Declaration vs Definition in C

 Section 18.1: Understanding Declaration and Definition

A declaration introduces an identifier and describes its type, be it a type, object, or function. A declaration is what the compiler needs to accept references to that identifier. These are declarations:

    extern int bar;

    extern int g(int, int);

    double f(int, double); /* extern can be omitted for function declarations */

    double h1(); /* declaration without prototype */

    double h2(); /* ditto */

A definition actually instantiates/implements this identifier. It's what the linker needs in order to link references to those entities. These are definitions corresponding to the above declarations:

    int bar;

    int g(int lhs, int rhs) {return lhs*rhs;}

    double f(int i, double d) {return i+d;}

    double h1(int a, int b) {return -1.5;}

    double h2() {} /* prototype is implied in definition, same as double h2(void) */

A definition can be used in the place of a declaration.

However, it must be defined exactly once. If you forget to define something that's been declared and referenced somewhere, then the linker doesn't know what to link references to and complains about a missing symbols. If youdefine something more than once, then the linker doesn't know which of the definitions to link references to and complains about duplicated symbols.

    Exception:

    extern int i = 0; /* defines i */

    extern int j; /* declares j */

This exception can be explained using concepts of "Strong symbols vs Weak symbols" (from a linker's perspective) .

    /* All are definitions. */

    struct S { int a; int b; }; /* defines S */

    struct X

     {     /* defines X */

            int x; /* defines non-static data member x */

    };

    struct X anX; /* defines anX */

 

Difference between Declaration and Definition in C

S.No.	Declaration	Definition
1	Tells compiler about name and type of variable, class, function, etc.	Tells compiler about what value stored in variable or working of function, class, etc.
2	Memory allocation is not done.	Memory allocation is done.
3	Can re-declare multiple times.	Can define only once.

    

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