Read Hex from a file and convert 2 Bytes into Signed int - c

Question

I have a file in this format:
F2,80,FF,CF,0F,00,A2,XXXX,XXXX,XXXX,01FE,00
I need to take bytes 3 and 4 and combine them into a signed integer.
For example I should extract FF and CF and combine them to 0xFFCF. This should give me a signed value of -49.
The code that I have is here:
int main()
{
char buffer[1024] ;
char *record,*line;
uint8_t val;
uint8_t msb, lsb;
int16_t rate;
int i=0,j=0;
int mat[100][100];
FILE *fstream = fopen("log1.txt","r");
if(fstream == NULL)
{
printf("\n file opening failed ");
return -1 ;
}
while((line=fgets(buffer,sizeof(buffer),fstream))!=NULL)
{
record = strtok(line,",");
int count = 0;
while(record != NULL)
{
count++;
if (count == 3)
{
printf("string:%s\n", record);
sscanf(record, "%02X", &msb);
printf("MSB: %01X\n",msb) ;
}
if (count == 4)
{
printf("string:%s\n", record);
sscanf(record, "%02X", &lsb);
printf("lsb: %01X\n",lsb);
}
if (count == 5)
{
int16_t value = (short)(((msb) & 0xFF) << 8 | (lsb) & 0xFF);
printf("rate: %.2f\n", value*0.03125);
getchar();
}
record = strtok(NULL,",");
}
++i ;
}
return 0;
}
The exact output I see from my code is:
string:FF
MSB: FF
string:CD
lsb: CD
HEX: 00CD
rate: 6.41
I would expect rate to come out as: -1.59
I never seem to see negative numbers, and the values I get are too small.

Answer 1

Rather than using different variable types to try and get the behaviour that you want, how about just being explicit about it? Like so:
#include <stdio.h>
int main(int argc, char *argv[])
{
int msb = 0xff;
int lsb = 0xcf;
int value = (((msb & 0xff) << 8) | (lsb & 0xff));
if (value >= 0x8000) value = -(0x10000 - value);
printf("%d\n", value);
return 0;
}

Answer 2

Here is how I got the code to work:
int16_t hexToInt(char* msb, char* lsb)
{
char toparse[50];
strcpy(toparse, msb);
strcat(toparse,lsb);
int16_t number = (int16_t)strtol(toparse, NULL, 16);
return number;
}
int main()
{
char buffer[1024] ;
char *record,*line;
uint8_t val;
char msb[16], lsb[16];
int16_t rate;
FILE *fstream = fopen("log1.txt","r");
if(fstream == NULL)
{
printf("\n file opening failed ");
return -1 ;
}
while((line=fgets(buffer,sizeof(buffer),fstream))!=NULL)
{
record = strtok(line,",");
int count = 0;
while(record != NULL)
{
count++;
if (count == 3)
{
printf("string:%s\n", record);
strcpy(msb, record);
}
if (count == 4)
{
printf("string:%s\n", record);
strcpy(lsb,record);
}
if (count == 5)
{
int16_t value = hexToInt(msb,lsb);
printf("rate: %.2f\n", value*0.03125);
getchar();
}
record = strtok(NULL,",");
}
++i ;
}
return 0;
}

Answer 3

The value is indeed 0xFFCF, but when multiplying with 0.03125 it is promoted to higher datatype because of which value looses its signedness.
Just change
printf("rate: %.2f\n", value*0.03125);
to
printf("rate: %.2f\n", ((short)value*0.03125));
With value being -49 or 0xFFCF the output will be
rate: -1.53

Answer 4

Code has undefined behavior. OP's code attempts to save an int sized result into 1-byte locations. A good compiler or one with warnings enabled would warn of this issue.
uint8_t msb, lsb;
...
sscanf(record, "%02X", &msb);
sscanf(record, "%02X", &lsb);
Code could use the correct scanf() specifier
#include <inttypes.h>
sscanf(record, "%02" SCNx8, &msb);
sscanf(record, "%02" SCNx8, &lsb);
or simply a different type
unsigned msb, lsb;
...
sscanf(record, "%02X", &msb);
sscanf(record, "%02X", &lsb);
OP's conversion is suspect:
uint8_t msb, lsb;
int16_t value = (short)(((msb) & 0xFF) << 8 | (lsb) & 0xFF);
printf("rate: %.2f\n", value*0.03125);
Suggest something like #John Bickers The following works even if int is 16-bit.
long value = msb;
value <<= 8;
value += lsb;
if (value >= 0x8000) value -= 0x10000;
printf("value %ld\n", value);
// -49 * 0.03125 --> "-1.53"
printf("rate: %.2f\n", value * 0.03125);
Since OP expects a scaled out of -15.6 from -49, perhaps scaling by 1/pi is needed rather than * 0.03125?