I am trying to communicate with a usb dongle via serial communication. My communication works, however I cant get the device to correctly parse the communication. My devices reads the message and compares it with a hardcoded c-string. It parses and recognizes that it's the correct string, but when I try to parse the value after the : character, it returns 0x00000000 and I have no idea why. I've tried using char cast and use atoi, I tried using a simple ascii translation, and even doing a bitwise addition operation as shown here: convert subset of vector<uint8_t> to int
For example:
I send "Heart Rate:55"
It parses and recognizes that "Heart Rate:" but when I tell it to go find the 55 and bring it back to do stuff with it, it gives me a 0x00000000
Heres a snippet:
const uint8_t hrmSet[] = "Heart Rate:";
/** Find the : character in the string and break it apart to find if it matches,
and determine the value of the value of the desired heart rate. **/
int parse(uint8_t *input, uint8_t size)
{
for (uint8_t i = 0; i < size; i++)
{
if (input[i] == ':')
{
if (compare_string(input, hrmSet, i) == 0)
{
int val = 0;
for (int j = i+1; j < size; j++)
{
if (!isdigit(input[j]))
{
for (int k = i; k < j; k++)
{
val <<= 8;
val |= input[k];
}
}
}
return val;
}
return -1;
}
}
return -1;
}
Compare string function
/** Compare the input with the const values byte by byte to determine if they are equal.**/
int compare_string(uint8_t *first, const uint8_t *second, int total)
{
for (int i = 0; i < total; i++)
{
if (*first != *second)
{
break;
}
if (*first == '\0' || *second == '\0')
{
break;
}
first++;
second++;
}
if (*first == ':' && *second == ':')
{
return 0;
}
else
{
return -1;
}
}
The problem here is that you are using nested loops to perform tasks that should be done with sequential loops.
For example, the i
loop searches for the colon, and then the loop in compare_string
searches for the colon again. You could run the i
loop first, and then call compare_string
after the i
loop finishes. But a better design is to have compare_string
search for the colon while comparing, and then return the index of the character after the colon (or -1 if the colon is not found).
The same is true of the j
and k
nested loops. The j
loop is searching for the end of the number. The k
loop only runs once after the j
loop is finished, and so the k
loop should be after the j
loop, not nested. But a better design is a single loop that converts the number while searching for the end of the number.
The code below demonstrates one possible implementation using the techniques I've described.
const uint8_t hrmSet[] = "Heart Rate:";
int compare_string( uint8_t *input, const uint8_t *expected, int size )
{
for ( int i = 0; i < size; i++ )
{
if ( *input != *expected || *expected == '\0' )
return( -1 );
if ( *input == ':' && *expected == ':' )
return( i + 1 );
input++;
expected++;
}
return( -1 );
}
int parse( uint8_t *input, uint8_t size )
{
int i, val;
if ( (i = compare_string( input, hrmSet, size )) < 0 )
return( -1 );
val = 0;
for ( ; i < size && isdigit( input[i] ); i++ )
val = val * 10 + input[i] - '0';
return( val );
}
int main( void )
{
uint8_t input[] = "Heart Rate:75";
int rate = parse( input, sizeof(input) - 1 );
printf( "%d\n", rate );
}