modbus_receive

Name

modbus_receive - receive an indication request

Synopsis

int modbus_receive(modbus_t *ctx, uint8_t *req);

Description

The modbus_receive() function shall receive an indication request from the socket of the context ctx. This function is used by a Modbus slave/server to receive and analyze indication request sent by the masters/clients.

If you need to use another socket or file descriptor than the one defined in the context ctx, see the function modbus_set_socket.

Return value

The function shall store the indication request in req and return the request length if successful. The returned request length can be zero if the indication request is ignored (eg. a query for another slave in RTU mode). Otherwise it shall return -1 and set errno.

Buffer size

The maximum size of the request depends on the used backend, in RTU the req array must be MODBUS_RTU_MAX_ADU_LENGTH bytes and in TCP it must be MODBUS_TCP_MAX_ADU_LENGTH bytes. If you want to write code compatible with both, you can use the constant MODBUS_MAX_ADU_LENGTH (maximum value of all libmodbus backends). Take care to allocate enough memory to store requests to avoid crashes of your server.

Example

uint8_t req[MODBUS_MAX_ADU_LENGTH];
rc = modbus_receive(ctx, req);