In C programming, a structure can be both declared and defined. While these two terms are often used interchangeably, they have distinct meanings:

• Declaration: Introduces the structure's name to the compiler, making it available for use.
• Definition: Specifies the members of the structure and reserves memory.

Declaration of a Structure

The declaration of a structure tells the compiler that a structure with a particular name exists, but it doesn’t allocate memory or specify its members. In the context of structures, the declaration is usually part of the definition.

A structure is declared using the struct keyword followed by its name:

struct Person;

This is an incomplete declaration of the structure Person. The members are not defined yet, and you can't use this to declare a structure variable.

Definition of a Structure

The definition of a structure provides the complete details, including the structure members. This involves specifying the types and names of all fields that the structure will have. The definition both declares the structure and describes its layout.

struct Person {
    char name[50];
    int age;
    float height;
};

Here, Person is defined with three members: name, age, and height. This is the full definition, and memory layout for each member is now known to the compiler.

Structure Variable Declaration

Once a structure is defined, you can declare variables of that structure type. This step involves allocating memory for the structure's members:

struct Person person1;  // Declare a variable of type struct Person

This declaration uses the struct Person type, and person1 is a variable that will store the values of name, age, and height.

You can also combine the definition and the declaration of structure variables:

struct Person {
    char name[50];
    int age;
    float height;
} person1, person2;  // Defining structure and declaring variables simultaneously

Summary of Declaration and Definition in C Structures

1. Structure Declaration:

   • Introduces the struct name to the compiler.
   • Does not allocate memory or define members.
   • Example: struct Person; (incomplete declaration).

2. Structure Definition:

   • Specifies the members and their types.
   • Allocates memory for structure variables when declared.
   • Example:

     struct Person {
         char name[50];
         int age;
         float height;
     };
     

3. Declaring Structure Variables:

   • After defining a structure, you can declare variables of that type.
   • Example:

     struct Person person1;  // Declares a structure variable
     

In general, structure definition is often paired with variable declaration, allowing full use of the defined structure immediately. For modular programming, especially when dealing with header files in larger projects, it's common to separate structure declaration and definition for a cleaner codebase.