Confused as to how to pass back to main

As the title states I'm a little confused as to how I'd pass some of the values within a module back to main. Someone please help me ASAP!

Here is my main (it is incomplete) and a module within it.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#define    pi 3.146
#define    brakeFactor 0.7
double readVal (char *carModel,double *rSlope, double *sLength);
double calculateSpeed (double rSlope, double sLength);
double averageSpeed(double sLengthArray,double speedArray);
void dataSort(double sLengthArray,double rSlopeArray,double speedArray)
void listValues ()
int main ()
{
   char option;
   
   listValue (); /* calling module, not sure what to put for the arguments*/
   while(option != d)
   {   
    printf ("Please enter the letter of your chosen option.\n);
    printf("a. Display the historical data\nb. Find the historical data for a specific car.\nc. Display the average speed.\nd. END");
    scanf ("%c", &option);
    if (option == a)
    {
              dataSort(double sLengthArray,double rSlopeArray,double speedArray)
              printf ("|\n");
              printf ("|\tHISTORICAL DATA FROM SPEEDING CARS\n");
              printf ("|\tSkidlength\tRoadSlope\tSpeed\n");
              for (i=1; i<=count; i++)
              {
                printf("|\t%.3lf\t\t%0.lf\t\t%.2lf\n", sLengthArray[i], rSlopeArray[i], speedArray[i]);
              }
              printf ("|\n");
    }
    if (option == b)
    {
              
    }
    if (option == c)
    {
               double averageSpeed(sLengthArray, speedArray);
    }
    if (option == d)
    {
               printf("You have chosen to end the program");
               break;
    }
   }
    system ("pause");
    return 0;
}


listValue () /* Called module,need to return the the arrays and sum and count for use in main*/
{
    double slopeRad, slopeTangent, speedArray[200]={0}, sum=0, speed, sLength, *sLengthP, sLengthArray[200]={0},rSlope, *rSlopeP,rSlopeArray[200]={0}; 
    char carModel[1000], carModelArray[200][1000],*carModelP;
    int count=0, i; 
    carModelP=carModel;
    sLengthP = &sLength;
    rSlopeP = &rSlope;
    
    
    while (sLength != 999)
    {
      readVal(carModel,&rSlope,&sLength);
      if (sLength == 999)
      {
         break;
      }
      else
      {
       speed = calculateSpeed(rSlope,sLength);
       sum = sum + speed;
       count = count + 1;
       for (i=0;i<count;i++)
        {
            sLengthArray[count]= sLength;
            rSlopeArray[count]= rSlope;
            strcpy(carModelArray[count],carModel);
            speedArray[count]= speed;
        }
       }
    }
    printf ("|\n");
    printf ("|\tHISTORICAL DATA FROM SPEEDING CARS\n");
    printf ("|\tSkidlength\tRoadSlope\tSpeed\n");
    for (i=1; i<=count; i++)
    {
      printf("|\t%.3lf\t\t%0.lf\t\t%.2lf\n", sLengthArray[i], rSlopeArray[i], speedArray[i]);
    }
    printf ("|\n");
}



double readVal (char *carModel,double *rSlope, double *sLength)
{
      double all [300];
      int i;
      printf ("Please enter the car model.\n");
      scanf ("%s", carModel);
      do
      {
  			printf ("Please enter a skid length.If 999 is entered, no calculations will be done.\n");
			scanf ("%lf", sLength);
      }
      while(sLength <0);            
      do
      {
  			printf ("Please enter a road slope between -30 and 30.\n");
            scanf ("%lf",rSlope);
      }
      while (*rSlope<-30||*rSlope>30);
}

double calculateSpeed (double rSlope, double sLength)
{
     double slopeRad, slopeTangent, speed;
     slopeRad = rSlope*(pi/180);
     slopeTangent= sin(slopeRad)/cos(slopeRad);
     speed = sqrt(30*sLength*(slopeTangent + brakeFactor));
     return speed;
}

You can't do that. ListValues is a function that has local variables. Once you return from the function, those local variables are GONE when your function returns (that is: The space those variables use when you are in the function is no longer available once you leave the function).

--
Mats

can I just declare the values that i need in main and them pass them to the module by reference?

Nope, but you can pass them by address/pointer.
No such thing as references in C...

There are 3 ways to pass values outside a functions

So you want this (for count and carModel):
1) Global variables

Code:

char carModel[1000];
int count; //automatically count initialized to 0
...
void listValues ();
int main ()
{
 ....
  listValues ();
}

void listValues ()
{
    //carModel and count are already declared
}

alternatively you can do this:

Code:

2) Passing with pointers
void listValues (char CharModel[1000], int* count);
int main ()
{
 ....
  char charModel[1000];
  int count = 0;
  listValues (charModel, &count);
}
void listValues (char CharModel[1000], int* count)
{
    //carModel and count are already declared in main
}

Lastly,
3) returning values with return. Only for one variable (except if you use a struct). But you have a lot, so I you don't want this. Just keep in mind if you wanted only count:

Code:

int listValues();
int main ()
{
 ....
  int count = listValues();
  
}
int listValues ()
{
    //declare all variables as you do
   return count;
}

Your 3rd example is broken, since the function takes no arguments, yet you pass two... and count is nowhere to be found, nor is it necessary since the count is returned...
Also, know that global variables should be avoided where possible because they can easily create difficult code to debug. Code that does not run as expected.

fixed