Structure within structure in C

Nested structure in C is nothing but structure within structure. One structure can be declared inside other structure as we declare structure members inside a structure. The structure variables can be a normal structure variable or a pointer variable to access the data. You can learn below concepts in this section.

1. Structure within structure in C using normal variable
2. Structure within structure in C using pointer variable

1. Structure within structure in C using normal variable:

           This program explains how to use structure within structure in C using normal variable. “student_college_detail’ structure is declared inside “student_detail” structure in this program. Both structure variables are normal structure variables.

Please note that members of “student_college_detail” structure are accessed by 2 dot(.) operator and members of “student_detail” structure are accessed by single dot(.) operator.

#include 
#include 

struct student_college_detail
{
    int college_id;
    char college_name[50];
};

struct student_detail 
{
    int id;
    char name[20];
    float percentage;
    // structure within structure
    struct student_college_detail clg_data;
}stu_data;

int main() 
{
    struct student_detail stu_data = {1, "Raju", 90.5, 71145,
                                       "Anna University"};
    printf(" Id is: %d \n", stu_data.id);
    printf(" Name is: %s \n", stu_data.name);
    printf(" Percentage is: %f \n\n", stu_data.percentage);

    printf(" College Id is: %d \n", 
                    stu_data.clg_data.college_id);
    printf(" College Name is: %s \n", 
                    stu_data.clg_data.college_name);
    return 0;
}

Output:

Id is: 1 Name is: Raju Percentage is: 90.500000 College Id is: 71145 College Name is: Anna University

Structure within structure in C using pointer variable:

           This program explains how to use structure within structure in C using pointer variable. “student_college_detail’ structure is declared inside “student_detail” structure in this program. one normal structure variable and one pointer structure variable is used in this program.

Please note that combination of .(dot) and ->(arrow) operators are used to access the structure member which is declared inside the structure.

#include 
#include 

struct student_college_detail
{
    int college_id;
    char college_name[50];
};

struct student_detail 
{
    int id;
    char name[20];
    float percentage;
    // structure within structure
    struct student_college_detail clg_data; 
}stu_data, *stu_data_ptr;

int main() 
{
  struct student_detail stu_data = {1, "Raju", 90.5, 71145, 
                                    "Anna University"};
    stu_data_ptr = &stu_data;

    printf(" Id is: %d \n", stu_data_ptr->id);
    printf(" Name is: %s \n", stu_data_ptr->name);
    printf(" Percentage is: %f \n\n", 
                         stu_data_ptr->percentage);

    printf(" College Id is: %d \n", 
                         stu_data_ptr->clg_data.college_id);
    printf(" College Name is: %s \n", 
                      stu_data_ptr->clg_data.college_name);

    return 0;
}

Output:

Id is: 1 Name is: Raju Percentage is: 90.500000 College Id is: 71145 College Name is: Anna University

C – Passing structure to function

• A structure can be passed to any function from main function or from any sub function.
• Structure definition will be available within the function only.
• It won’t be available to other functions unless it is passed to those functions by value or by address(reference).
• Else, we have to declare structure variable as global variable. That means, structure variable should be declared outside the main function. So, this structure will be visible to all the functions in a C program.

Passing structure to function in C:

It can be done in below 3 ways.

1. Passing structure to a function by value
2. Passing structure to a function by address(reference)
3. No need to pass a structure – Declare structure variable as global

Example program – passing structure to function in C by value:

           In this program, the whole structure is passed to another function by value. It means the whole structure is passed to another function with all members and their values. So, this structure can be accessed from called function. This concept is very useful while writing very big programs in C.

#include 
#include 

struct student 
{
            int id;
            char name[20];
            float percentage;
};

void func(struct student record);

int main() 
{
            struct student record;

            record.id=1;
            strcpy(record.name, "Raju");
            record.percentage = 86.5;

            func(record);
            return 0;
}

void func(struct student record)
{
            printf(" Id is: %d \n", record.id);
            printf(" Name is: %s \n", record.name);
            printf(" Percentage is: %f \n", record.percentage);
}

Output:

Id is: 1 Name is: Raju Percentage is: 86.500000

Example program – Passing structure to function in C by address:

           In this program, the whole structure is passed to another function by address. It means only the address of the structure is passed to another function. The whole structure is not passed to another function with all members and their values. So, this structure can be accessed from called function by its address.

