There are variables of different data types in C, such as ints, chars, and floats. And they let you store data.
And we have arrays to group together a collection of data of the same data type.
But in reality, we will not always have the luxury of having data of only one type. That's where a structure comes into the picture. In this article, we'll learn more about structured data types in C.
Table of Contents
A. Fundamentals
- Definition and Declaration
- Initialization and Accessing the Members of a Structure
- Operating with Structure Variable
- Array Of Structure
- Nested Structure
B. Memory Allocation
C. Pointers
D. Functions
E. Self-Referential Structures
F. Conclusion
Let's get started, shall we?
Fundamentals
1. Definition and Declaration
A structure is a collection of one or more variables, possibly of different types, grouped under a single name. It is a user-defined data type.
They help to organize complicated data in large programs, as they allow a group of logically related variables to be treated as one.
For example, a student can have properties of name, age, gender and marks. We could create a character array for name, an integer variable for roll, a character variable for gender, and an integer array for marks.
But if there are 20 or 100 students, it will be difficult to handle those variables.
We can declare a structure using the struct keyword following the syntax as below:
/* Syntax */
struct structureName
{
dataType memberVariable1;
datatype memberVariable2;
...
};
/* Example */
struct student
{
char name[20];
int roll;
char gender;
int marks[5];
};
The above statement defines a new data type struct student. Each variable of this data type will consist of name[20], roll, gender and marks[5]. These are known as members of the structure.
Once a structure is declared as a new data type, then the variables of that data type can be created.
/* Variable declaration */
struct structureName structureVariable;
/* Example /*
struct student st1;
struct student st2,st3,st4;
Each variable of struct student has its copies of the members.
A few important nuggets:
- The members of the structure do not occupy memory until a structure variable is created.
- You might have noticed we are using
structas a variable declaration as well. Isn't that tedious?
Using the typedef keyword in the structure declaration, we can prevent having to write struct again.
typedef struct students
{
char name[20];
int roll;
char gender;
int marks[5];
} STUDENT;
/* or */
typedef struct
{
char name[20];
int roll;
char gender;
int marks[5];
} STUDENT;
STUDENT st1,st2,st3,st4;
By convention, uppercase letters are used for type definitions (such as STUDENT).
3. The structure definition and variable declaration can be combined as below.
struct student
{
char name[20];
int roll;
chat gender;
int marks[5];
}st1, st2, st3, st4;
4. The use of structureName is optional. The code below is completely valid.
struct
{
char name[20];
int roll;
char gender;
int marks[5];
}st1, st2, st3, st4;
5. Structures are generally declared at the top of the source code file, even before defining functions (you'll see why).
6. C doesn't allow variable initialization inside a structure declaration.
2. Initialization and Accessing the Members of a Structure
Like any other variable, a structure variable can also be initialized where they are declared. There is a one-to-one relationship between the members and their initializing values.
/* Variable Initialization */
struct structureName = { value1, value2,...};
/* Example */
typedef struct
{
char name[20];
int roll;
char gender;
int marks[5];
}STUDENT;
void main(){
STUDENT st1 = { "Alex", 43, 'M', {76, 78, 56, 98, 92}};
STUDENT st2 = { "Max", 33, 'M', {87, 84, 82, 96, 78}};
}
To access the members, we have to use . (the dot operator).
/* Accessing Memebers of a Structure */
structureVariable.memberVariable
/* Example */
printf("Name: %s\n", st1.name);
printf("Roll: %d\n", st1.roll);
printf("Gender: %c\n", st1.gender);
for( int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n", i, st1.marks[i]);
/* Output */
Name: Alex
Roll: 43
Gender: M
Marks in 0th subject: 76
Marks in 1th subject: 78
Marks in 2th subject: 56
Marks in 3th subject: 98
Marks in 4th subject: 92
The members can be initialized in the variable declaration in any order using ..
STUDENT st3 = { .gender = 'M', .roll = 23, .name = "Gasly", .marks = { 99, 45, 67, 78, 94}};
We can also initialize the first few members and leave the remaining blank. However, the uninitialized members should be only at the end of the list.
