Why my variadic vsscanf function has undefined behaviour? - c

Question

I created a variadic function to parse strings which might have different arguments.
In current case, the incoming string would be "38400,8,N,1\n".
static int parse_response(const char *buff, char *format, ...){
uint16_t retval=0;
va_list arg_ptr;
va_start(arg_ptr, format);
retval = vsscanf((const char*)buff, format, arg_ptr);
va_end(arg_ptr);
return retval;
}
here is the function where I called my vsscanf function.
satel_return_typedef Satel_GetBaudrate(satel_typedef* Satel, uint32_t* Baudrate, uint32_t timeout_ms){
int Len;
Len = printf_SLCommandString(Satel->tx_data_buffer, Satel->max_size, "SL%%B?");
uint8_t DataBits; char Parity; uint8_t StopBit;
if(Len < 0 || Len > (int)Satel->max_size) {
return R_API_ERROR;
}
if(Satel->Platform->Comm_Send(Satel->tx_data_buffer, Len, NULL)<0)
return R_PLATFORM_ERROR;
if(Satel_WaitForCommResponse(Satel,timeout_ms)==R_COMM_RESPONSE_VALUE){
parse_response((char*)Satel->rx_data_buffer,"%d,%d,%c,%d\n", Baudrate, &DataBits, &Parity, &StopBit);
return R_OP_SUCCESSED;
}
return R_OP_ERROR;
}
I know, there are some rules to not fall into undefined behaviour but, My mind confused about which rules are breaking here.
typedef void (*Write_Enamod_Pin)(satel_power_typedef Config);
typedef void (*Write_Service_Pin)(satel_service_typedef Config);
typedef int32_t (*Send_Message)(const uint8_t *data_to_send, uint32_t bytes_to_send, void *custom); /* -1 IO Error*/
typedef int32_t (*Read_Message)(uint8_t *data_to_read, uint32_t bytes_to_read, void *custom); /* -1 IO Error*/
typedef uint32_t (*Get_TickCount)(void);
typedef void (*Delay_ms)(uint32_t milisec);
typedef struct{
Write_Enamod_Pin Power;
Write_Service_Pin Service;
Send_Message Comm_Send;
Read_Message Comm_Read;
Get_TickCount Get_TickCount;
Delay_ms Delay_Ms;
}satel_hw_typedef;
typedef struct{
satel_state_typedef State;
uint8_t* tx_data_buffer;
uint8_t* rx_data_buffer;
uint32_t max_size;
satel_hw_typedef* Platform;
}satel_typedef;

Answer 1

You've got a mismatch of format specifiers.
The %d format specifier expect an int *. In the 3 places you're using it, you're passing a uint32_t *, a uint8_t *, and a uint8_t *. Those mismatches trigger undefined behavior.
You need to use u for an unsigned integer argument for all 3, and you want to use the hh modifier for the uint8_t (i.e. unsigned char) arguments. Also, you want to remove \n from the format string as that prevents reading from stopping until you press some other non-whitespace character after pressing ENTER.
"%u,%hhu,%c,%hhu"