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From: stevesu@copper.UUCP
Newsgroups: comp.lang.c
Subject: Re: Fortran vs. C strings
Message-ID: <1142@copper.TEK.COM>
Date: Fri, 19-Jun-87 01:14:41 EDT
Article-I.D.: copper.1142
Posted: Fri Jun 19 01:14:41 1987
Date-Received: Sat, 20-Jun-87 07:34:44 EDT
References: <6480@dartvax.UUCP>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 83
Summary: Use descriptors

I was going to reply by mail as requested, but since
misinformation has already been posted, I'll present a
quick synopsis here.

VMS, in spite of its faults, embodies a nice concept called the
VAX Calling Standard, which virtually guarantees that programs
written in any language can call each other fairly easily.
The calling standard describes not only the stack frame (which is
implemented by the VAX CALLS and CALLG instructions), but also
conventions for passing typed data, which is the responsibility
of the various compilers (or, in the case of a low-level high
level language like C, the programmer).

Character strings are passed by descriptor.  A descriptor is an
8-byte structure that looks like this:
	 _______________________________
	|__c_l_a_s_s__|__d_t_y_p_e__|_____l_e_n_g_t_h_____|
	|______________p_o_i_n_t_e_r___________|

or, as a C struct:

	struct descriptor
		{
		unsigned short int dsc_length;
		unsigned char dsc_dtype;
		unsigned char dsc_class;
		char *dsc_pointer;
		};

When passing a string to another routine, it is usually
sufficient to fill in the dtype and class as 0.  (Purists may
wish to use more appropriate values; see references below.)

Here is a quick example.  SYS$ASSIGN is not written in Fortran,
but the descriptor-passing mechanism is the same.

	#define IO$_WRITEVBLK 0x30

	main()
	{
	struct descriptor d;
	short int channel;

	d.dsc_pointer = "sys$output";
	d.dsc_length = 10;
	d.dsc_dtype = d.dsc_class = 0;

	sys$assign(&d, &channel, 0, 0);
	sys$qiow(0, channel, IO$_WRITEVBLK, 0, 0, 0, "Hello, world!\r", 14,
								0, 0, 0, 0);
	}

Note that descriptors are almost always passed by reference
(struct descriptor *).

Passing a descriptor which is to be filled in by the caller, and
accepting a descriptor that has been passed by another routine,
are trickier because you have to deal with some more unusual
descriptor classes, including one in which the string is stored
two bytes beyond the pointer, the pointer pointing to a word
holding the string's current length.  I believe that
LIB$ANALYZE_SDESC can be used to deal with arbitrary string
descriptors in a general way, without having to know about all of
the descriptor classes.

There are some header files or something that come with VAXC,
DEC's VMS C compiler, that make handling descriptors a bit
easier.

                                           Steve Summit
                                           stevesu@copper.tek.com

References:

	VAX-11 Guide to Creating Modular Library Procedures
		(Obsolete VMS V3 manual), section 4.5
	VAX/VMS Run-Time Library Routines Reference Manual,
		section 2.6
	Introduction to VAX/VMS System Routines, section 2.9.1

Disclaimer: This entire article has of course been concerned
solely with VMS-related issues.  Strings are passed to and from
Fortran programs on Unix in a completely different way.