Uninitialized integers and floating-point numbers have a default value of 0. It is \0( NULL) for characters and strings.
STUDENT st4 = { "Kviyat", 65};
/* same as { "Kviyat", 65, '\0', { 0, 0, 0, 0, 0} } */
3. Operating with the Structure Variable
Like variables of primitive data types, we cannot perform arithmetic operations such as +, -, *, /, and so on. Likewise, relational and equality operators cannot be used with structure variables.
But, we can copy one structure variable to another, provided they belong to the same structure.
/* Invalid Operations */
st1 + st2
st1 - st2
st1 == st2
st1 != st2 etc.
/* Valid Operation */
st1 = st2
We will have to compare the structure members individually to compare structure variables.
#include <stdio.h>
#include <string.h>
struct student
{
char name[20];
double roll;
char gender;
int marks[5];
}st1,st2;
void main()
{
struct student st1= { "Alex", 43, 'M', {76, 78, 56, 98, 92}};
struct student st2 = { "Max", 33, 'M', {87, 84, 82, 96, 78}};
if( strcmp(st1.name,st2.name) == 0 && st1.roll == st2.roll)
printf("Both are the records of the same student.\n");
else printf("Different records, different students.\n");
/* Copiying the structure variable */
st2 = st1;
if( strcmp(st1.name,st2.name) == 0 && st1.roll == st2.roll)
printf("\nBoth are the records of the same student.\n");
else printf("\nDifferent records, different students.\n");
}
/* Output */
Different records, different students.
Both are the records of the same student.
4. Array of a Structure
You have already seen how we had to create 4 different struct student type variables to store the records of 4 students.
A better way would be to create an array of struct student (just like an array of ints).
struct student
{
char name[20];
double roll;
char gender;
int marks[5];
};
struct student stu[4];
To access the elements of the array stu and the members of each element, we can use loops.
/* Taking values for the user */
for(int i = 0; i < 4; i++)
{
printf("Enter name:\n");
scanf("%s",&stu[i].name);
printf("Enter roll:\n");
scanf("%d",&stu[i].roll);
printf("Enter gender:\n");
scanf(" %c",&stu[i].gender);
for( int j = 0; j < 5; j++)
{
printf("Enter marks of %dth subject:\n",j);
scanf("%d",&stu[i].marks[j]);
}
printf("\n-------------------\n\n");
}
/* Finding the average marks and printing it */
for(int i = 0; i < 4; i++)
{
float sum = 0;
for( int j = 0; j < 5; j++)
{
sum += stu[i].marks[j];
}
printf("Name: %s\nAverage Marks = %.2f\n\n", stu[i].name,sum/5);
}
5. Nested Structure
Nesting a structure means having one or more structure variables inside another structure. Like we declare an int member or char member, we can also declare a structure variable as a member.
struct date
{
int date;
int month;
int year;
};
struct student
{
char name[20];
int roll;
char gender;
int marks[5];
struct birth birthday;
};
void main(){
struct student stu1;
The structure variable birthday of type struct birth is nested inside struct student. It should be clear that you cannot nest a structure variable of type struct student inside struct student.
Note that the structure to be nested has to be declared first. Using ., we can access the members contained within the inner structure as well as other members.
/* Example */
stu1.birthday.date
stu1.birthday.month
stu1.birthday.year
stu1.name
Structure variables of different types can be nested as well.
struct birth
{
int date;
int month;
int year;
};
struct relation
{
char fathersName[20];
char mothersName[20];
};
struct student
{
char name[20];
int roll;
char gender;
int marks[5];
struct birth birthday;
struct relation parents;
};
Memory Allocation
When a structure variable of some type is declared, structure members are allocated contiguous (adjacent) memory locations.
struct student
{
char name[20];
int roll;
char gender;
int marks[5];
} stu1;
Here, the memory will be allocated to name[20], followed by roll, gender and marks[5]. This implies that the size of the st1 or struct student will be the sum of the size of its members, right? Let's check.
void main()
{
printf("Sum of the size of members = %I64d bytes\n", sizeof(stu1.name) + sizeof(stu1.roll) + sizeof(stu1.gender) + sizeof(stu1.marks));
printf("Using sizeof() operator = %I64d bytes\n",sizeof(stu1));
}
/* Output */
Sum of the size of members = 45 bytes
Using sizeof() operator = 48 bytes
Since thesizeof()operator returnslong long unsigned int, use%I64das a format specifier. You might have to use%lluor%llddepending upon your compiler.
