How can I write a function in Fortran which takes both input and output as arguments? For example:
fun(integer input,integer output)
I want to make use of the output value. I have tried something like this but the output variable is not holding the value.
Specifically, I am calling a C function from Fortran which takes input and output as parameters. I am able to pass input values successfully, but the output variable is not acquiring a value.
In Fortran, your fun() is called a subroutine. A function is a value-returning thing like this:
sin_of_x = sin(x)
So your first decision is which approach your Fortran code will take. You probably want to use a subroutine. Then sort out the intent of your arguments.
An example. if you want a function that returns void you should use a subroutine instead.
function foo(input, output)
implicit none
integer :: foo
integer, intent(in) :: input
integer, intent(out) :: output
output = input + 3
foo = 0
end function
program test
implicit none
integer :: a, b, c, foo
b = 5
a = foo(b, c)
print *,a,b, c
end program
If you are calling a C routine, then the signature makes use of references.
$ cat test.f90
program test
implicit none
integer :: a, b, c, foo
b = 5
a = foo(b, c)
print *,a,b, c
end program
$ cat foo.c
#include <stdio.h>
int foo_(int *input, int *output) {
printf("I'm a C routine\n");
*output = 3 + *input;
return 0;
}
$ g95 -c test.f90
$ gcc -c foo.c
$ g95 test.o foo.o
$ ./a.out
I'm a C routine
0 5 8
if you use strings, things gets messy.