#include 
#include 

struct student 
{
           int id;
           char name[20];
           float percentage;
};

void func(struct student *record);

int main() 
{
          struct student record;

          record.id=1;
          strcpy(record.name, "Raju");
          record.percentage = 86.5;

          func(&record);
          return 0;
}

void func(struct student *record)
{
          printf(" Id is: %d \n", record->id);
          printf(" Name is: %s \n", record->name);
          printf(" Percentage is: %f \n", record->percentage);
}

Output:

Id is: 1 Name is: Raju Percentage is: 86.500000

Example program to declare a structure variable as global in C:

           Structure variables also can be declared as global variables as we declare other variables in C. So, When a structure variable is declared as global, then it is visible to all the functions in a program. In this scenario, we don’t need to pass the structure to any function separately.

#include 
#include 

struct student 
{
            int id;
            char name[20];
            float percentage;
};
struct student record; // Global declaration of structure

void structure_demo();

int main() 
{
            record.id=1;
            strcpy(record.name, "Raju");
            record.percentage = 86.5;

            structure_demo();
            return 0;
}

void structure_demo()
{
            printf(" Id is: %d \n", record.id);
            printf(" Name is: %s \n", record.name);
            printf(" Percentage is: %f \n", record.percentage);
}

Output:

Id is: 1 Name is: Raju Percentage is: 86.500000

Memory : Stack vs Heap

7. Memory : Stack vs Heap

---

Stack vs Heap

So far we have seen how to declare basic type variables such as int, double, etc, and complex types such as arrays and structs. The way we have been declaring them so far, with a syntax that is like other languages such as MATLAB, Python, etc, puts these variables on the stack in C.

The Stack

What is the stack? It's a special region of your computer's memory that stores temporary variables created by each function (including the main() function). The stack is a "FILO" (first in, last out) data structure, that is managed and optimized by the CPU quite closely. Every time a function declares a new variable, it is "pushed" onto the stack. Then every time a function exits, all of the variables pushed onto the stack by that function, are freed (that is to say, they are deleted). Once a stack variable is freed, that region of memory becomes available for other stack variables.
The advantage of using the stack to store variables, is that memory is managed for you. You don't have to allocate memory by hand, or free it once you don't need it any more. What's more, because the CPU organizes stack memory so efficiently, reading from and writing to stack variables is very fast.
A key to understanding the stack is the notion that when a function exits, all of its variables are popped off of the stack (and hence lost forever). Thus stack variables are local in nature. This is related to a concept we saw earlier known as variable scope, or local vs global variables. A common bug in C programming is attempting to access a variable that was created on the stack inside some function, from a place in your program outside of that function (i.e. after that function has exited).
Another feature of the stack to keep in mind, is that there is a limit (varies with OS) on the size of variables that can be store on the stack. This is not the case for variables allocated on the heap.
To summarize the stack:

• the stack grows and shrinks as functions push and pop local variables
• there is no need to manage the memory yourself, variables are allocated and freed automatically
• the stack has size limits
• stack variables only exist while the function that created them, is running

The Heap

The heap is a region of your computer's memory that is not managed automatically for you, and is not as tightly managed by the CPU. It is a more free-floating region of memory (and is larger). To allocate memory on the heap, you must use malloc() or calloc(), which are built-in C functions. Once you have allocated memory on the heap, you are responsible for using free() to deallocate that memory once you don't need it any more. If you fail to do this, your program will have what is known as a memory leak. That is, memory on the heap will still be set aside (and won't be available to other processes). As we will see in the debugging section, there is a tool called valgrind that can help you detect memory leaks.
Unlike the stack, the heap does not have size restrictions on variable size (apart from the obvious physical limitations of your computer). Heap memory is slightly slower to be read from and written to, because one has to use pointers to access memory on the heap. We will talk about pointers shortly.
Unlike the stack, variables created on the heap are accessible by any function, anywhere in your program. Heap variables are essentially global in scope.

Stack vs Heap Pros and Cons

Stack

• very fast access
• don't have to explicitly de-allocate variables
• space is managed efficiently by CPU, memory will not become fragmented
• local variables only
• limit on stack size (OS-dependent)
• variables cannot be resized

Heap

• variables can be accessed globally
• no limit on memory size
• (relatively) slower access
• no guaranteed efficient use of space, memory may become fragmented over time as blocks of memory are allocated, then freed
• you must manage memory (you're in charge of allocating and freeing variables)
• variables can be resized using realloc()

Examples

Here is a short program that creates its variables on the stack. It looks like the other programs we have seen so far.

