Fortran Wiki
procedure (Rev #2, changes)

Subroutine

A subroutine looks similar to a program except that it also takes input parameters. It is invoked by a oneline command call.

Outline

<keywords> subroutine <name> (<parameters>)
 <statements> 
end subroutine <name>

Invocation

...
 call <subroutine_name>(<parameters>)
...

Parameter intents

Each parameter must have its intent declared. The intent shows what the subroutine can do with each parameter. Possible intents are: in, out, inout.

• in: The parameter will have an initial value and its value is not allowed to change.
• out: The parameter will not have an initial value and one can set its value. The parameter retains the final value even after the procedure has ended.
• inout (can also we written as in out): An initial value is set and one can change its value. The parameter retains the final value even after the procedure has ended.

Additionally, one can set the value attribute. This allows the parameter to be changed inside of the procedure but the after its end the parameter will change back to its original value. This is useful for describing C functions.

Keywords

Recursive

Only procedures with the keyword recursive are permitted to call themselves.

Elemental

For easy vectorization use the keyword elemental. See also elemental.

Pure

The pure keyword indicates that the procedure has no side effects, e.g. does not alter gloabl variables, or prints messages.