LEN]){})