Using%dwill give a warning - format '%d' expects argument of type 'int', but argument 2 has type 'long long unsigned int'.
Using the sizeof() operator gives 3 more bytes than the sum of the size of the members. Why? Where are those 3 bytes in the memory?
Let's answer the second question first. We can print the addresses of the members to find the addresses of those 3 bytes.
void main()
{
printf("Address of member name = %d\n", &stu1.name);
printf("Address of member roll = %d\n", &stu1.roll);
printf("Address of member gender = %d\n", &stu1.gender);
printf("Address of member marks = %d\n", &stu1.marks);
}
/* Output */
Address of member name = 4225408
Address of member roll = 4225428
Address of member gender = 4225432
Address of member marks = 4225436
We can see that the array marks[5] instead of being allocated from 4225433 has been allocated from 4224536. But why?
1. Data Alignment
Before looking at data alignment, it is important to know how the processor reads data from the memory.
A processor reads one word in one cycle. This word is 4 bytes for a 32-bit processor and 8 bytes for a 64-bit processor. The lower the number of cycles, the better is the performance of the CPU.
One way to achieve this is by aligning the data. Aligning means that a variable of any primitive data type of size t will always (by default) have an address that is a multiple of t. This essentially is data alignment. This takes place every time.
Aligned addresses for some data types
| Data types | Size (in bytes) | Address |
|---|---|---|
| char | 1 | multiple of 1 |
| short | 2 | multiple of 2 |
| int, float | 4 | multiple of 4 |
| double, long, * (pointers) | 8 | multiple of 8 |
| long double | 16 | multiple of 16 |
2. Structure Padding
You may need to insert some extra bytes between the members of the structure to align the data. These extra bytes are known as padding.
In our above example, the 3 bytes acted as padding. Without them, marks[0] which is of type int (address multiple of 4) would have its base address as 4225433 (not a multiple of 4).
You can now probably see why structures can't be compared directly.
3. Structure Member Alignment
To explain this, we will take another example (you'll understand why).
struct example
{
int i1;
double d1;
char c1;
} example1;
void main()
{
printf("size = %I64d bytes\n",sizeof(example1));
}
What would be the output? Let's apply what we know.
i1 is of 4 bytes. It will be followed by a padding of 4 bytes because the address of d1 should be divisible by 8.
This will be followed by 8 and 1 byte respectively for d1 and c1. Thus, the output should be 4 + 4 + 8 + 1 = 17 bytes.
/* Output */
size = 24 bytes
What? Wrong again! How? Through an array of struct example, we can understand better. We will also print the address of the members of example2[0].
void main()
{
struct example example2[2];
printf("Address of example2[0].i1 = %d\n", &example2[0].i1);
printf("Address of example2[0].d1 = %d\n", &example2[0].d1);
printf("Address of example2[0].c1 = %d\n", &example2[0].c1);
}
/* Output */
Address of example2[0].i1 = 4225408
Address of example2[0].d1 = 4225416
Address of example2[0].c1 = 4225424
Let's suppose the size of example2[0] is 17 bytes. This implies that the address of example2[1].i1 will be 4225425. This isn't possible since the address of int should be a multiple of 4.
Logically, the possible address for example2[1].i1 seems to be 4225428, a multiple of 4.
This is wrong as well. Do you know why? The address of example2[1].d1 now will be ( 28 + 4 ( i1) + 3 ( padding)) 4225436 which is not a multiple of 8.
In order to avoid such misalignment, the compiler introduces alignment to every structure. This is done by adding extra bytes after the last member, known as structure member alignment.
In the example discussed at the start of this section, this wasn't required (which is why we needed this other example).
A simple way to remember is through this rule: Address of structure and structure length must be multiples of t_max. Here, t_max is the maximum size taken by a member in the structure.
