Thread: function returns weird value

I'm having trouble getting a function to work right. it's been bugging me badly.. I have a crystalfontz 633 lcd display connected to the serial port with a dallas one wire temperature sensor connected to the display. it sends a temperature report back to the computer once every second. the ConvertTempReport function is used to decode the sensor's output and return it as degrees fahrenheit. if I use printf inside the function, it works perfectly. but I want it to return the temperature as the functions output. when I do that, it gives me weird numbers. I can't figure out why it's doing that or what I'm doing wrong. someone please help. I'm running centos 5 server.

Code:

while(1){
  usleep(100000);  // 1/10 second
  if(check_for_packet()){
  //Terminate the incoming data so C handles it well in case it is a string.
  incoming_response.data[incoming_response.data_length]=0;

  double tempf;
  tempf=ConvertTempReport(incoming_response.data);
  printf("%f\n",tempf);

  }
}


double ConvertTempReport(ubyte data[4]){
  double temp;
  temp=((data[2] << 8) | data[1])/16.0;
  if(temp!=0){temp=(1.8 * temp)+32.0;}
  return temp;
}

this is the output:

14336.000000
12320.000000
10272.000000
8224.000000
6176.000000
4128.000000
19809.000000
17761.000000
32097.000000
30049.000000
28001.000000
25953.000000

Originally Posted by dwks

Did you prototype the function? If you didn't, the compiler would assume that the function returned an int, and as you're actually returning a double, that could mess things up.

What's a prototype? Just try putting this line before both code snippets you posted, globally, perhaps just after your #includes . . . .

Code:

double ConvertTempReport(ubyte data[4]);

sweet, thank you. I knew it was most likely something simple, I just needed an outside point of view. thanks :-)

Originally Posted by Elysia

OK, listen to warnings. This will definitely warn.
To be able to call a function without a prototype is just pure idiocy IMO. It should not be allowed.

it wasn't giving me any warnings. I thought of adding a prototype but didn't think it would cause the problem. now I have a better understanding of it all. I'm learning as fast as I can.

now, I'm trying to add a log file to my program. this is giving me some trouble now too. again, probably something simple. it creates the new file, but it's always empty.

Code:

FILE *f1=NULL;

int main(int argc, char* argv[]){

f1=fopen("temp.log","w");
if(f1==NULL){printf("Error opening temp.log for writing");}

fputs("test",f1);

fclose(f1);
}