> @@ -118,6 +123,10 @@ struct globals {
>  	int unix_sock;
>  	const char *unix_path;
>  
> +	const char *update_command;
> +	struct list_head changed_data_types;
> +	uint8_t changed_data_type_count;
> +

uint8_t is not enough to store the amount of changed data types. If
data_type 0-255 are changed then it would have a count of 256 - which
overflows changed_data_type_count and then ends up again as 0. This would
completely kill this functionality because the list will never change
again (it already has all types and can never remove any types) and
thus this value will also never change again.

> @@ -61,6 +61,7 @@ static void alfred_usage(void)
>  	printf("\n");
>  	printf("  -u, --unix-path [path]              path to unix socket used for client-server\n");
>  	printf("                                      communication (default: \""ALFRED_SOCK_PATH_DEFAULT"\")\n");
> +	printf("  -c, --update-command                path to command to call on data change\n");

command is not something with a path. So "path to command" doesn't sound
right. Maybe just drop the "path to" part. 

> +static void execute_update_command(struct globals *globals)
> +{
> +	pid_t script_pid;
> +	size_t command_len;
> +	char *command;
> +	struct changed_data_type *data_type, *is;
> +	/* data type is uint8_t, so 255 is maximum (3 chars)
> +	 * space for appending + NULL
> +	 */
> +	char buf[5]; 
> +
> +

Please remove the extra trailing whitespace after "buf[5];".

Please remove the extra (second) blank line.

> +	if (!globals->update_command || !globals->changed_data_type_count)
> +		return;
> +
> +	/* length of script + 4 bytes per data type (space +3 chars)
> +	 * + 1 for NULL
> +	 */

Can you change the NULL in the comment to "terminating null byte"?
NULL just looks like it has something to do with pointers.
(same in the comment above for buf)

> +	command_len = strlen(globals->update_command);
> +	command_len += 4 * globals->changed_data_type_count + 1;
> +	command = malloc(command_len);
> +	if (!command)
> +		return;
> +
> +	strncpy(command, globals->update_command, command_len - 1);
> +	list_for_each_entry_safe(data_type, is, &globals->changed_data_types,
> +				 list) {
> +		/* append the datatype to command line */
> +		snprintf(buf, sizeof(buf), " %d", data_type->data_type);
> +		buf[sizeof(buf)-1] = '\0';

Please add a space before and after the '-' operator.

> +
> +		strncat(command, buf, command_len - strlen(command) - 1);
> +
> +		/* clean the list */
> +		list_del(&data_type->list);
> +		free(data_type);
> +	}
> +	globals->changed_data_type_count = 0;
> +	command[command_len - 1] = '\0';

This \0 has to be moved directly after the strncpy to be useful.
The strncat is already expecting command to be \0 terminated.

Kind regards,
	Sven