For struct example, 8 bytes is the maximum size of d1. Therefore, there is a padding of 7 bytes to the end of the structure, making its size 24 bytes.
Following these two rules, you can easily find the size of any structure:
- Any data type stores its value at an address that is a multiple of its size.
- Any structure takes the size which is a multiple of the maximum bytes taken by a member.
Though we are able to lower the CPU cycles, there is a significant amount of memory going to waste.
One way to decrease the amount of padding to a possible minimum is by declaring the member variables in decreasing order of their size.
If we follow this in struct example, the size of the structure reduces to 16 bytes. The padding gets reduced from 7 to 3 bytes.
struct example
{
double d1;
int i1;
char c1;
} example3;
void main()
{
printf("size = %I64d bytes\n",sizeof(example3));
}
/* Output */
size = 16 bytes
4. Structure Packing
Packing is the opposite of padding. It prevents the compiler from padding and removes the unallocated memory.
In the case of Windows, we use the #pragma pack directive, which specifies the packing alignment for structure members.
#pragma pack(1)
struct example
{
double d1;
int i1;
char c1;
} example4;
void main()
{
printf("size = %I64d bytes\n",sizeof(example4));
}
/* Output */
size = 13 bytes
This ensures that the members are aligned on a 1-byte boundary. In other words, the address of any data type has to be a multiple of 1 byte or their size (whichever is lower).
Pointers
If you want to brush up pointers before going forward, here is a link to an article covering pointers in-depth.
1. Pointer as a Member
A structure can have pointers as members as well.
struct student
{
char *name;
int *roll;
char gender;
int marks[5];
};
void main()
{ int alexRoll = 44;
struct student stu1 = { "Alex", &alexRoll, 'M', { 76, 78, 56, 98, 92 }};
}
Using . (the dot operator), we can again access the members. Since roll now has the address of alexRoll, we will have to dereference stu1.roll to get the value (and not stu1.(*roll) ).
printf("Name: %s\n", stu1.name);
printf("Roll: %d\n", *(stu1.roll));
printf("Gender: %c\n", stu1.gender);
for( int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n", i, stu1.marks[i]);
/* Output */
Name: Alex
Roll: 43
Gender: M
Marks in 0th subject: 76
Marks in 1th subject: 78
Marks in 2th subject: 56
Marks in 3th subject: 98
Marks in 4th subject: 92
2. Pointer to Structure
Like integer pointers, array pointers, and function pointers, we have pointer to structures or structure pointers as well.
struct student {
char name[20];
int roll;
char gender;
int marks[5];
};
struct student stu1 = {"Alex", 43, 'M', {76, 98, 68, 87, 93}};
struct student *ptrStu1 = &stu1;
Here, we have declared a pointer ptrStu1 of type struct student. We have assigned the address of stu1 to ptrStu1.
ptrStu1 stores the base address of stu1, which is the base address of the first member of the structure. Incrementing by 1 would increase the address by sizeof(stu1) bytes.
printf("Address of structure = %d\n", ptrStu1);
printf("Adress of member `name` = %d\n", &stu1.name);
printf("Increment by 1 results in %d\n", ptrStu1 + 1);
/* Output */
Address of structure = 6421968
Adress of member 'name' = 6421968
Increment by 1 results in 6422016
We can access the members of stu1 using ptrStu1 in two ways. Using * (indirection operator) or using -> (infix or arrow operator).
With *, we will continue to use the .(dot operator) whereas with -> we won't need the dot operator.
printf("Name w.o using ptrStu1 : %s\n", stu1.name);
printf("Name using ptrStu1 and * : %s\n", (*ptrStu1).name);
printf("Name using ptrStu1 and -> : %s\n", ptrStu1->name);
/* Output */
Name without using ptrStu1: Alex
Name using ptrStu1 and *: Alex
Name using ptrStu1 and ->: Alex
Similarly, we can access and modify other members as well. Note that the brackets are necessary while using * since the dot operator(.) has higher precedence over *.
