Pass structure pointer as an argument to a function in c

Question

I am developing a code in which I need to pass a structure pointer value to a function as a parameter. I am calling function "proto485Compare()" in "RS485TaskSlave()". "rs485Msg" contains my data for which I need to verify for checksum, I am checking it in "RS485TaskSlave()" using "if(proto485ValidCheckSum(rs485_message_t ((uint8_t*)message)))" but it is showing error on this line.

Please help me to get the values of "rs485Msg" & check for checksum on this line

Below is my code snippet:

 typedef struct
 {
 uint8_t AB; ///< Address byte (Start byte)
 uint8_t CB; ///< Command (index) "ASCII-byte"
 uint8_t SI; ///< Action (sub index) "ASCII-byte"
 uint8_t PH1; ///< Payload "ASCII-byte" of the high nibble of the high byte of the raw payload
 uint8_t PH0; ///< Payload "ASCII-byte" of the low nibble of the high byte of the raw payload
 uint8_t PL1; ///< Payload "ASCII-byte" of the high nibble of the low byte of the raw payload
 uint8_t PL0; ///< Payload "ASCII-byte" of the low nibble of the low byte of the raw payload
 uint8_t CS1; ///< Checksum "ASCII-byte" of the high nibble of the raw checksum
 uint8_t CS0; ///< Checksum "ASCII-byte" of the low nibble of the raw checksum
 } rs485_message_t;
 uint8_t proto485ValidCheckSum(rs485_message_t message)
 { 
 uint8_t rawChecksum = (message.CB - OFFSET_ASCII) + (message.SI - OFFSET_ASCII) +
 (message.PH1 - OFFSET_ASCII) + (message.PH0 - OFFSET_ASCII) +
 (message.PL1 - OFFSET_ASCII) + (message.PL0 - OFFSET_ASCII);
 uint8_t res = (((rawChecksum & 0xF0) >> 4) + OFFSET_ASCII) == message.CS1 &&
 ((rawChecksum & 0x0F) + OFFSET_ASCII) == message.CS0;
 return res;
 }
 void RS485TaskSlave(void *p_arg) //b - RS485 communication Task
 {
 uint8_t res; 
 rs485_message_t rs485Msg;
 uint8_t command, action, value;
 (void)p_arg;
 while(1)
 { 
 res = driver485Read((uint8_t *)&rs485Msg, RS485_MSG_LENGTH); 
 if(RS485_MSG_LENGTH == 9)
 {
 res = proto485Compare((uint8_t *)&rs485Msg, command, action, value); 
 }
 } 
 }
 int16_t proto485Compare(uint8_t* message, uint8_t* comparec, uint8_t* comparea, uint8_t* comaprev)
 { int j = 0;
 rs485_message_t rs485Msg;
 char res; 
 uint8_t FWmsg[9] = {0x09,0x31,0x30,0x30,0x30,0x30,0x33,0x30,0x34}; 
 uint8_t arduinodata[9] = {0x09,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30}; 
 if(proto485ValidCheckSum(rs485_message_t ((uint8_t*)message))) //
/*GETTING ERRORVON THIS LINE please help how to pass message value to verify checksum at this line*/
 {
 printf("compare command..........");
 for (j=0; j<9; j++)
 {
 arduinodata[j] = message[j];
 printf("%d ",arduinodata[j]);
 }
 printf("\n");
 if(compareArray(FWmsg,arduinodata,7)==0)
 { 
 uint8_t add, fwc, fwa;
 uint16_t fwv; 
 res = GetABFWversion(&add, &fwc, &fwa, &fwv);
 res = proto485OnlyWrite(add,fwc, fwa, fwv);
 }
 }
 else
 {
 printf("Arrays have different elements.\n");
 } 
 return res;
 }

Answer 1

Remove your casting, and change your message parameter to be rs485_message_t *:

int16_t proto485Compare(rs485_message_t * message, uint8_t* comparec, uint8_t* comparea, uint8_t* comaprev) {
 int j = 0;
 rs485_message_t rs485Msg;
 char res; 
 uint8_t FWmsg[9] = {0x09,0x31,0x30,0x30,0x30,0x30,0x33,0x30,0x34}; 
 uint8_t arduinodata[9] = {0x09,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30}; 
 if(proto485ValidCheckSum(message))) { ...

And when you call it, simply:

res = proto485Compare(&rs485Msg, command, action, value); 

To get at the individual bytes you can then cast a new variable:

uint8_t *messageAsBytes = (uint8_t *)message;

Then you can use:

messageAsBytes[i]

etc.

However another cleaner way is to create a union in your structure:

struct rs485_message_t {
 union {
 uint8_t bytes[9];
 struct {
 uint8_t AB; ///< Address byte (Start byte)
 uint8_t CB; ///< Command (index) "ASCII-byte"
 uint8_t SI; ///< Action (sub index) "ASCII-byte"
 uint8_t PH1; ///< Payload "ASCII-byte" of the high nibble of the high byte of the raw payload
 uint8_t PH0; ///< Payload "ASCII-byte" of the low nibble of the high byte of the raw payload
 uint8_t PL1; ///< Payload "ASCII-byte" of the high nibble of the low byte of the raw payload
 uint8_t PL0; ///< Payload "ASCII-byte" of the low nibble of the low byte of the raw payload
 uint8_t CS1; ///< Checksum "ASCII-byte" of the high nibble of the raw checksum
 uint8_t CS0; ///< Checksum "ASCII-byte" of the low nibble of the raw checksum
 };
 };
}; 

So you can now access message->CS0, etc, as well as message->bytes[3].

You should also pass a pointer when you call proto485ValidCheckSum - purely to save memory.

uint8_t proto485ValidCheckSum(rs485_message_t *message)
{ 
 uint8_t rawChecksum = (message->CB - OFFSET_ASCII) + (message->SI - OFFSET_ASCII) +
 (message->PH1 - OFFSET_ASCII) + (message->PH0 - OFFSET_ASCII) +
 (message->PL1 - OFFSET_ASCII) + (message->PL0 - OFFSET_ASCII);
 uint8_t res = (((rawChecksum & 0xF0) >> 4) + OFFSET_ASCII) == message->CS1 &&
 ((rawChecksum & 0x0F) + OFFSET_ASCII) == message->CS0;
 return res;
}

And again call it with a pointer, which the first block of code does anyway.

In general, it's better to pass a single pointer to a struct around from function to function instead of cloning it each time.

You can also use "pass by reference" if you like, but I find it's better to stick to pointers when passing something around alot like this.

• swanand: Thank you for reply, I have made above mentioned changed & run the code it is showing below errors.... 1) on line "if(proto485ValidCheckSum(rs485_message_t ((uint8_t*)message)))" it gives "type name is not allowed" and on line " arduinodata[j] = message[j];" it shows that "a value of type "rs485_message_t" cannot be assigned to an entity of tpe "uint8_t" "
• Majenko: @swanand Read my first code block and compare it to yours.
• swanand: Sorry my bad, i have changed for message element reading it is working fine but I am still getting error on line " if(proto485ValidCheckSum(rs485_message_t ((uint8_t*)message)))" t gives "type name is not allowed" Please inform If I am doing something wrong....
• Majenko: @swanand Read my first code block and compare it to yours.
• swanand: Thank you so much once again for providing guidance......