Implement an strerror-like helper function

Returns a human-redable error string.
Signed-off-by: David Wagner <david.wagner@easymile.com>

src/MQTTAsync.c

@@ -3329,3 +3329,42 @@ MQTTAsync_nameValue* MQTTAsync_getVersionInfo(void)
 	libinfo[i].value = NULL;
 	return libinfo;
 }
+
+
+const char* MQTTAsync_strerror(int code)
+{
+  switch (code) {
+    case MQTTASYNC_SUCCESS:
+      return "Success";
+    case MQTTASYNC_FAILURE:
+      return "Failure";
+    case MQTTASYNC_PERSISTENCE_ERROR:
+      return "Persistence error";
+    case MQTTASYNC_DISCONNECTED:
+      return "Disconnected";
+    case MQTTASYNC_MAX_MESSAGES_INFLIGHT:
+      return "Maximum in-flight messages amount reached";
+    case MQTTASYNC_BAD_UTF8_STRING:
+      return "Invalid UTF8 string";
+    case MQTTASYNC_NULL_PARAMETER:
+      return "Invalid (NULL) parameter";
+    case MQTTASYNC_TOPICNAME_TRUNCATED:
+      return "Topic containing NULL characters has been truncated";
+    case MQTTASYNC_BAD_STRUCTURE:
+      return "Bad structure";
+    case MQTTASYNC_BAD_QOS:
+      return "Invalid QoS value";
+    case MQTTASYNC_NO_MORE_MSGIDS:
+      return "Too many pending commands";
+    case MQTTASYNC_OPERATION_INCOMPLETE:
+      return "Operation discarded before completion";
+    case MQTTASYNC_MAX_BUFFERED_MESSAGES:
+      return "No more messages can be buffered";
+    case MQTTASYNC_SSL_NOT_SUPPORTED:
+      return "SSL is not supported";
+    case MQTTASYNC_BAD_PROTOCOL:
+      return "Invalid protocole scheme";
+  }
+
+  return NULL;
+}

src/MQTTAsync.h

@@ -1187,6 +1187,13 @@ typedef struct
   */
 DLLExport MQTTAsync_nameValue* MQTTAsync_getVersionInfo(void);
 
+/**
+ * Returns a pointer to the string representation of the error or NULL.
+ *
+ * Do not free after use. Returns NULL if the error code is unknown.
+ */
+DLLExport const char* MQTTAsync_strerror(int code);
+
 
 /**
   * @cond MQTTAsync_main

src/MQTTClient.c

@@ -2098,6 +2098,37 @@ MQTTClient_nameValue* MQTTClient_getVersionInfo(void)
 }
 
 
+const char* MQTTClient_strerror(int code)
+{
+  switch (code) {
+    case MQTTCLIENT_SUCCESS:
+      return "Success";
+    case MQTTCLIENT_FAILURE:
+      return "Failure";
+    case MQTTCLIENT_DISCONNECTED:
+      return "Disconnected";
+    case MQTTCLIENT_MAX_MESSAGES_INFLIGHT:
+      return "Maximum in-flight messages amount reached";
+    case MQTTCLIENT_BAD_UTF8_STRING:
+      return "Invalid UTF8 string";
+    case MQTTCLIENT_NULL_PARAMETER:
+      return "Invalid (NULL) parameter";
+    case MQTTCLIENT_TOPICNAME_TRUNCATED:
+      return "Topic containing NULL characters has been truncated";
+    case MQTTCLIENT_BAD_STRUCTURE:
+      return "Bad structure";
+    case MQTTCLIENT_BAD_QOS:
+      return "Invalid QoS value";
+    case MQTTCLIENT_SSL_NOT_SUPPORTED:
+      return "SSL is not supported";
+    case MQTTCLIENT_BAD_PROTOCOL:
+      return "Invalid protocole scheme";
+  }
+
+  return NULL;
+}
+
+
 /**
  * See if any pending writes have been completed, and cleanup if so.
  * Cleaning up means removing any publication data that was stored because the write did

src/MQTTClient.h

@@ -993,6 +993,13 @@ DLLExport void MQTTClient_free(void* ptr);
   */
 DLLExport void MQTTClient_destroy(MQTTClient* handle);
 
+/**
+ * Returns a pointer to the string representation of the error or NULL.
+ *
+ * Do not free after use. Returns NULL if the error code is unknown.
+ */
+DLLExport const char* MQTTClient_strerror(int code);
+
 #endif
 #ifdef __cplusplus
      }