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 = %d\n", 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 = %d\n", result);

    return 0;
}

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

Example Output #

Result = 30

C Standard Behavior #

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.