---
title: 6.14 Implicit Function Declarations
description: "An implicit function declaration occurs when a function is called without a prior declaration that specifies its interface. Older C standards permitted certain implicit declarations, but **C99 and later do not allow implicit function declarations**. ``` #include int main(void) {…"
url: "https://edcret.in/docs/c-programming/c-functions/6-14-implicit-function-declarations/"
updated: "2026-10-27"
category: C Programming / Functions
---

# 6.14 Implicit Function Declarations

An implicit function declaration occurs when a function is called without a prior declaration that specifies its interface. Older C standards permitted certain implicit declarations, but **C99 and later do not allow implicit function declarations**.

```
#include <stdio.h>

int main(void)
{
    int result = calculate(10, 20);

    printf("Result = %dn", result);

    return 0;
}

int calculate(int a, int b)
{
    return a + b;
}
```

### Example Output

The program is not valid C99-or-later code as written. A conforming implementation must diagnose the undeclared call to `calculate()`.

### Explanation

The function call appears before the compiler has seen a declaration:

```
int result = calculate(10, 20);
```

The definition appears later:

```
int calculate(int a, int b)
{
    return a + b;
}
```

A later definition does not retroactively provide a declaration for an earlier call.

### Corrected Version

Declare the function before calling it:

```
#include <stdio.h>

int calculate(int a, int b);

int main(void)
{
    int result = calculate(10, 20);

    printf("Result = %dn", result);

    return 0;
}

int calculate(int a, int b)
{
    return a + b;
}
```

### Example Output

```
Result = 30
```

### C Standard Behavior

| Standard | Implicit function declarations |
| --- | --- |
| C90 | Permitted in certain cases, with an assumed return type of `int` |
| C99 | Removed |
| C11 | Not permitted |
| C17 | Not permitted |
| C23 | Not permitted |

Implicit declarations were unsafe because the assumed function interface could differ from the actual definition, potentially causing incorrect behavior. Modern C code should declare functions before calling them.