Thread: Program stops working when it gets to function

Step 1 is learn how to indent code consistently.
If the code is clearly laid out, it's a lot easier for others to read, and easier for you to spot problems.

Code:

#include <stdio.h>
//#include <conio.h>

void Rent(void);
float entry(int);
float discount(int, float);

int j = 0;

struct rental {
  char firstname[20];
  char lastname[20];
  int number[10];
  char address[40];
  int age;
} rents[40];

int main()
{
  int num1;
  printf("Welcome to Ollivierre's Car Rental Service!\n"
         "Do you want to rent a car or are you returning one?\n"
         "1.Renting\n"
         "2.Returning\n"
         "0.Exit\n");
  scanf("%d", &num1);

  while (num1 != 0) {
    switch (num1) {
    case 1:
      Rent();
      break;
    }
  }
  printf("Have a nice day!");
  return 0;
}

void Rent()
{
  int entrance, num2, num3, num4, car;
  float rate, discop;
  FILE *rentPtr;
  rentPtr = fopen("Rental.txt", "w");

  printf("You have chosen to rent a car\n");
  printf("Please Enter your First name: \n");
  scanf("%s", &rents[j].firstname);
  printf("Please Enter your Last name: \n");
  scanf("%s", &rents[j].lastname);
  printf("Please Enter your Address: \n");
  scanf("%s", &rents[j].address);
  printf("Please Enter your Telephone Number: \n");
  scanf("%d", &rents[j].number);
  printf("Please Enter your Age(Must be at least age 20)): \n");
  scanf("%d", &rents[j].age);
  entrance = rents[j].age;

  if (entrance > 20) {
    printf("What type of vehicle would you like?\n"
           "These are the vehicles available.\n");
    printf("1.Toyota Camry($30)\n"
           "2.Mitsubishi lancer($60)\n"
           "3.Bugatti veyron($95)\n"
           "4.Lamborghini gallardo ($90)\n"
           "5.Nissan Altima($80)\n"
           "6.mitsubishi galant ($45)\n");
    printf("Which vehicle would you like to rent?\n");
    scanf("%d", num2);
    car = entry(num2);
    printf("The price of your car per day is %0.2f", car);
    printf("How many days will you be renting this car for?: \n");
    scanf("%d", num4);
    rate = car * num4;
    if (num4 > 10) {
      discop = discount(num4, rate);
      printf("You are renting the car for: %d day(s).\n"
             "The price would be: %0.2f\n", num4, discop);
    } else {
      if (num4 < 10) {
        printf("You are renting the car for: %d day(s).\n"
        "The price would be %0.2f", num4, rate);
      }
    }
    fprintf(rentPtr, "Firstname\tLastname\tAddress\tNumber\tAge");
    fprintf(rentPtr, "\n");
    fprintf(rentPtr, "%s\t%s\t%s\t%d\t%d", rents[j].firstname, rents[j].lastname, 
            rents[j].address, rents[j].number, rents[j].age);
    fclose(rentPtr);
  } else {
    if (entrance <= 19) {
      printf("You are too young to rent a car!\n");
    }
  }
  printf("Have a nice day!");
}

float entry(int a)
{
  float price;

  if (a == 1) {
    price = 30;
  } else {
    if (a == 2) {
      price = 60;
    } else {
      if (a == 3) {
        price = 95;
      } else {
        if (a == 4) {
          price = 90;
        } else {
          if (a == 5) {
            price = 80;
          } else {
            if (a == 6) {
              price = 45;
            }
          }
        }
      }
    }
  }

  return price;
}

float discount(int b, float c)
{
  float disco;
  if ((b > 10) && (b < 21)) {
    disco = c - (c * .02);
  } else {
    if ((b > 20) && (b < 41)) {
      disco = c - (c * .05);
    } else {
      if (b > 40) {
        disco = c - (c * .10);
      }
    }
  }
  return disco;
}

Next, get a compiler that can tell you a lot more about what is wrong with your program before you try to run it.

Code:

$ gcc -Wall -Wextra foo.c
foo.c: In function ‘Rent’:
foo.c:48:9: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘char (*)[20]’ [-Wformat=]
   scanf("%s", &rents[j].firstname);
         ^
foo.c:50:9: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘char (*)[20]’ [-Wformat=]
   scanf("%s", &rents[j].lastname);
         ^
foo.c:52:9: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘char (*)[40]’ [-Wformat=]
   scanf("%s", &rents[j].address);
         ^
foo.c:54:9: warning: format ‘%d’ expects argument of type ‘int *’, but argument 2 has type ‘int (*)[10]’ [-Wformat=]
   scanf("%d", &rents[j].number);
         ^
foo.c:69:11: warning: format ‘%d’ expects argument of type ‘int *’, but argument 2 has type ‘int’ [-Wformat=]
     scanf("%d", num2);
           ^
foo.c:71:12: warning: format ‘%f’ expects argument of type ‘double’, but argument 2 has type ‘int’ [-Wformat=]
     printf("The price of your car per day is %0.2f", car);
            ^
foo.c:73:11: warning: format ‘%d’ expects argument of type ‘int *’, but argument 2 has type ‘int’ [-Wformat=]
     scanf("%d", num4);
           ^
foo.c:87:22: warning: format ‘%d’ expects argument of type ‘int’, but argument 6 has type ‘int *’ [-Wformat=]
     fprintf(rentPtr, "%s\t%s\t%s\t%d\t%d", rents[j].firstname, rents[j].lastname, 
                      ^
foo.c:41:23: warning: unused variable ‘num3’ [-Wunused-variable]
   int entrance, num2, num3, num4, car;
                       ^
foo.c:69:5: warning: ‘num2’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     scanf("%d", num2);
     ^
foo.c:73:5: warning: ‘num4’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     scanf("%d", num4);
     ^

You have a lot of printf / scanf calls where the format string is inconsistent with the parameter available.