Implicit function declaration

A subroutine or intrinsic function was called without having been declared.

This usually indicates that the header file that declares this subroutine was not #included. When a subroutine is used without having been declared, many C and FORTRAN compilers generate an implicit declaration as "function returning integer." This behavior is a common cause of errors, because the compiler may generate incorrect code if the implicit declaration does not match the actual definition.

ID	Code Location	Description
1	Call site	The call that provoked the implicit declaration

Example

          
#include <stdio.h>

// Note no #include for <math.h> !

int main(int argc, char **argv)
{
    // sin is not declared, so compiler assumes int sin();
    // next assignment converts "int" return value to double,
    // leading to unexpected result
    double d = sin(1.0);
    printf("sin(1) = %f\n", d);
    return 0;
}