sd_bus_message_read_basic man page
sd_bus_message_read_basic — Read a basic type from a message
int sd_bus_message_read_basic(sd_bus_message *m, char type, void *p);
sd_bus_message_read_basic() reads a basic type from a message and advances the read position in the message. The set of basic types and their ascii codes passed in type are described in the D-Bus Specification.
If p is not NULL, it should contain a pointer to an appropriate object. For example, if type is 'y', the object passed in p should have type uint8_t *. If type is 's', the object passed in p should have type const char **. Note that, if the basic type is a pointer (e.g., const char * in the case of a string), the pointer is only borrowed and the contents must be copied if they are to be used after the end of the messages lifetime. Similarly, during the lifetime of such a pointer, the message must not be modified.
If there is no object of the specified type at the current position in the message, an error is returned.
On success, sd_bus_message_read_basic() returns 0 or a positive integer. On failure, it returns a negative errno-style error code.