To: J3                                                     J3/18-258r1
From: Dan Nagle & Steve Lionel
Subject: cstring-fstring
Date: 2018-October-16



I Introduction

When interoperating between C and Fortran, strings may need to be passed
between procedures written in C and those written in Fortran.  This
raises the issue of how to convert strings between the representation
used by either language.

Fortran strings have a length, which is stored as metadata.
C strings have a starting address and may be terminated
by a null character.  (This "null-terminated string" is not mandated
by the C standard, but is a convention employed by several library
procedures of C.)


II Use-cases


Fortran to C

15.5.2.4p3 makes an exception to the requirement that length type
parameters of the actual and dummy arguments agree "for the case
of the character length parameter of an actual argument of type
character with default kind or C character kind (18.2.2)
associated with a dummy argument that is not assumed-shape or
assumed-rank. " This means that one typically declares character
dummy arguments for C functions to be arrays of characters.

C, however, typically wants to see a NUL at the end of the string
to indicate its length, so Fortran code will usually pass

TRIM(char-value)//C_NULL_CHAR

Having an intrinsic that performs this function would allow for
cleaner code. Some users might not want the TRIM operation
so it would make sense to have this as an option.


Fortran to C

When a C function passes a string to a Fortran interoperable procedure
it can't be received as type CHARACTER except as an array of single
characters. There is no straightforward way to convert this to a
Fortran character value of the proper length (determined by searching
for a NUL.) An intrinsic that takes such a character array as an
argument and returns a character value of the correct length would
make this much less painful.


III Specification

Add two new procedures to intrinsic module ISO_C_BINDING to provide
the conversion. All arguments are INTENT(IN).

C_F_STRING (ARRAY, MAXLEN)

Class: Transformational function

ARRAY shall be a rank-1 character array of default kind or kind
C_CHAR, and with a length type parameter of 1

MAXLEN shall be a scalar integer

Result characteristics: The result is a character scalar with the same
kind type parameter as ARRAY.

Result value:

I am having trouble coming up with proper wording for this. The idea is:

Each character of the result value is the value of the corresponding
element of ARRAY, in element sequence order, until a value of CHAR(0)
is found, or MAXLEN elements have been examined, or the number of
elements in ARRAY have been examined (if not assumed-size), whichever
comes first. The length of the result value is one less than the
index of the ARRAY sequence element that contains CHAR(0).

The reason for requiring MAXLEN is to help avoid the security issues of
walking off the end of the value.


F_C_STRING (STRING [, TRIM])

Class: Transformational function

STRING shall be a scalar of default kind or of kind C_CHAR

TRIM (optional) shall be a logical scalar

Result characteristics: The result is of type character with
the same kind type parameter as STRING.

Result value:

Case (i): if TRIM is absent or has the value true, the value
of the result is the value of STRING with any trailing blanks
removed, and a NUL character (CHAR(0)) appended to the trimmed
value. The length type parameter is one greater than the length
of STRING less the number of trailing blanks in STRING.

Case (ii): if TRIM is present with the value false, the
value of the result is the value of STRING with a NUL character
(CHAR(0)) appended. The length type parameter of the result is one
greater than the length of STRING.