3. Pointer and Array of Structure
We can create an array of type struct student and use a pointer to access the elements and their members.
struct student stu[10];
/* Pointer to the first element (structure) of the array */
struct student *ptrStu_type1 = stu;
/* Pointer to an array of 10 struct student */
struct student (*ptrStu_type2)[10] = &stu;
Note that ptrStu_type1 is a pointer to stu[0] whereas ptrStu_type2 is a pointer to the whole array of 10 struct student. Adding 1 to ptrStu_type1 would point to stu[1].
We can use ptrStu_type1 with a loop to traverse through the elements and their members.
for( int i = 0; i < 10; i++)
printf("%s, %d\n", ( ptrStu_type1 + i)->name, ( ptrStu_type1 + i)->roll);
Functions
1. Function as a Member
Functions can not be a member of a structure. However, using function pointers, we can call functions using . . Just keep in mind that this is not recommended.
struct example
{
int i;
void (*ptrMessage)(int i);
};
void message(int);
void message(int i)
{
printf("Hello, I'm a member of a structure. This structure also has an integer with value %d", i);
}
void main()
{
struct example eg1 = {6, message};
eg1.ptrMessage(eg1.i);
}
We have declared two members, an integer i and a function pointer ptrMessage inside struct example. The function pointer points to a function that takes an integer and returns void.
message is one such function. We initialized eg1 with 6 and message. Then we use . to call the function using ptrMessage and pass eg1.i.
2. Structure as a Function Argument
Like variables, we can pass individual structure members as arguments.
#include <stdio.h>
struct student {
char name[20];
int roll;
char gender;
int marks[5];
};
void display(char a[], int b, char c, int marks[])
{
printf("Name: %s\n", a);
printf("Roll: %d\n", b);
printf("Gender: %c\n", c);
for(int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n",i,marks[i]);
}
void main()
{
struct student stu1 = {"Alex", 43, 'M', {76, 98, 68, 87, 93}};
display(stu1.name, stu1.roll, stu1.gender, stu1.marks);
}
/* Output */
Name: Alex
Roll: 43
Gender: M
Marks in 0th subject: 76
Marks in 1th subject: 98
Marks in 2th subject: 68
Marks in 3th subject: 87
Marks in 4th subject: 93
Note that the structure struct student is declared outside main(), at the very top. This is to ensure that it is available globally and display() can use it.
If the structure is defined inside main(), its scope will be limited to main().
Passing structure members is not efficient when there are a large number of them. Then structure variables can be passed to a function.
void display(struct student a)
{
printf("Name: %s\n", a.name);
printf("Roll: %d\n", a.roll);
printf("Gender: %c\n", a.gender);
for(int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n",i,a.marks[i]);
}
void main()
{
struct student stu1 = {"Alex", 43, 'M', {76, 98, 68, 87, 93}};
display(stu1);
}
If the size of the structure is large, then passing a copy of it won't be very efficient. We could pass a structure pointer to a function. In this case, the address of the structure is passed as an actual argument.
void display(struct student *p)
{
printf("Name: %s\n", p->name);
printf("Roll: %d\n", p->roll);
printf("Gender: %c\n", p->gender);
for(int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n",i,p->marks[i]);
}
void main()
{
struct student stu1 = {"Alex", 43, 'M', {76, 98, 68, 87, 93}};
struct student *ptrStu1 = &stu1;
display(ptrStu1);
}
Passing an array of structure to a function is similar to passing an array of any type to a function. The name of the array, which is the base address of the array of the structure, is passed to the function.
void display(struct student *p)
{
for( int j = 0; j < 10; j++)
{
printf("Name: %s\n", (p+j)->name);
printf("Roll: %d\n", (p+j)->roll);
printf("Gender: %c\n", (p+j)->gender);
for(int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n",i,(p+j)->marks[i]);
}
}
void main()
{
struct student stu1[10];
display(stu1);
}
3. Structure as a Function Return
We can return a structure variable, just like any other variable.
