Fortran向C传递NULL值

在很多C或C++的头文件定义中，NULL被指定定义为0，这里不再具体展开

gfortran的手册关于iso c binding的章节，定义NULL如下

Moreover, the following two named constants are defined:

Name	Type
C_NULL_PTR	C_PTR
C_NULL_FUNPTR	C_FUNPTR

Both are equivalent to the value NULL in C.

据此，从Fortran向C或C++传递空指针还是需要一些技巧，也即需要把传入参数设置为指针类型C_PTR

接下来是代码：

#include "stdio.h"

int test(int *a, int num)
{
    if(!a){
        printf("This is null pointer
");
    }
    else{
        for(int i=0; i<num; i++){
            printf("Array[%d]=%d
", i+1, a[i]);
        }
    }
    return 0;
}

program main
use, intrinsic:: iso_c_binding
implicit none
interface
    integer(c_int) function func(array, n) bind(c, name="test")
    import
    implicit none
    integer(c_int), intent(in), value:: n
    integer(c_int), intent(in):: array(n)
    end function
    
    integer(c_int) function func2(pt, n) bind(c, name="test")
    import
    implicit none
    integer(c_int), intent(in), value:: n
    type(c_ptr), value:: pt
    end function
end interface

type(c_ptr) :: a1
integer(c_int), target:: abc(5)
integer:: i

abc = [12, 12, 35, 67, 11]
a1 = c_loc(abc(1))
! Pass array directly
i = func(abc, size(abc))
! Pass address
i = func2(c_loc(abc(1)), 5)
! Pass null pointer
i = func2(c_null_ptr, 5)
! Pass null function pointer
i = func2(c_null_funptr, 5)
end program

gfortran code_c.c code_for.f90 -o test

运行结果：

Array[1]=12
Array[2]=12
Array[3]=35
Array[4]=67
Array[5]=11

Array[1]=12
Array[2]=12
Array[3]=35
Array[4]=67
Array[5]=11

This is null pointer

This is null pointer