---
title: 6.7 Function Declarations and Definitions
description: "A function declaration specifies a function's interface, including its return type and parameter types. A function definition provides the implementation that executes when the function is called. ``` #include int subtract(int a, int b); /* Declaration */ int main(void) {…"
url: "https://edcret.in/docs/c-programming/c-functions/6-7-function-declarations-and-definitions/"
updated: "2026-10-27"
category: C Programming / Functions
---

# 6.7 Function Declarations and Definitions

A function declaration specifies a function's interface, including its return type and parameter types. A function definition provides the implementation that executes when the function is called.

```
#include <stdio.h>

int subtract(int a, int b);  /* Declaration */

int main(void)
{
    int result = subtract(20, 8);

    printf("Result = %d\n", result);

    return 0;
}

int subtract(int a, int b)   /* Definition */
{
    return a - b;
}
```

### Example Output

```
Result = 12
```

### Explanation

The declaration:

```
int subtract(int a, int b);
```

informs the compiler that `subtract()` returns an `int` and accepts two `int` parameters. It does not contain the function body.

The definition:

```
int subtract(int a, int b)
{
    return a - b;
}
```

provides the implementation. It calculates the difference between the arguments and returns the result.

### Declaration vs. Definition

| Feature | Function declaration | Function definition |
| --- | --- | --- |
| Specifies return type | Yes | Yes |
| Specifies parameter types | Yes | Yes |
| Contains function body | No | Yes |
| Ends with a semicolon after the parameter list | Yes | No |
| Provides the implementation | No | Yes |

A function can be declared more than once when the declarations are compatible, but a function must not have multiple definitions in the same program where that violates C's definition rules.

### Function Defined Before Its Call

A separate prototype is not necessary when the function definition itself appears before the call:

```
#include <stdio.h>

int subtract(int a, int b)
{
    return a - b;
}

int main(void)
{
    printf("%d\n", subtract(20, 8));
    return 0;
}
```

Here, the definition makes the function's interface known before `main()` calls it.