Thread: problem with passing value of a function

Hello

I'm having a problem with this code I'm stumped as to why the function Get_Distance() is returning the wrong value back to the caller. When I run a test in my IDE I watch the values of each variable at breakpoints. When executed distance is 1391.261 which is correct. When the value is returned back to the caller it turns into -6057.000 and assigns -6057.000 to distance2pet and that is what is displayed on my LCD. If I make distance2Pet a global and just make everything 1 big source file then it works fine, but I have a lot more code to do and don't want 5,000 lines of code in 1 file. Thank you for any input!

File 1 mainDemo.C

Code:

#include <p24FJ128GA010.h>
#include <stdio.h>



_CONFIG1(JTAGEN_OFF & GCP_OFF & GWRP_OFF & BKBUG_OFF & ICS_PGx2 & FWDTEN_OFF)
_CONFIG2(IESO_OFF & FCKSM_CSDCMD & OSCIOFNC_ON &  FNOSC_FRC & POSCMOD_NONE)


char  distance2PetString[15];

main ()
{
     float distance2Pet;  //distance from pet to base
	
     LCDInit(); //initialize LCD

     LCDClear(); //clear the LCD
	
     distance2Pet =  Get_Distance(41.0344, 85.1662, 41.0444, 85.1762);

     sprintf(distance2PetString, "%.3f", distance2Pet);
	
     while(1) {
			
         displayPetDistance();

     } //end while

} //end main

displayPetDistance()
{
     int d;

     LCDL1Home();

     for(d = 0; d<15; d++){

     LCDPut(distance2PetString[d]);
     }
}

File 2
gps_read.c

Code:

#include <p24fj128ga010.h>
#include <math.h>
#include "gps_read.h"

#define EarthRadius	        (6378100)      // meters
#define Degrees_2_Radians	(0.017453293)  // pi/180
#define Radians_2_Degrees	(57.29577951)  // 180/pi

 float Get_Distance( float lat1,  float long1,  float lat2,  float long2)
 {
	
	float dLat, dLong, sideA, sideC;
	
	lat1  *= Degrees_2_Radians;  // 0.716185  value of variable at this point
	long1 *= Degrees_2_Radians;   // 1.48643
	lat2  *= Degrees_2_Radians;   // 0.71636
	long2 *= Degrees_2_Radians; // 1.48661
	
	dLat  = lat2  - lat1;   // 0.000175
	dLong = long2 - long1;  // 0.00018
	
	sideA = (sinf(dLat/2)) * (sinf(dLat/2)); // 7.65625e-9

	sideA = sideA + ((cosf(lat1)) * (cosf(lat2)) * (sinf(dLong/2)) * (sinf(dLong/2))); 

	sideC = 2 * atan2f( sqrt(sideA), sqrt(1-sideA) ); 

	distance = (EarthRadius*sideC); // meters 1391.261
	
	return distance;
}

header file gps_read.h

Code:

float distance;

Are you including the header file that has the prototype for your Get_Distance function, or does p24FJ128GA010.h take care of this?

I see nothing wrong with your code. I even tested what you were doing on a small scale and had no issues.

no I didn't include the gps_read.h in the main source I'll try that. I have made distance a local variable in gps_read.c and then tried it as a global in gps_read.c and deleted the header file and still got the same results.

The p24fj128ga010.h configures all the registers and things inside the microcontroller.

What really stumps me is that I can make distance an extern variable type and the program will work with 2 source files, or I can put all the code in 1 file and make it work correctly.

I tried including gps_read.h in the main but no go. I'm trying to stay away from extern if i can but I figured I would give it a try just to do some troubleshooting. Maybe it's a compiler issue, I've just never had a value "change" when it is returned to the calling function.

As many suggest, staying away from extern is ideal. Instead of using a public variable, create a pointer and access it's address when you need to read/write to it. This example shows how to using an int, but you can do the same with a float or double.

Example:
main.c

Code:

#include <stdio.h>
#include "other.h"

int main (void)
{
    int mynum = 0;

    modifyvar(&mynum);

    printf("%d\n", mynum);
    return 0;
}

other.h

Code:

void modifyvar(int *myvar);

other.c

Code:

#include "other.h"

void modifyvar(int *myvar)
{
    *myvar = 5;
}

I've tried every combination of pointer that I could think of and couldn't get anywhere. I have made other functions where a value is passed back to the caller and assigned to a local variable in that function, it is just frustrating why this one isn't working.