#include <stdio.h>
struct student {
char name[20];
int roll;
char gender;
int marks[5];
};
struct student increaseBy5(struct student p)
{
for( int i =0; i < 5; i++)
if(p.marks[i] + 5 <= 100)
{
p.marks[i]+=5;
}
return p;
}
void main()
{
struct student stu1 = {"Alex", 43, 'M', {76, 98, 68, 87, 93}};
stu1 = increaseBy5(stu1);
printf("Name: %s\n", stu1.name);
printf("Roll: %d\n", stu1.roll);
printf("Gender: %c\n", stu1.gender);
for(int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n",i,stu1.marks[i]);
}
/* Output */
Name: Alex
Roll: 43
Gender: M
Marks in 0th subject: 81
Marks in 1th subject: 98
Marks in 2th subject: 73
Marks in 3th subject: 92
Marks in 4th subject: 98
The function increaseBy5() increases the marks by 5 for subjects where, after increasing the marks, it is less than or equal to 100. Note that the return type is a structure variable of type struct student.
While returning a structure member the return type has to be that of the member.
A structure pointer can also be returned by a function.
#include <stdio.h>
#include <stdlib.h>
struct rectangle {
int length;
int breadth;
};
struct rectangle* function(int length, int breadth)
{
struct rectangle *p = (struct rectangle *)malloc(sizeof(struct rectangle));
p->length = length;
p->breadth = breadth;
return p;
}
void main()
{
struct rectangle *rectangle1 = function(5,4);
printf("Length of rectangle = %d units\n", rectangle1->length);
printf("Breadth of rectangle = %d units\n", rectangle1->breadth);
printf("Area of rectangle = %d square units\n", rectangle1->length * rectangle1->breadth);
}
/* Output */
Length of rectangle = 5 units
Breadth of rectangle = 4 units
Area of rectangle = 20 square units
Notice we have allocated the memory of size struct rectangle dynamically using malloc(). Since it returns a void pointer, we have to typecast it to a struct rectangle pointer.
Self-Referential Structure
We discussed that pointers can be a member of a structure too. What if the pointer is a structure pointer? The structure pointer can either be of the same type as the structure or different.
Self-referential structures are those which have structure pointer(s) of the same type as their member(s).
struct student {
char name[20];
int roll;
char gender;
int marks[5];
struct student *next;
};
This is a self-referential structure where next is a struct student type structure pointer.
We will now create two structure variables stu1 and stu2 and initialize them with values. We will then store the address of stu2 in next member of stu1.
void main()
{
struct student stu1 = {"Alex", 43, 'M', {76, 98, 68, 87, 93}, NULL};
struct student stu2 = { "Max", 33, 'M', {87, 84, 82, 96, 78}, NULL};
stu1.next = &stu2;
}
We can now access the members of stu2 using stu1 and next.
void main()
{
printf("Name: %s\n", stu1.next->name);
printf("Roll: %d\n", stu1.next->roll);
printf("Gender: %c\n", stu1.next->gender);
for(int i = 0; i < 5; i++)
printf("Marks in %dth subject: %d\n",i,stu1.next->marks[i]);
}
/* Output */
Name: Max
Roll: 33
Gender: M
Marks in 0th subject: 87
Marks in 1th subject: 84
Marks in 2th subject: 82
Marks in 3th subject: 96
Marks in 4th subject: 78
Suppose we want a different structure variable after stu1, that is insert another structure variable between stu1 and stu2. This can be done easily.
void main()
{
struct student stuBetween = { "Gasly", 23, 'M', {83, 64, 88, 79, 91}, NULL};
st1.next = &stuBetween;
stuBetween.next = &stu2;
}
Now stu1.next stores the address of stuBetween. And stuBetween.next has the address of stu2. We can now access all three structures using stu1.
printf("Roll Of %s: %d\n", stu1.next->name, stu1.next->roll);
printf("Gender Of %s: %c\n", stu1.next->next->name, stu1.next->next->gender);
/* Output */
Roll Of Gasly: 23
Gender Of Max: M
Notice how we have formed a link between stu1, stuBetween and stu3. What we have discussed here is the starting point of a Linked List.
Self-Referential Structures are very useful in creating data structures such as linked list, stacks, queues, graphs and so on.
Conclusion
Done! We have covered everything from the definition of a structure to the usage of self-referential structures.
Try to recap all the sub-topics that you read. If you can recollect them, well done! Read the ones you can't remember again.
The next logical step would be to learn more about linked lists and various other data structures which have been used here.
Keep learning. Stay home and stay safe.