#include 

double multiplyByTwo (double input) {
  double twice = input * 2.0;
  return twice;
}

int main (int argc, char *argv[])
{
  int age = 30;
  double salary = 12345.67;
  double myList[3] = {1.2, 2.3, 3.4};

  printf("double your salary is %.3f\n", multiplyByTwo(salary));

  return 0;
}

double your salary is 24691.340

On lines 10, 11 and 12 we declare variables: an int, a double, and an array of three doubles. These three variables are pushed onto the stack as soon as themain() function allocates them. When the main() function exits (and the program stops) these variables are popped off of the stack. Similarly, in the functionmultiplyByTwo(), the twice variable, which is a double, is pushed onto the stack as soon as the multiplyByTwo() function allocates it. As soon as themultiplyByTwo() function exits, the twice variable is popped off of the stack, and is gone forever.
As a side note, there is a way to tell C to keep a stack variable around, even after its creator function exits, and that is to use the static keyword when declaring the variable. A variable declared with the static keyword thus becomes something like a global variable, but one that is only visible inside the function that created it. It's a strange construction, one that you probably won't need except under very specific circumstances.
Here is another version of this program that allocates all of its variables on the heap instead of the stack:

#include 
#include 

double *multiplyByTwo (double *input) {
  double *twice = malloc(sizeof(double));
  *twice = *input * 2.0;
  return twice;
}

int main (int argc, char *argv[])
{
  int *age = malloc(sizeof(int));
  *age = 30;
  double *salary = malloc(sizeof(double));
  *salary = 12345.67;
  double *myList = malloc(3 * sizeof(double));
  myList[0] = 1.2;
  myList[1] = 2.3;
  myList[2] = 3.4;

  double *twiceSalary = multiplyByTwo(salary);

  printf("double your salary is %.3f\n", *twiceSalary);

  free(age);
  free(salary);
  free(myList);
  free(twiceSalary);

  return 0;
}

As you can see, using malloc() to allocate memory on the heap and then using free() to deallocate it, is no big deal, but is a bit cumbersome. The other thing to notice is that there are a bunch of star symbols * all over the place now. What are those? The answer is, they are pointers. The malloc() (and calloc() andfree()) functions deal with pointers not actual values. We will talk more about pointers shortly. The bottom line though: pointers are a special data type in C that store addresses in memory instead of storing actual values. Thus on line 5 above, the twice variable is not a double, but is a pointer to a double. It's an address in memory where the double is stored.

When to use the Heap?

When should you use the heap, and when should you use the stack? If you need to allocate a large block of memory (e.g. a large array, or a big struct), and you need to keep that variable around a long time (like a global), then you should allocate it on the heap. If you are dealing with realtively small variables that only need to persist as long as the function using them is alive, then you should use the stack, it's easier and faster. If you need variables like arrays and structs that can change size dynamically (e.g. arrays that can grow or shrink as needed) then you will likely need to allocate them on the heap, and use dynamic memory allocation functions like malloc(), calloc(), realloc() and free() to manage that memory "by hand". We will talk about dynamically allocated data structures after we talk about pointers.

Return multiple values in C

Method 1 : Using Array

• If more than two variables of same type is to be returned then we can use array .
• Store each and every value to be returned in an array and return base address of that array.

Method 2 : Using Pointer and One Return Statement

• Pointer Variable can updated directly using  Value at ['*'] Operator.
• Usually Function can return single value.
• If we need to return more than two variables then update 1 variable directly using pointer and return second variable using ‘return Statement‘. [ Call by Value + Call by Reference ]

EX:

#include

int* function2(int a){

    int* array = (int*)malloc(2);

    int b=(65*a)/100;

    int c=5*8;

    array[0]=b;

    array[1]=c;

    return array;

}

int main(void)

{

    int a=18,i;

    int* array;

    array=function2(a);

   for(i=0;i<2 div="" i="">

   {

        printf("array[%d]: %d\n",i,array[i]);

    }

    free(array);

    

     getch();  

}

Method 3 : Using Structure 

• Construct a Structure containing values to be returned and then return the base address of the structure to the calling function.

EX:

#include

struct dont { int x; double y; };

struct dont fred(void)
{
        struct dont b;
        b.x = 1;
        b.y = 91.99919;
        return b;
}

int main(int argc, char **argv)
{
        struct dont look = fred();
        printf("look.x = %d, look.y = %lf\n", look.x, look.y);
        getch();
        return 0;
}

Array of Pointers in C

#include
#include

struct name {
   int a;
   float b;
   char c[30];
};

int main(){
   struct name *ptr;
 
   int i,n;
   printf("Enter n: ");
   scanf("%d",&n);
 
   ptr=(struct name*)malloc(n*sizeof(struct name));
/* Above statement allocates the memory for n structures with pointer ptr pointing to base address */

   for(i=0;i       printf("Enter string, integer and floating number  respectively:\n");
       scanf("%s%d%f",&(ptr+i)->c,&(ptr+i)->a,&(ptr+i)->b);
     
   }
   printf("Displaying Infromation:\n");
   for(i=0;i       printf("%s\t%d\t%.2f\n",(ptr+i)->c,(ptr+i)->a,(ptr+i)->b);
     
       getch();
   return 0;
}