Segmentation fault with optional arguments in Fortran functions

Question

I can use Fortran optional argumenrs with subroutines with intent(in) and intent(inout), but with functions optional arguments work only with intent(in), right? With intent(inout) I get segmentation faults in the following code:

real function foo(x, tol) 
    real, intent(in) :: x
    real, optional, intent(inout) :: tol
    if( .not. present(tol) ) tol = 1e-6
    !...
end function foo

inout should work see here stackoverflow.com/questions/3121954/… — agentp
– agentp, Commented Aug 26, 2013 at 21:35
Maybe the segfault comes from not testing for presence of the argument? — user1220978
– user1220978, Commented Aug 27, 2013 at 6:21

Vladimir F Героям слава · Accepted Answer · 2019-01-15 12:08:23Z

5

I found the problem, I used the variable even when not present on the fourth line (in tol = 1e-6):

real function foo(x, tol) 
    real, intent(in) :: x
    real, optional, intent(inout) :: tol
    if( .not. present(tol) ) tol = 1e-6
    !...
end function foo

But I would like to use it even when not present and set a default value, like when in C++ we do something like that

double foo(double x, double tol=1e-6)

Unfortunately, it seems it is not possible in Fortran.

edited Jan 15, 2019 at 12:08

Vladimir F Героям слава

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answered Aug 27, 2013 at 12:15

rosilho

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3 Comments

user1220978 Over a year ago

You can't define/assign/use an optional argument that is not present. You have to use another variable. For example, name you optional var opt_tol, and define tol in your program (if present(opt_tol) then tol=opt_tol else tol=default value). See section 12.4.1.6 of the Fortran 2003 standard.

agentp Over a year ago

stackoverflow.com/questions/18302083/…

Steve Lionel Over a year ago

A frequent mistake of those just learning about OPTIONAL is not realizing that the caller of a procedure with OPTIONAL requires an explicit interface to the called routine. See software.intel.com/en-us/forums/topic/275071#comment-1548437 and software.intel.com/en-us/blogs/2012/01/05/… for more info.

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